Low Noise, Low Drift
FET Op Amp
Improved Replacement for Burr-Brown
OPA-111 and OPA-121 Op Amp
2 V p-p max, 0.1 Hz to 10 Hz
10 nV/√Hz max at 10 kHz
11 fA p-p Current Noise 0.1
HIGH DC ACCURACY
250 V max Offset Voltage
1 V/؇C max Drift
1.5 pA max Input Bias Current
114 dB Open-Loop Gain
Available in Plastic Mini-DIP, 8-Pin Header Packages, or
Low Noise Photodiode Preamps
Precision I-V Converters
The AD645 is a low noise, precision FET input op amp. It of-
fers the pico amp level input currents of a FET input device
coupled with offset drift and input voltage noise comparable to a
high performance bipolar input amplifier.
The AD645 has been improved to offer the lowest offset drift in
a FET op amp, 1 µV/°C. Offset voltage drift is measured and
trimmed at wafer level for the lowest cost possible. An inher-
ently low noise architecture and advanced manufacturing tech-
niques result in a device with a guaranteed low input voltage
noise of 2 µV p-p, 0.1 Hz to 10 Hz. This level of dc performance
along with low input currents make the AD645 an excellent
choice for high impedance applications where stability is of
1 10 100 1k 10k
FREQUENCY – Hz
Figure 1. AD645 Voltage Noise Spectral Density vs.
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
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
8-Pin Plastic Mini-DIP
TO-99 (H) Package
NC = NO CONNECT
– IN 2
3 AD645 5
+IN 4 OFFSET
NOTE: CASE IS CONNECTED
TO PIN 8
The AD645 is available in six performance grades. The AD645J
and AD645K are rated over the commercial temperature range
of 0°C to +70°C. The AD645A, AD645B, and the ultra-
precision AD645C are rated over the industrial temperature
range of –40°C to +85°C. The AD645S is rated over the military
temperature range of –55°C to +125°C and is available
processed to MIL-STD-883B.
The AD645 is available in an 8-pin plastic mini-DIP, 8-pin
header, or in die form.
1. Guaranteed and tested low frequency noise of 2 µV p-p max
and 20 nV/√Hz at 100 Hz makes the AD645C ideal for low
noise applications where a FET input op amp is needed.
2. Low VOS drift of 1 µV/°C max makes the AD645C an excel-
lent choice for applications requiring ultimate stability.
3. Low input bias current and current noise (11 fA p-p 0.1 Hz to
10 Hz) allow the AD645 to be used as a high precision
preamp for current output sensors such as photodiodes, or as
a buffer for high source impedance voltage output sensors.
–2.5 –2.0 –1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 2.0 2.5
INPUT OFFSET VOLTAGE DRIFT– µV/°C
Figure 2. Typical Distribution of Average Input Offset
Voltage Drift (196 Units)
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