AD6645 Datasheet
80 MSPS/105 MSPS A/D Converter

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14-Bit, 80 MSPS/105 MSPS
A/D Converter
AD6645
FEATURES
SNR = 75 dB, fIN 15 MHz, up to 105 MSPS
SNR = 72 dB, fIN 200 MHz, up to 105 MSPS
SFDR = 89 dBc, fIN 70 MHz, up to 105 MSPS
100 dBFS multitone SFDR
IF sampling to 200 MHz
Sampling jitter: 0.1 ps
1.5 W power dissipation
Differential analog inputs
Pin compatible to AD6644
Twos complement digital output format
3.3 V CMOS compatible
Data-ready for output latching
APPLICATIONS
Multichannel, multimode receivers
Base station infrastructures
AMPS, IS-136, CDMA, GSM, W-CDMA
Single channel digital receivers
Antenna array processing
Communications instrumentation
Radars, infrared imaging
Instrumentation
GENERAL DESCRIPTION
The AD6645 is a high speed, high performance, monolithic 14-bit
analog-to-digital converter (ADC). All necessary functions,
including track-and-hold (T/H) and reference, are included on the
chip to provide a complete conversion solution. The AD6645
provides CMOS-compatible digital outputs. It is the fourth
generation in a wideband ADC family, preceded by the AD9042
(12-bit, 41 MSPS), the AD6640 (12-bit, 65 MSPS, IF sampling),
and the AD6644 (14-bit, 40 MSPS/65 MSPS).
Designed for multichannel, multimode receivers, the AD6645 is
part of the Analog Devices, Inc., SoftCell® transceiver chipset.
The AD6645 maintains 100 dB multitone, spurious-free dynamic
range (SFDR) through the second Nyquist band. This breakthrough
performance eases the burden placed on multimode digital
receivers (software radios) that are typically limited by the ADC.
Noise performance is exceptional; typical signal-to-noise ratio
(SNR) is 74.5 dB through the first Nyquist band.
The AD6645 is built on the Analog Devices extra fast
complementary bipolar (XFCB) process and uses an innovative,
multipass circuit architecture. Units are available in thermally
enhanced 52-lead PowerQuad 4 (LQFP_PQ4) and 52-lead
exposed pad (TQFP_EP) packages specified from −40°C to
+85°C at 80 MSPS and −10°C to +85°C at 105 MSPS.
PRODUCT HIGHLIGHTS
1. IF Sampling. The AD6645 maintains outstanding ac
performance up to input frequencies of 200 MHz, suitable
for multicarrier 3G wideband cellular IF sampling receivers.
2. Pin Compatibility. The ADC has the same footprint and
pin layout as the AD6644 14-bit, 40 MSPS/65 MSPS ADC.
3. SFDR Performance and Oversampling. Multitone SFDR
performance of 100 dBFS can reduce the requirements of
high end RF components and allows the use of receive
signal processors, such as the AD6620, AD6624/AD6624A,
or AD6636.
AVCC
DVCC
FUNCTIONAL BLOCK DIAGRAM
AIN
A1 TH1
AIN
TH2
AD6645
A2 TH3
TH4
TH5 ADC3
VREF
2.4V
ENCODE
ENCODE
INTERNAL
TIMING
ADC1
DAC1
5
ADC2
DAC2
5
DIGITAL ERROR CORRECTION LOGIC
6
GND
DMID OVR DRY D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
MSB
LSB
Figure 1.
Rev. D
Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarksandregisteredtrademarksarethepropertyoftheirrespectiveowners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.461.3113 ©2002–2008 Analog Devices, Inc. All rights reserved.


AD6645 Datasheet
80 MSPS/105 MSPS A/D Converter

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AD6645* Product Page Quick Links
Last Content Update: 11/01/2016
Comparable Parts
View a parametric search of comparable parts
Evaluation Kits
• AD6645 Evaluation Board
Documentation
Application Notes
• AN-1142: Techniques for High Speed ADC PCB Layout
• AN-282: Fundamentals of Sampled Data Systems
• AN-345: Grounding for Low-and-High-Frequency Circuits
• AN-501: Aperture Uncertainty and ADC System
Performance
• AN-586: LVDS Outputs for High Speed A/D Converters
• AN-715: A First Approach to IBIS Models: What They Are
and How They Are Generated
• AN-737: How ADIsimADC Models an ADC
• AN-741: Little Known Characteristics of Phase Noise
• AN-756: Sampled Systems and the Effects of Clock Phase
Noise and Jitter
• AN-807: Multicarrier WCDMA Feasibility
• AN-808: Multicarrier CDMA2000 Feasibility
• AN-835: Understanding High Speed ADC Testing and
Evaluation
• AN-905: Visual Analog Converter Evaluation Tool Version
1.0 User Manual
• AN-935: Designing an ADC Transformer-Coupled Front
End
Data Sheet
• AD6645: 14-Bit, 80 MSPS/105 MSPS A/D Converter Data
Sheet
Software and Systems Requirements
• Military Part Cross-Reference Guide
• Military Products by Function
• Military Products by GENERIC Part Number
Tools and Simulations
• Visual Analog
• AD6645 IBIS Models
Reference Designs
• CN0109
Reference Materials
Customer Case Studies
• Ball Aerospace Case Study
Technical Articles
• Buffer Adapts Single-ended Signals for Differential Inputs
• Cascaded Transformers Achieve Better Frequency
Performance in High-Speed ADC
• Converters for 3G are Optimized for Cost, Size and Power
• Correlating High-Speed ADC Performance to Multicarrier
3G Requirements
• DNL and Some of its Effects on Converter Performance
• MS-2210: Designing Power Supplies for High Speed ADC
• MS-2735: Maximizing the Dynamic Range of Software-
Defined Radio
Design Resources
• AD6645 Material Declaration
• PCN-PDN Information
• Quality And Reliability
• Symbols and Footprints
Discussions
View all AD6645 EngineerZone Discussions
Sample and Buy
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Technical Support
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number
* This page was dynamically generated by Analog Devices, Inc. and inserted into this data sheet. Note: Dynamic changes to
the content on this page does not constitute a change to the revision number of the product data sheet. This content may be
frequently modified.


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80 MSPS/105 MSPS A/D Converter

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AD6645
TABLE OF CONTENTS
Features .............................................................................................. 1 
Applications....................................................................................... 1 
General Description ......................................................................... 1 
Product Highlights ........................................................................... 1 
Functional Block Diagram .............................................................. 1 
Revision History ............................................................................... 2 
Specifications..................................................................................... 3 
DC Specifications ......................................................................... 3 
Digital Specifications ................................................................... 4 
AC Specifications.......................................................................... 4 
Switching Specifications .............................................................. 5 
Absolute Maximum Ratings............................................................ 7 
Thermal Resistance ...................................................................... 7 
REVISION HISTORY
10/08—Rev. C to Rev. D
Added TQFP_EP Package ............................................ Throughout
Renamed Thermal Characteristics Section Thermal Resistance
Section................................................................................................ 7
Added Table 6; Renumbered Sequentially .................................... 7
Moved Equivalent Circuits Section.............................................. 14
Moved Terminology Section......................................................... 15
Changes to Table 9.......................................................................... 20
Updated Outline Dimensions ....................................................... 24
Changes to Ordering Guide .......................................................... 24
12/06—Rev. B to Rev. C
Updated Format..................................................................Universal
Changes to Specifications ................................................................ 3
Changes to Jitter Considerations Section .................................... 19
Changes to Table 8, Bill of Materials............................................ 20
Changes to Figure 43, Evaluation Board Schematic .................. 21
Changes to Figure 44 and Figure 46............................................. 22
Updated Outline Dimensions ....................................................... 23
Changes to Ordering Guide .......................................................... 23
Explanation of Test Levels............................................................7 
ESD Caution...................................................................................7 
Pin Configuration and Function Descriptions..............................8 
Typical Performance Characteristics ..............................................9 
Equivalent Circuits......................................................................... 14 
Terminology .................................................................................... 15 
Theory of Operation ...................................................................... 17 
Applying the AD6645 ................................................................ 17 
Layout Information ........................................................................ 19 
Jitter Considerations .................................................................. 19 
Outline Dimensions ....................................................................... 24 
Ordering Guide .......................................................................... 24 
7/03—Rev. A to Rev. B.
Changes to Title ................................................................................1
Changes to Features ..........................................................................1
Changes to Product Description .....................................................1
Changes to Specifications.................................................................3
Changes to Absolute Maximum Ratings........................................7
Changes to Ordering Guide .......................................................... 24
Updated Outline Dimensions....................................................... 20
6/02—Rev. 0 to Rev. A.
Change to DC Specifications ...........................................................3
Rev. D | Page 2 of 24


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AD6645
SPECIFICATIONS
DC SPECIFICATIONS
AVCC = 5 V, DVCC = 3.3 V; TMIN and TMAX at rated speed grade, unless otherwise noted.
Table 1.
Parameter
RESOLUTION
ACCURACY
No Missing Codes
Offset Error
Gain Error
Differential Nonlinearity (DNL)
Integral Nonlinearity (INL)
TEMPERATURE DRIFT
Offset Error
Gain Error
POWER SUPPLY REJECTION RATIO
(PSRR)
REFERENCE OUT (VREF)1
ANALOG INPUTS (AIN, AIN)
Differential Input Voltage Range
Differential Input Resistance
Differential Input Capacitance
POWER SUPPLY
Supply Voltages
AVCC
DVCC
Supply Current
IAVCC (AVCC = 5.0 V)
IDVCC (DVCC = 3.3 V)
Rise Time2
AVCC
POWER CONSUMPTION
Temp
Full
Full
Full
Full
Full
Full
Full
25°C
Full
Full
Full
25°C
Full
Full
Full
Full
Full
Full
Test Level
II
II
II
II
V
V
V
V
V
V
V
II
II
II
II
IV
II
AD6645ASQ-80/AD6645ASV-80
Min Typ
Max
14
Guaranteed
−10 +1.2
+10
−10 0
+10
−1.0 ±0.25
+1.5
±0.5
1.5
48
±1.0
2.4
2.2
1
1.5
4.75 5.0
3.0 3.3
275
32
1.5
5.25
3.6
320
45
250
1.75
AD6645ASQ-105/AD6645ASV-105
Min Typ Max
14
Guaranteed
−10 +1.2 +10
−10 0
+10
−1.0 ±0.5 +1.5
±1.5
1.5
48
±1.0
2.4
2.2
1
1.5
4.75 5.0
3.0 3.3
275
32
5.0
1.5
5.25
3.6
320
45
250
1.75
Unit
Bits
mV
% FS
LSB
LSB
ppm/°C
ppm/°C
mV/V
V
V p-p
pF
V
V
mA
mA
ms
W
1 VREF is provided for setting the common-mode offset of a differential amplifier, such as the AD8138, when a dc-coupled analog input is required. VREF should be
buffered if used to drive additional circuit functions.
2 Specified for dc supplies with linear rise time characteristics.
Rev. D | Page 3 of 24


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AD6645
DIGITAL SPECIFICATIONS
AVCC = 5 V, DVCC = 3.3 V; TMIN and TMAX at rated speed grade, unless otherwise noted.
Table 2.
Parameter
ENCODE INPUTS (ENCODE, ENCODE)
Differential Input Voltage1
Differential Input Resistance
Differential Input Capacitance
LOGIC OUTPUTS (D13 to D0, DRY, OVR)
Logic Compatibility
Logic 1 Voltage (DVCC = 3.3 V)2
Logic 0 Voltage (DVCC = 3.3 V)2
Output Coding
DMID
Temp
Full
25°C
25°C
Full
Full
Full
Test
Level
IV
V
V
II
II
V
AD6645ASQ-80/AD6645ASV-80
Min Typ
Max
0.4
10
2.5
CMOS
2.85 DVCC − 2
0.2 0.5
Twos complement
DVCC/2
AD6645ASQ-105/AD6645ASV-105
Min Typ
Max
0.4
10
2.5
CMOS
2.85 DVCC − 2
0.2
Twos complement
DVCC/2
0.5
Unit
V p-p
pF
V
V
V
1 All ac specifications tested by driving ENCODE and ENCODE differentially.
2 Digital output logic levels: DVCC = 3.3 V, CLOAD = 10 pF. Capacitive loads >10 pF degrades performance.
AC SPECIFICATIONS
All ac specifications tested by driving ENCODE and ENCODE differentially. AVCC = 5 V, DVCC = 3.3 V; ENCODE, ENCODE, TMIN and
TMAX at rated speed grade, unless otherwise noted.
Table 3.
Parameter
SNR
Analog Input @ −1 dBFS
SINAD
Analog Input @ −1 dBFS
WORST HARMONIC (SECOND OR THIRD)
Analog Input @ −1 dBFS
Temp
Test
Level
AD6645ASQ-80/
AD6645ASV-80
Min Typ Max
AD6645ASQ-105/
AD6645ASV-105
Min Typ Max
Unit
Conditions
25°C V
Full II
25°C I
Full II
25°C V
25°C V
75.0
72.5 74.5
72.0 73.5
73.0
72.0
75.0
72.5 74.5
72.0 73.5
73.0
72.0
dB At 15.5 MHz
dB At 30.5 MHz
dB At 37.7 MHz
dB At 70.0 MHz
dB At 150.0 MHz
dB At 200.0 MHz
25°C V
Full II
25°C I
Full V
25°C V
25°C V
75.0
72.5 74.5
73.0
68.5
62.5
75.0
72.5 74.5
73.0
67.5
62.5
dB At 15.5 MHz
dB At 30.5 MHz
dB At 37.7 MHz
dB At 70.0 MHz
dB At 150.0 MHz
dB At 200.0 MHz
25°C V
Full II
25°C I
Full V
25°C V
25°C V
93.0
85.0 93.0
89.0
70.0
63.5
93.1
85.0 93.0
87.0
70.0
63.5
dBc At 15.5 MHz
dBc At 30.5 MHz
dBc At 37.7 MHz
dBc At 70.0 MHz
dBc At 150.0 MHz
dBc At 200.0 MHz
Rev. D | Page 4 of 24


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AD6645
Parameter
WORST HARMONIC (FOURTH OR HIGHER)
Analog Input @ −1 dBFS
TWO-TONE SFDR
TWO-TONE IMD REJECTION2, 3
F1, F2 @ −7 dBFS
ANALOG INPUT BANDWIDTH
Temp
Test
Level
AD6645ASQ-80/
AD6645ASV-80
Min Typ Max
AD6645ASQ-105/
AD6645ASV-105
Min Typ Max
Unit
Conditions
25°C V
Full II
25°C I
Full V
25°C V
25°C V
25°C V
25°C V
25°C V
96.0
85.0 95.0
90.0
90.0
88.0
100
100
96.0
86.0 95.0
90.0
90.0
88.0
98.0
98.0
98.0
dBc
dBc
dBc
dBc
dBc
dBc
dBFS
dBFS
dBFS
At 15.5 MHz
At 30.5 MHz
At 37.7 MHz
At 70.0 MHz
At 150.0 MHz
At 200.0 MHz
At 30.5 MHz1, 2
At 55.0 MHz1, 3
At 70.0 MHz1, 4
25°C V
25°C V
90
270
90 dBc
270 MHz
1 Analog input signal power swept from −10 dBFS to −100 dBFS.
2 F1 = 30.5 MHz, F2 = 31.5 MHz.
3 F1 = 55.25 MHz, F2 = 56.25 MHz.
4 F1 = 69.1 MHz, F2 = 71.1 MHz.
SWITCHING SPECIFICATIONS
AVCC = 5 V, DVCC = 3.3 V; ENCODE, ENCODE, TMIN and TMAX at rated speed grade, unless otherwise noted.
Table 4.
Parameter
ENCODE INPUT PARAMETERS1
Maximum Conversion Rate
Minimum Conversion Rate
ENCODE Pulse Width High, tENCH2
ENCODE Pulse Width Low, tENCL2
ENCODE Period1
ENCODE/DATA-READY
ENCODE Rising to Data-Ready Falling
ENCODE Rising to Data-Ready Rising
50% Duty Cycle
ENCODE/DATA (D13:0), OVR
ENCODE to DATA Falling Low
ENCODE to DATA Rising Low3
ENCODE to DATA Delay3 (Hold Time)
ENCODE to DATA Delay (Setup Time)
50% Duty Cycle
AD6645ASQ-80/
Test AD6645ASV-80
Symbol Temp Level Min Typ
Max
AD6645ASQ-105/
AD6645ASV-105
Min Typ
Max
Unit
Full II
80
105 MSPS
Full IV 30 30 MSPS
Full IV
5.625
4.286
ns
Full V
6.25
4.75 ns
Full IV
5.625
4.286
ns
Full V
6.25
4.75 ns
tENC Full V
12.5
9.5 ns
tDR Full V 1.0 2.0 3.1 1.0 2.0 3.1 ns
tE_DR
Full V
tENCH + tDR
tENCH + tDR
ns
Full V 7.3 8.3
9.4 5.7 6.75
7.9 ns
tE_FL Full V 2.4 4.7
7.0 2.4 4.7
7.0 ns
tE_RL Full V 1.4 3.0
4.7 1.4 3.0
4.7 ns
tH_E Full V 1.4 3.0
4.7 1.4 3.0
4.7 ns
tS_E
Full V
tENC
tE_FL(max)
tENC
tE_FL(max)
ns
tENC
tE_FL(typ)
tENC
tE_FL(typ)
ns
tENC
tE_FL(min)
tENC
tE_FL(min)
ns
Full V 5.3 7.6
10.0 2.3
4.8
7.0 ns
Rev. D | Page 5 of 24


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AD6645
Parameter
DATA-READY (DRY4)/DATA(D13:0),, OVR
Data-Ready to DATA Delay (Hold Time)
50% Duty Cycle
Data-Ready to DATA Delay (Setup Time)
50% Duty Cycle
APERTURE DELAY
APERTURE UNCERTAINTY (JITTER)
Symbol
tH_DR
tS_DR
tA
tJ
Temp
Full
Full
Full
Full
25°C
25°C
Test
Level
V
V
V
V
V
V
AD6645ASQ-80/
AD6645ASV-80
Min Typ
Max
Note 55
6.6 7.2
7.9
Note 55
2.1 3.6
5.1
−500
0.1
AD6645ASQ-105/
AD6645ASV-105
Min Typ
Max
Note 55
5.1 5.7
6.4
Note 55
0.6 2.1
3.5
−500
0.1
1 Several timing parameters are a function of tENC and tENCH.
2 Several timing parameters are a function of tENCL and tENCH.
3 ENCODE TO DATA Delay (Hold Time) is the absolute minimum propagation delay through the ADC, tE_RL = tH_E.
4 DRY is an inverted and delayed version of the encode clock. Any change in the duty cycle of the clock will correspondingly change the duty cycle of DRY.
5 Data-ready to DATA Delay (tH_DR and tS_DR) is calculated relative to rated speed grade and is dependent on tENC and duty cycle.
Unit
ns
ns
ps
ps rms
tA
N
N+3
AIN
N+1
N+2
ENCODE,
ENCODE
tE_RL
D[13:0], OVR
DRY
tENC
N
tE_FL
tENCH
N+1
N–3
tENCL
N–2
N+2
tE_DR
tS_DR
N+3
tS_E
N–1
tH_DR
tDR
Figure 2. Timing Diagram
N+4
N+4
tH_E
N
Rev. D | Page 6 of 24


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ABSOLUTE MAXIMUM RATINGS
Table 5.
Parameter
Electrical
AVCC Voltage
DVCC Voltage
Analog Input Voltage
Analog Input Current
Digital Input Voltage
Digital Output Current
Environmental
Operating Temperature Range (Ambient)
AD6645-80
AD6645-105
Maximum Junction Temperature
Lead Temperature (Soldering, 10 sec)
Storage Temperature Range (Ambient)
Rating
0 V to 7 V
0 V to 7 V
0 V to AVCC
25 mA
0 V to AVCC
4 mA
−40°C to +85°C
−10°C to +85°C
150°C
300°C
−65°C to +150°C
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
THERMAL RESISTANCE
The heat sink of the AD6645ASVZ, 52-lead TQFP_EP (SV-52-1)
package must be soldered to the PCB GND plane to meet thermal
specifications.
Table 6. Thermal Characteristics
Package Type
Rating
52-Lead TQFP_EP
θJA (0 m/sec airflow)1, 2, 3
θJMA (1.0 m/sec airflow)2, 3, 4, 5
θJC6, 7
23°C/W, soldered heat sink
17°C/W, soldered heat sink
2°C/W, soldered heat sink
52-Lead LQFP_PQ4
θJA (0 m/sec airflow)1, 2, 3
θJMA (1.0 m/sec airflow)2, 3, 4, 5
θJA (0 m/sec airflow)1, 2, 3
θJMA (1.0 m/sec airflow)2, 3, 4, 5
θJC6, 7
30°C/W, unsoldered heat sink
24°C/W, unsoldered heat sink
23°C/W, soldered heat sink
17°C/W, soldered heat sink
2°C/W
1 Per JEDEC JESD51-2 (heat sink soldered to PCB).
2 2S2P JEDEC test board.
3 Values of θJA are provided for package comparison and PCB design
considerations.
4 Per JEDEC JESD51-6 (heat sink soldered to PCB).
5 Airflow increases heat dissipation, effectively reducing θJA. Furthermore, the
more metal that is directly in contact with the package leads from metal
traces, throughholes, ground, and power planes, the more θJA is reduced.
6 Per MIL-STD-883, Method 1012.1.
7 Values of θJC are provided for package comparison and PCB design
considerations when an external heat sink is required.
AD6645
Values of θJA are provided for package comparison and PCB
design considerations. θJA can be used for a first-order
approximation of TJ by the equation
TJ = TA + (θJA × PD)
where:
TA is the ambient temperature (°C).
PD is the power dissipation (W).
EXPLANATION OF TEST LEVELS
I. 100% production tested.
II. 100% production tested at 25°C and guaranteed by design
and characterization at temperature extremes.
III. Sample tested only.
IV. Parameter is guaranteed by design and characterization
testing.
V. Parameter is a typical value only.
ESD CAUTION
Rev. D | Page 7 of 24


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AD6645
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
52 51 50 49 48 47 46 45 44 43 42 41 40
DVCC 1
GND 2
VREF 3
GND 4
ENCODE 5
ENCODE 6
GND 7
AVCC 8
AVCC 9
GND 10
AIN 11
AIN 12
GND 13
PIN 1
IDENTIFIER
AD6645
TOP VIEW
(Not to Scale)
39 D3
38 D2
37 D1
36 D0 (LSB)
35 DMID
34 GND
33 DVCC
32 OVR
31 DNC
30 AVCC
29 GND
28 AVCC
27 GND
14 15 16 17 18 19 20 21 22 23 24 25 26
NOTES
1. DNC = DO NOT CONNECT.
2. EXPOSED PAD. CONNECT THE EXPOSED PAD TO GND.
Figure 3. Pin Configuration
Table 7. Pin Function Descriptions
Pin Number
Mnemonic
1, 33, 43
2, 4, 7, 10, 13, 15, 17, 19, 21, 23, 25,
27, 29, 34, 42
DVCC
GND
3 VREF
5 ENCODE
6 ENCODE
8, 9, 14, 16, 18, 22, 26, 28, 30
11
AVCC
AIN
12 AIN
20 C1
24 C2
31 DNC
32 OVR
35 DMID
36 D0 (LSB)
37 to 41, 44 to 50
D1 to D5, D6 to D12
51 D13 (MSB)
52 DRY
53 (EPAD)
Exposed Paddle (EPAD)
Description
3.3 V Power Supply (Digital) Output Stage Only.
Ground.
2.4 V Reference. Bypass to ground with a 0.1 μF microwave chip capacitor.
Encode Input. Conversion initiated on rising edge.
Complement of ENCODE, Differential Input.
5 V Analog Power Supply.
Analog Input.
Complement of AIN, Differential Analog Input.
Internal Voltage Reference. Bypass to ground with a 0.1 μF chip capacitor.
Internal Voltage Reference. Bypass to ground with a 0.1 μF chip capacitor.
Do not connect this pin.
Overrange Bit. A logic level high indicates analog input exceeds ±FS.
Output Data Voltage Midpoint. Approximately equal to (DVCC)/2.
Digital Output Bit (Least Significant Bit); Twos Complement.
Digital Output Bits in Twos Complement.
Digital Output Bit (Most Significant Bit); Twos Complement.
Data-Ready Output.
Exposed Pad. Connect the exposed pad to GND.
Rev. D | Page 8 of 24


AD6645 Datasheet
80 MSPS/105 MSPS A/D Converter

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TYPICAL PERFORMANCE CHARACTERISTICS
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
–130
0
ENCODE = 80MSPS
AIN = 2.2MHz @ –1dBFS
SNR = 75.0dB
SFDR = 93.0dBc
3
2
5
46
5 10 15 20 25 30 35 40
FREQUENCY (MHz)
Figure 4. Single Tone @ 2.2 MHz
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
–130
0
5
5
ENCODE = 80MSPS
AIN = 15.5MHz @ –1dBFS
SNR = 75.0dB
SFDR = 93.0dBc
64
3
2
10 15 20 25 30 35 40
FREQUENCY (MHz)
Figure 5. Single Tone @ 15.5 MHz
0
–10
ENCODE = 80MSPS
AIN = 29.5MHz @ –1dBFS
–20
SNR = 74.5dB
SFDR = 93.0dBc
–30
–40
–50
–60
–70
–80
–90
–100
–110
3
5
2
6
–120
–130
0
5 10 15 20 25
FREQUENCY (MHz)
30
Figure 6. Single Tone @ 29.5 MHz
4
35 40
AD6645
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
–130
0
ENCODE = 80MSPS
AIN = 69.1MHz @ –1dBFS
SNR = 73.5dB
SFDR = 89.0dBc
3
2
6 54
5 10 15 20 25 30 35 40
FREQUENCY (MHz)
Figure 7. Single Tone @ 69.1 MHz
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
–130
0
ENCODE = 80MSPS
AIN = 150MHz @ –1dBFS
SNR = 73.0dB
SFDR = 70.0dBc
3
2
5
4
6
5 10 15 20 25 30 35 40
FREQUENCY (MHz)
Figure 8. Single Tone @ 150 MHz
0
–10
ENCODE = 80MSPS
AIN = 200MHz @ –1dBFS
–20
SNR = 72.0dB
SFDR = 64.0dBc
–30
–40
–50
–60
–70 2
–80
4
–90 6
–100
–110
–120
–130
0
5 10 15 20 25
FREQUENCY (MHz)
5
30
Figure 9. Single Tone @ 200 MHz
3
35
40
Rev. D | Page 9 of 24


AD6645 Datasheet
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75.5
75.0
T = –40°C
74.5
T = +85°C
74.0
T = +25°C
73.5
73.0
72.5
ENCODE = 80MSPS @ AIN = –1dBFS
TEMP = –40°C, +25°C, +85°C
72.0
0 10 20 30 40
FREQUENCY (MHz)
50
60
Figure 10. Signal-to-Noise Ratio (SNR) vs. Frequency
70
94
92
90 T = +25°C
88 T = –40°C, +85°C
86
84
82
ENCODE = 80MSPS @ AIN = –1dBFS
TEMP = –40°C, +25°C, +85°C
80
0 10 20 30 40
50
ANALOG INPUT FREQUENCY (MHz)
60
70
Figure 11. Worst-Case Harmonics vs. Analog Input Frequency
76
75
74
73
72
71
ENCODE = 80MSPS @ AIN = –1dBFS
TEMP = 25°C
70
0 20 40 60 80 100 120 140 160 180 200
ANALOG FREQUENCY (MHz)
Figure 12. Signal-to-Noise Ratio (SNR) vs. Analog Frequency (IF)
100
95
WORST OTHER SPUR
90
85
80
HARMONICS
75 (SECOND, THIRD)
70
65
ENCODE = 80MSPS @ AIN = –1dBFS
TEMP = 25°C
60
0 20 40 60
80 100 120 140 160 180
ANALOG FREQUENCY (MHz)
Figure 13. Harmonics vs. Analog Frequency (IF)
200
120
110
dBFS
100
90 ENCODE = 80MSPS
80 AIN = 30.5MHz
70
60 dBc
50
40
SFDR = 90dB
REFERENCE LINE
30
20
10
0
–90 –80 –70 –60 –50 –40 –30 –20 –10
ANALOG INPUT POWER LEVEL (dBFS)
Figure 14. Single-Tone SFDR @ 30.5 MHz
0
120
110
dBFS
100
90 ENCODE = 80MSPS
80 AIN = 69.1MHz
70
60 dBc
50 SFDR = 90dB
REFERENCE LINE
40
30
20
10
0
–90 –80 –70 –60 –50 –40 –30 –20 –10
ANALOG INPUT POWER LEVEL (dBFS)
Figure 15. Single-Tone SFDR @ 69.1 MHz
0
Rev. D | Page 10 of 24


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0
ENCODE = 80MSPS
–10 AIN = 30.5MHz,
–20
31.5MHz (–7dBFS)
NO DITHER
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
–130
0
5 10 15 20 25 30 35
FREQUENCY (MHz)
Figure 16. Two-Tone SFDR @ 30.5 MHz and 31.5 MHz
40
110
100
90
ENCODE = 80MSPS
80 F1 = 30.5MHz
F2 = 31.5MHz
70
60
50
40
dBFS
dBc
SFDR = 90dB
REFERENCE LINE
30
20
10
0
–77 –67 –57 –47 –37 –27 –17
INPUT POWER LEVEL (F1 = F2 dBFS)
–7
Figure 17. Two-Tone SFDR @ 30.5 MHz and 31.5 MHz
100
95 WORST SPUR @ AIN = 2.2MHz
90
85
80
SNR @ AIN = 2.2MHz
75
70
65
15
30 45 60 75 90
ENCODE FREQUENCY (MHz)
105
Figure 18. SNR, Worst-Case Spurious vs. Encode @ 2.2 MHz
AD6645
0
ENCODE = 80MSPS
–10 AIN = 55.25MHz,
–20
56.25MHz (–7dBFS)
NO DITHER
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
–130
0
5 10 15 20 25 30 35 40
FREQUENCY (MHz)
Figure 19. Two-Tone SFDR @ 55.25 MHz and 56.25 MHz
110
100
dBFS
90
80 ENCODE = 80MSPS
F1 = 55.25MHz
70 F2 = 56.25MHz
60
dBc
50 SFDR = 90dB
REFERENCE LINE
40
30
20
10
0
–77 –67 –57 –47 –37 –27 –17
INPUT POWER LEVEL (F1 = F2 dBFS)
–7
Figure 20. Two-Tone SFDR @ 55.25 MHz and 56.25 MHz
95
WORST SPUR @ AIN = 69.1MHz
90
85
80
75 SNR @ AIN = 69.1MHz
70
65
15
30 45 60 75 90
ENCODE FREQUENCY (MHz)
105
Figure 21. SNR, Worst-Case Spurious vs. Encode @ 69.1 MHz
Rev. D | Page 11 of 24


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AD6645
0
ENCODE = 80.0MSPS
–10 AIN = 30.5MHz @ –29.5dBFS
–20 NO DITHER
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
3
5
2
6
4
–130
0
5 10 15 20 25 30 35 40
FREQUENCY (MHz)
Figure 22. 1 M Sample FFT Without Dither
110
ENCODE = 80.0MSPS
100 AIN = 30.5MHz
NO DITHER
90
80
70
60
50
40
30 SFDR = 90dB
REFERENCE LINE
20
10
0
90 80 70 60 50 40 30 20
ANALOG INPUT LEVEL (dBFS)
10
Figure 23. SFDR Without Dither
0
0
–10
ENCODE = 76.8MSPS
AIN = 69.1MHz @ –1dBFS
–20
SNR = 73.5dB
SFDR = 89.0dBc
–30
–40
–50
–60
–70
–80
–90
–100
–110
35
2
6
4
–120
–130
0
5 10 15 20 25 30 35 40
FREQUENCY (MHz)
Figure 24. Single Tone @ 69.1 MHz, Encode = 76.8 MSPS
0
ENCODE = 80.0MSPS
–10 AIN = 30.5MHz @ –29.5dBFS
WITH DITHER @ –19.2 dBm
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
53
2
6
4
–130
0
5 10 15 20 25 30 35 40
FREQUENCY (MHz)
Figure 25. 1 M Sample FFT with Dither
110
ENCODE = 80.0MSPS
100 AIN = 30.5MHz
WITH DITHER @ –19.2dBm
90
80
70
SFDR = 100dB
60 REFERENCE LINE
50
40
30
SFDR = 90dB
REFERENCE LINE
20
10
0
–90 –80 –70 –60 –50 –40 –30 –20 –10
ANALOG INPUT LEVEL (dBFS)
Figure 26. SFDR with Dither
0
0
–10
ENCODE = 76.8MSPS
AIN = W-CDMA @ 69.1MHz
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
2
3
64
5
–120
–130
0
5 10 15 20 25 30 35 40
FREQUENCY (MHz)
Figure 27. W-CDMA Tone @ 69.1 MHz, Encode = 76.8 MSPS
Rev. D | Page 12 of 24


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0
–10
ENCODE = 76.8MSPS
AIN = 2W-CDMA @ 59.6MHz
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
–130
0
5 10 15 20 25 30 35 40
FREQUENCY (MHz)
Figure 28. Two W-CDMA Carriers @ 59.6 MHz, Encode = 76.8 MSPS
0
ENCODE = 61.44MSPS
–10 AIN = 4W-CDMA @ 46.08MHz
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
–130
0
2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0
FREQUENCY (MHz)
Figure 29. Four W-CDMA Carriers @ 46.08 MHz, Encode = 61.44 MSPS
AD6645
0
ENCODE = 76.8MSPS
–10 AIN = W-CDMA @ 140MHz
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
65
42
3
–120
–130
0
5 10 15 20 25 30 35 40
FREQUENCY (MHz)
Figure 30. W-CDMA Tone @ 140 MHz, Encode = 76.8 MSPS
0
ENCODE = 61.44MSPS
–10 AIN = W-CDMA @ 190MHz
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
2
36
45
–120
–130
0
2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0
FREQUENCY (MHz)
Figure 31. W-CDMA Tone @ 190 MHz, Encode = 61.44 MSPS
Rev. D | Page 13 of 24


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AD6645
EQUIVALENT CIRCUITS
VCH AVCC
AIN BUF
VCL
VCH AVCC
500
500
BUF
AIN BUF
T/H
VREF
T/H
VCL
Figure 32. Analog Input Stage
ENCODE
AVCC
AVCC
10k
10k
LOADS
AVCC
AVCC
10k
10k
ENCODE
LOADS
Figure 33. Encode Inputs
AVCC
VREF
AVCC
AVCC
CURRENT
MIRROR
C1, C2
Figure 34. Compensation Pin, C1 or C2
DVCC
CURRENT
MIRROR
DVCC
VREF
D0 TO D13,
OVR, DRY
CURRENT
MIRROR
Figure 35. Digital Output Stage
2.4V
AVCC
AVCC
100µA
VREF
Figure 36. 2.4 V Reference
10k
DVCC
DMID
10k
Figure 37. DMID Reference
Rev. D | Page 14 of 24


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TERMINOLOGY
Analog Bandwidth
The analog input frequency at which the spectral power of the
fundamental frequency (as determined by the FFT analysis) is
reduced by 3 dB.
Aperture Delay
The delay between the 50% point of the rising edge of the
encode command and the instant at which the analog input is
sampled.
Aperture Uncertainty (Jitter)
The sample-to-sample variation in aperture delay.
Differential Analog Input Resistance, Differential Analog
Input Capacitance, and Differential Analog Input Impedance
The real and complex impedances measured at each analog
input port. The resistance is measured statically and the
capacitance and differential input impedances are measured
with a network analyzer.
Differential Analog Input Voltage Range
The peak-to-peak differential voltage that must be applied to
the converter to generate a full-scale response. The peak differential
voltage is computed by observing the voltage on a single pin and
subtracting the voltage from the other pin, which is 180° out of
phase. The peak-to-peak differential is computed by rotating the
inputs’ phase 180°and taking the peak measurement again. The
difference is then computed between both peak measurements.
Differential Nonlinearity
The deviation of any code width from an ideal 1 LSB step.
Encode Pulse Width/Duty Cycle
Pulse width high is the minimum amount of time that the
encode pulse should be left in a high state to achieve rated
performance; pulse width low is the minimum time that
the encode pulse should be left in a low state. See timing
implications of changing tENCH in Table 4. At a given clock rate,
these specifications define an acceptable encode duty cycle.
Full-Scale Input Power
The full-scale input power is expressed in dBm and can be
calculated by using the following equation:
V
2Full
Scale rms
PowerFull
Scale
=
10 log
Z Input
0.001
⎢⎥
⎢⎣ ⎥⎦
AD6645
Harmonic Distortion, Second
The ratio of the rms signal amplitude to the rms value of the
second harmonic component, reported in dBc.
Harmonic Distortion, Third
The ratio of the rms signal amplitude to the rms value of the
third harmonic component, reported in dBc.
Integral Nonlinearity
The deviation of the transfer function from a reference line
measured in fractions of 1 LSB using a best straight line
determined by a least square curve fit.
Maximum Conversion Rate
The encode rate at which parametric testing is performed.
Minimum Conversion Rate
The encode rate at which the SNR of the lowest analog signal
frequency drops by no more than 3 dB below the guaranteed limit.
Noise (for Any Range Within the ADC)
VNOISE =
Z × 0.001×10⎜⎛ FSdBm SNRdBc SignaldBFS ⎟⎞
10
where:
Z is the input impedance.
FS is the full scale of the device for the frequency in question.
SNR is the value for the particular input level.
Signal is the signal level within the ADC reported in dB below
full scale. This value includes both thermal noise and quantiza-
tion noise.
Output Propagation Delay
The delay between a differential crossing of ENCODE and
ENCODE and the time when all output data bits are within
valid logic levels.
Power Supply Rejection Ratio (PSSR)
The ratio of a change in input offset voltage to a change in
power supply voltage.
Power Supply Rise Time
The time from when the dc supply is initiated until the supply
output reaches the minimum specified operating voltage for the
ADC. The dc level is measured at the supply pin(s) of the ADC.
Signal-to-Noise-and-Distortion (SINAD)
The ratio of the rms signal amplitude (set at 1 dB below full scale)
to the rms value of the sum of all other spectral components,
including harmonics, but excluding dc.
Signal-to-Noise Ratio (Without Harmonics)
The ratio of the rms signal amplitude (set at 1 dB below full scale)
to the rms value of the sum of all other spectral components,
excluding the first five harmonics and dc.
Rev. D | Page 15 of 24


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Spurious-Free Dynamic Range (SFDR)
The ratio of the rms signal amplitude to the rms value of the
peak spurious spectral component. The peak spurious component
may or may not be a harmonic. May be reported in dBc (that is,
degrades as signal level is lowered) or dBFS (always related back
to converter full scale).
Two-Tone Intermodulation Distortion Rejection
The ratio of the rms value of either input tone to the rms value
of the worst third-order intermodulation product, reported in dBc.
Two-Tone SFDR
The ratio of the rms value of either input tone to the rms value
of the peak spurious component. The peak spurious component
may or may not be an IMD product, and may be reported in
dBc (that is, degrades as signal level is lowered) or in dBFS
(always related back to converter full scale).
Worst Other Spur
The ratio of the rms signal amplitude to the rms value of the
worst spurious component (excluding the second and third
harmonics), reported in dBc.
Rev. D | Page 16 of 24


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AD6645
THEORY OF OPERATION
The AD6645 ADC employs a three-stage subrange architecture.
This design approach achieves the required accuracy and speed
while maintaining low power and small die size.
As shown in the functional block diagram (see Figure 1), the
AD6645 has complementary analog input pins, AIN and AIN.
Each analog input is centered at 2.4 V and should swing ±0.55 V
around this reference (see Figure 32). Because AIN and AIN are
180° out of phase, the differential analog input signal is 2.2 V p-p.
Both analog inputs are buffered prior to the first track-and-hold,
TH1. The high state of the encode pulse places TH1 in hold
mode. The held value of TH1 is applied to the input of a 5-bit
coarse ADC1. The digital output of ADC1 drives a 5-bit digital-
to-analog converter, DAC1. DAC1 requires 14 bits of precision
that is achieved through laser trimming. The output of DAC1 is
subtracted from the delayed analog signal at the input of TH3 to
generate a first residue signal. TH2 provides an analog pipeline
delay to compensate for the digital delay of ADC1.
The first residual signal is applied to a second conversion stage
consisting of a 5-bit ADC2, a 5-bit DAC2, and a pipeline TH4.
The second DAC requires 10 bits of precision, which is met by
the process with no trim. The input to TH5 is a second residual
signal generated by subtracting the quantized output of DAC2
from the first residual signal held by TH4. TH5 drives a final
6-bit ADC3.
The digital outputs from ADC1, ADC2, and ADC3 are added
together and corrected in the digital error correction logic to
generate the final output data. The result is a 14-bit parallel
digital CMOS-compatible word, coded as twos complement.
APPLYING THE AD6645
Encoding the AD6645
The AD6645 encode signal must be a high quality, extremely
low phase noise source to prevent degradation of performance.
Maintaining 14-bit accuracy places a premium on encode clock
phase noise. SNR performance can easily degrade by 3 dB to
4 dB with 70 MHz analog input signals when using a high jitter
clock source. See the AN-501 application note, Aperture
Uncertainty and ADC System Performance, for complete details.
For optimum performance, the AD6645 must be clocked
differentially. The encode signal is usually ac-coupled into the
ENCODE and ENCODE pins via a transformer or capacitors.
These pins are biased internally and require no additional bias.
Figure 38 shows one preferred method for clocking the AD6645.
The clock source (low jitter) is converted from single-ended to
differential using an RF transformer. The back-to-back Schottky
diodes across the transformer secondary limit excessive amplitude
swings from the clock into the AD6645 to approximately 0.8 V p-p
differential. This helps to prevent the large voltage swings of the
clock from feeding through to other portions of the AD6645
and limits the noise presented to the encode inputs.
CLOCK
SOURCE
0.1µF
T1-4T
ENCODE
AD6645
HSMS2812
DIODES
ENCODE
Figure 38. Crystal Clock Oscillator, Differential Encode
If a low jitter clock is available, another option is to ac-couple a
differential ECL/PECL signal to the encode input pins, as
shown in Figure 39. The MC100EL16 (or same family) from
ON Semiconductor offers excellent jitter performance.
VT
ECL/
PECL
0.1µF
ENCODE
AD6645
ENCODE
0.1µF
VT
Figure 39. Differential ECL for Encode
Driving the Analog Inputs
As with most new high speed, high dynamic range ADCs, the
analog input to the AD6645 is differential. Differential inputs
improve on-chip performance as signals are processed through
attenuation and gain stages. Most of the improvement is a result
of differential analog stages having high rejection of even-order
harmonics. There are also benefits at the PCB level. First,
differential inputs have high common-mode rejection of stray
signals, such as ground and power noise. Second, they provide
good rejection of common-mode signals, such as local oscillator
feedthrough.
The AD6645 analog input voltage range is offset from ground
by 2.4 V. Each analog input connects through a 500 Ω resistor to
the 2.4 V bias voltage and to the input of a differential buffer (see
Figure 32). The resistor network on the input properly biases the
followers for maximum linearity and range. Therefore, the analog
source driving the AD6645 should be ac-coupled to the input pins.
Because the differential input impedance of the AD6645 is 1 kΩ,
the analog input power requirement is only −2 dBm, simplifying
the driver amplifier in many cases. To take full advantage of this
high input impedance, a 20:1 RF transformer is required. This is a
large ratio and can result in unsatisfactory performance. In this
case, a lower step-up ratio can be used. The recommended method
for driving the differential analog input of the AD6645 is to use
a 4:1 RF transformer. For example, if RT is set to 60.4 Ω and RS is set
to 25 Ω, along with a 4:1 impedance ratio transformer, the input
would match to a 50 Ω source with a full-scale drive of 4.8 dBm.
Series resistors (RS) on the secondary side of the transformer
should be used to isolate the transformer from the A/D.
Rev. D | Page 17 of 24


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AD6645
This limits the amount of dynamic current from the A/D
flowing back into the secondary of the transformer. The 50 Ω
impedance matching can also be incorporated on the secondary
side of the transformer, as shown in the evaluation board
schematic (see Figure 43).
ANALOG INPUT
SIGNAL
RT
ADT4-1WT RS
RS
0.1µF
AIN
AD6645
AIN
Figure 40. Transformer-Coupled Analog Input Circuit
In applications where dc coupling is required, a differential
output op amp, such as the AD8138, can be used to drive the
AD6645 (see Figure 41). The AD8138 op amp provides single-
ended-to-differential conversion, which reduces overall system
cost and minimizes layout requirements.
CF
5V
499
VIN
499
25
VOCM AD8138
AIN
AD6645
DIGITAL
OUTPUTS
AIN VREF
499
25
499
CF
Figure 41. DC-Coupled Analog Input Circuit
Power Supplies
Care should be taken when selecting a power source. The use of
linear dc supplies with rise times of <45 ms is highly recommended.
Switching supplies tend to have radiated components that can
be received by the AD6645. Decouple each of the power supply
pins as close to the package as possible using 0.1 μF chip capacitors.
The AD6645 has separate digital and analog power supply pins.
The analog supplies are AVCC and the digital supply pins are
DVCC. Although analog and digital supplies can be tied together,
the best performance is achieved when the supplies are separate
because the fast digital output swings can couple switching
currents back into the analog supplies. Note that AVCC must be
held within 5% of 5 V. The AD6645 is specified for DVCC = 3.3 V, a
common supply for digital ASICs.
Digital Outputs
Care must be taken when designing the data receivers for the
AD6645. It is recommended that the digital outputs drive a
series resistor followed by a gate, such as the 74LCX574.
To minimize capacitive loading, there should be only one gate
on each output pin. An example of this is shown in the evaluation
board schematic of Figure 43. The digital outputs of the AD6645
have a constant output slew rate of 1 V/ns. A typical CMOS gate
combined with a PCB trace have a load of approximately 10 pF.
Therefore, as each bit switches, 10 mA (10 pF × 1 V ÷ 1 ns) of
dynamic current per bit flow in or out of the device. A full-scale
transition can cause up to 140 mA (14 bits × 10 mA/bit) of current
to flow through the output stages. Place the series resistors as close
to the AD6645 as possible to limit the amount of current that can
flow into the output stage. These switching currents are confined
between ground and DVCC. Standard TTL gates should be avoided
because they can add appreciably to the dynamic switching
currents of the AD6645. Note that extra capacitive loading
increases output timing and invalidates timing specifications.
Digital output timing is guaranteed for output loads up to
10 pF. Digital output states for given analog input levels are
shown in Table 8.
Grounding
For optimum performance, it is highly recommended that a
common ground be used between the analog and digital power
planes. The primary concern with splitting grounds is that
dynamic currents may be forced to travel significant distances
in the system before recombining back at the common source
ground. This can result in a large, undesirable ground loop. The
most common place for this to occur is on the digital outputs of
the ADC. Ground loops can contribute to digital noise being
coupled back onto the ADC front end. This can manifest itself
as either harmonic spurs, or very high-order spurious products
that can cause excessive spikes on the noise floor. This noise
coupling is less likely to occur at lower clock speeds because the
digital noise has more time to settle between samples. In general,
splitting the analog and digital grounds can frequently contribute
to undesirable EMI-RFI and should, therefore, be avoided.
Conversely, if not properly implemented, common grounding
can actually impose additional noise issues because the digital
ground currents ride on top of the analog ground currents in
close proximity to the ADC input. To further minimize the
potential for noise coupling, it is highly recommended that
multiple ground return traces/vias be placed such that the
digital output currents do not flow back toward the analog front
end but are routed quickly away from the ADC. This does not
require a split in the ground plane and can be accomplished by
simply placing substantial ground connections directly back to
the supply at a point between the analog front end and the
digital outputs. In addition, the judicious use of ceramic chip
capacitors between the power supply and ground planes helps
to suppress digital noise. The layout should incorporate enough
bulk capacitance to supply the peak current requirements
during switching periods.
Rev. D | Page 18 of 24


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LAYOUT INFORMATION
The schematic of the evaluation board (see Figure 43)
represents a typical implementation of the AD6645. A multi-
layer board is recommended to achieve best results. It is highly
recommended that high quality, ceramic chip capacitors be
used to decouple each supply pin to ground directly at the
device. The pinout of the AD6645 facilitates ease of use in the
implementation of high frequency, high resolution design practices.
All of the digital outputs are segregated to two sides of the chip,
with the inputs on the opposite side for isolation purposes.
Care should be taken when routing the digital output traces. To
prevent coupling through the digital outputs into the analog
portion of the AD6645, minimal capacitive loading should be
placed on these outputs. It is recommended that a fanout of
only one gate should be used for all AD6645 digital outputs.
The layout of the encode circuit is equally critical. Any noise
received on this circuitry results in corruption in the digitization
process and lower overall performance. The encode clock must be
isolated from the digital outputs and the analog inputs.
Table 8. Twos Complement Output Coding
AIN Level
AIN Level
Output State Output Code
VREF + 0.55 V VREF − 0.55 V Positive FS
01 1111 1111 1111
VREF
VREF
Midscale
00 … 0/11 … 1
VREF − 0.55 V VREF + 0.55 V Negative FS 10 0000 0000 0000
AD6645
JITTER CONSIDERATIONS
The SNR for an ADC can be predicted. When normalized to
ADC codes, the following equation accurately predicts the SNR
based on three terms: jitter, average DNL error, and thermal
noise. Each of these terms contributes to the noise within the
converter.
SNR = 1.76
( )
20 log2π × fANALOG × t j rms
2
+
⎜⎝⎛
1+ε
2n
⎟⎠⎞2
+
⎜⎛
⎜⎝
2×
where:
2
×VNOISE rms
⎟⎞2
1/
2
2n
⎟⎠
fANALOG is the analog input frequency.
tj rms is the rms jitter of the encode (rms sum of encode source
and internal encode circuitry).
ε is the average DNL of the ADC (typically 0.41 LSB).
n is the number of bits in the ADC.
VNOISE rms is the voltage rms thermal noise that refers to the
analog input of the ADC (typically 0.9 LSB rms).
For a 14-bit ADC, such as the AD6645, aperture jitter can
greatly affect the SNR performance as the analog frequency is
increased. Figure 42 shows a family of curves that demonstrate the
expected SNR performance of the AD6645 as jitter increases.
The chart is derived from the preceding equation.
For a complete discussion of aperture jitter, see the AN-756
application note, Sampled Systems and the Effects of Clock Phase
Noise and Jitter. The AN-756 application note can be found on
www.analog.com.
80
75
70
AIN = 110MHz
65
AIN = 150MHz
AIN = 190MHz
60
AIN = 30MHz
AIN = 70MHz
55
0 0.1 0.2 0.3 0.4 0.5 0.6
JITTER (ps)
Figure 42. SNR vs. Jitter
Rev. D | Page 19 of 24


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AD6645
Table 9. AD6645/PCB Bill of Materials
Quantity Quantity
80 MSPS 105 MSPS Reference ID
11
PCB
44
C1, C2, C31, C38
88
99
00
C3, C7 to C10, C16,
C301, C32
C4, C15, C22 to
C26, C29, (C33)2, 3,
(C34)2, 3, C39
(C5, C6)2, 3
10 10
00
C11 to C14,
C17 to C21, C40
(C27, C28)2
11
CR13
11
E1
55
11
11
11
22
F1 to F5
J1
J1
J2
(J3)2, J4, J5
11
00
L1
(R1)2, 3
00
(R2)2
22
22
(R3 to R5)1,2, (R8)1,2,
R9, R10
R6, R7
00
(R11)2, 3, (R13)2, 3
00
(R12)2, 3, (R14)2, 3
11
R151
11
R35
44
RN1 to RN4
22
T23, T31
10
01
22
00
U1
U1
U2, U7
(U3)1, 2
22
U4, U6
Description
Printed circuit board, AD6645
engineering evaluation board
Capacitor, tantalum, SMT
BCAPTAJC, 10 μF, 16 V, 10%
Capacitor, ceramic, SMT 0508,
0.1 μF, 16 V, 10%
Capacitor, ceramic, SMT 0805,
0.1 μF, 25 V, 10%
Manufacturer
PCSM
Kemet
Presidio Components
Panasonic
Capacitor, ceramic, SMT 0805,
0.01 μF, 50 V, 10%
Capacitor, ceramic, SMT 0508,
0.01 μF, 50 V, 0.2%
Capacitor, ceramic, SMT 0805, limits
amp bandwidth as warranted
Diode, dual Schottky HSMS2812,
SOT-23, 30 V, 20 mA
Install jumper wire (across OPT_LAT
and BUFLAT)
EMI suppression ferrite chip, SMT 0805
Header, 6-pin, pin strip, 5 mm pitch
Pin strip, 6-pin, 5 mm pitch
Header, 40-pin, male, right angle
Connector, gold, female, coax., SMA,
vertical
Inductor, SMT, 1008-ct package, 4.7 nH
Resistor, thick film, SMT 0402, 100 Ω,
1/16 W, 1%
Resistor, thick film, SMT 1206, 60.4 Ω,
1/4 W, 1%
Resistor, thick film, SMT 0805, 500 Ω,
1/8 W, 1%
Resistor, thick film, SMT 0805, 25.5 Ω,
1/8 W, 1%
Resistor, thick film, SMT 0805, 66.5 Ω,
1/8 W, 1%
Resistor, thick film, SMT 0805, 100 Ω,
1/8W, 1%
Resistor, thick film, SMT 0402, 178 Ω,
1/16 W, 1%
Resistor, thick film, SMT 0805, 49.9 Ω,
1/8 W, 1%
Resistor array, SMT 0402; 100 Ω;
8 ISO RES.,1/4 W; 5%
Transformer, ADT4-1WT, CD542,
2 MHz to 775 MHz
IC, 14-bit, 80 MSPS ADC
IC, 14-bit, 105 MSPS ADC
IC, SOIC-20, Octal D-type flip-flop
IC, SOIC-8, low distortion differential
ADC driver
IC, SOT-23, tiny logic UHS 2 input
OR gate
Panasonic
Presidio Components
Panasonic
Steward
Wieland
Wieland
Samtec
Johnson Components
Coilcraft
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
Mini-Circuits
Analog Devices
Analog Devices
Fairchild
Analog Devices
Fairchild
Supplier Part No.
6645EE01D REV D
T491C106K016AS
0508X7R104K16VP3
ECJ-2VB1E104K
ECJ-2YB1H103K
0508X7R103M2P3
MA716-(TX)
HZ0805E601R-00
Z5.530.0625.0
25.602.2653.0
TSW-120-08-T-D-RA
142-0701-201
1008CT-040X-J
ERJ-2RKF1000
ERJ-8ENF60R4V
ERJ-6ENF4990V
ERJ-6ENF25R5V
ERJ-6ENF66R5V
ERJ-6ENF1000V
ERJ-2RKF1780X
ERJ-6ENF49R9V
EXB2HV101JV
ADT4-1WT
AD6645ASQ/ASV-80
AD6645ASQ/ASV-105
74LCX574WM
AD8138AR
NC7SZ32
Rev. D | Page 20 of 24


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AD6645
Quantity
80 MSPS
0
1
4
4
Quantity
105 MSPS
0
0
4
4
Reference ID
(U8)2, 3
Y1
Y1
Description
IC, SOIC-8, differential receiver
Clock oscillator, 80 MHz
Pin sockets, closed end
Circuit board support
Manufacturer
Motorola
CTS Reeves
AMP/Tyco Electronics
Richco, Inc.
Supplier Part No.
MC100LVEL16
MXO45-80
5-330808-3
CBSB-14-01
1 AC-coupled AIN is standard: R3, R4, R5, R8, and U3 are not installed. If dc-coupled AIN is required, C30, R15, and T3 are not installed.
2 Reference designators in parentheses are not installed on standard units.
3 AC-coupled encode is standard: C5, C6, C33, C34, R1, R11 to R14, and U8 are not installed. If PECL encode is required, CR1 and T2 are not installed.
Rev. D | Page 21 of 24


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AD6645
Figure 43. Evaluation Board Schematic
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AD6645
Figure 44. Top Signal Level
Figure 46. Ground Plane Layer 2 and Ground Plane Layer 5
Figure 45. 5.0 V Plane Layer 3 and 3.3 V Plane Layer 4
Figure 47. Bottom Signal Layer
Rev. D | Page 23 of 24


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AD6645
OUTLINE DIMENSIONS
1.60
0.75 MAX
0.60
0.45
52
1
12.20
12.00 SQ
11.80
40
39
2.65
2.50 (4 PLCS)
2.35
40
39
2.35
2.20 (4 PLCS)
2.05
52
1
PIN 1
TOP VIEW
(PINS DOWN)
1.45
1.40
1.35
0.15
0.05
SE ATING
PLANE
0.20
0.08
0.10 MAX
COPLANARITY
VIEW A
ROTATED 90° CCW
13
14
27
26
VIEW A
0.65
BSC
LEAD PITCH
0.38
0.32
0.22
27
26
EXPOSED
HEAT SINK
(CENTERED)
BOTTOM VIEW
(PINS UP)
6.05
5.90 SQ
5.75 10.20
10.00 SQ
9.80
13
14
FOR PROPER CONNECTION OF
THE EXPOSED PAD, REFER TO
THE PIN CONFIGURATION AND
FUNCTION DESCRIPTIONS
SECTION OF THIS DATA SHEET.
COMPLIANT TO JEDEC STANDARDS MS-026-BCC-HD
Figure 48. 52-Lead Low Profile Quad Flat Package, PowerQuad [LQFP_PQ4]
(SQ-52-1)
Dimensions shown in millimeters
0.75
0.60
0.45
SEATING
PLANE
1.20
MAX
12.00 BSC
SQ
52
1
PIN 1
40
39
40
39
52
1
TOP VIEW
(PINS DOWN)
10.00
BSC SQ
EXPOSED
PAD
6.50 BSC
SQ
1.05
1.00
0.95
0.15
0.05
0° MIN
0.20
0.09
3.5°
0.08 MAX
COPLANARITY
VIEW A
ROTATED 90° CCW
13
14
VIEW A
27
26
27
26
BOTTOM VIEW
(PINS UP)
0.65
BSC
LEAD PITCH
0.38
0.32
0.22
COMPLIANT TO JEDEC STANDARDS MS-026-ACC
Figure 49. 52-Lead Thin Quad Flat Package, Exposed Pad [TQFP_EP]
(SV-52-1)
Dimensions shown in millimeters
13
14
FOR PROPER CONNECTION OF
THE EXPOSED PAD, REFER TO
THE PIN CONFIGURATION AND
FUNCTION DESCRIPTIONS
SECTION OF THIS DATA SHEET.
ORDERING GUIDE
Model
Temperature Range
AD6645ASQ-80
−40°C to +85°C
AD6645ASQZ-801
−40°C to +85°C
AD6645ASVZ-801
−40°C to +85°C
AD6645ASQ-105
−10°C to +85°C
AD6645ASQZ-1051 −10°C to +85°C
AD6645ASVZ-1051 −10°C to +85°C
AD6645-80/PCBZ1
AD6645-105/PCBZ1
Package Description
52-Lead Low Profile Quad Flat Package, PowerQuad (LQFP_PQ4)
52-Lead Low Profile Quad Flat Package, PowerQuad (LQFP_PQ4)
52-Lead Thin Quad Flat Package, Exposed Pad [TQFP_EP]
52-Lead Low Profile Quad Flat Package, PowerQuad (LQFP_PQ4)
52-Lead Low Profile Quad Flat Package, PowerQuad (LQFP_PQ4)
52-Lead Thin Quad Flat Package, Exposed Pad [TQFP_EP]
Evaluation Board
Evaluation Board
Package Option
SQ-52-1
SQ-52-1
SV-52-1
SQ-52-1
SQ-52-1
SV-52-1
1 Z = RoHS Compliant Part.
©2002–2008 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D02647-0-10/08(D)
Rev. D | Page 24 of 24



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