The modulation transistor of the 13.56 MHz devices is
placed between antenna B and V SS. This transistor
creates a junction cap acitance between the two p ads.
This junction cap acitance is relatively lossy comp ared
to the on-board cap acitors. However , the MCRF450,
MCRF451, MCRF455 and MCRF360 are not af fected
by the junction cap acitance. For the MCRF452, this
lossy junction capacitance is in parallel with its second
50 pF internal resonant capacitor. The resulting loaded
circuit Q of the MCRF452 is about 5~10% lower than
that of the other devices.
The cap acitance or induct ance also can be trimmed
within a few percent using proper tuning mechanism.
Various tag design assistance and tuning methods that
are the subject of pending patent applications are avail-
able from Microchip Technology Inc.
The Q factor of a t ag’s antenna circuit is primarily
governed by the antenna’ s resistance. Therefore, the
antenna must be designed to have minimum resisat nce
within a given physical constraint. The antenna resis-
tance becomes smaller with thicker gauge wire or
etched metallic traces with a wider trace width. Etched
antennas with a four-turn spiral inductor on an ISO card
sized dimension can easily be made to be less than
1 ohm with a proper dimensional choice. In this case,
the unloaded Q (antenna only) can be greater than
100. The loaded Q (antenna with device) needs to be
greater than 50 for long range applications.
Microchip’s 13.56 MHz devices consume less than
200 µW of power during reading. This is about 20 times
less than other similar devices available from competi-
tors in the industry today . This reduced power
consumption means there will be more available power
for backscattering (re-radiation), which result s in a
longer read range.
In conventional RFID tags, data is sent by damping and
undamping the antenna voltage. However, for a high Q
circuit, it is very dif ficult to damp the volt age with high
speed (high data rate). Therefore, it is dif ficult to send
data with 100% AM modulation with the conventional
method. To overcome this problem, Microchip’s current
13.56 MHz devices are designed to shift the circuit’ s
tuning fr equency ins tead of damping the c oil v oltage
directly. This can be achieved by shorting and un-short-
ing one element in the resonant antenna circuit.
Microchip’s devices have a modulation transistor
between antenna B and V SS. The frequency tuning
element should be placed in p arallel with the modula-
tion transistor (antenna B and V SS). The modulation
transistor shorts the frequency switching element when
it turns on (sending dat a “Hi”), and releases when it
turns of f (sending dat a “Lo”). When the switching
element is released, the circuit tunes to the reader ’s
carrier frequency causing the circuit to develop
maximum volt age. When the switching element is
shorted, the circuit tunes away from the carrier
frequency, therefore, developing less volt age. The
reader monitors the changes in the t ag’s coil volt age,
and reconstruct s the modulation dat a. Refer to
Microchip’s application Note AN707 (DS00707) for
more details of this feature.
The de vice req uires three co nnection p oints to the
external antenna circuit: antenna A, antenna B, and
VSS. This is in order to switch the resonant frequency
(tuned and detuned) by shorting and un-shorting the
element between antenna B and VSS. In the MCRF452,
the antenna B is internally connected to the second
internal 50 pF capacitor between antenna B and V SS.
See Figure 1 for various external c ircuit configurations
for each device.
The resonant frequency of the tag is determined by the
LC component combination between antenna A and
VSS. The circuit must be tuned precisely to the reade’rs
carrier frequency for best read range performance.
Microchip’s 13.56 MHz devices are designed to send
data with 100% modulation with the appropriate exter-
nal circuit configuration. The modulation depth is deter-
mined how much the tag’s coil voltage is changed when
it sends dat a from “Hi” to “ Lo” or vice versa. In the
13.56 MHz devices, this is directly related to the sep a-
ration between tuned and detuned frequencies. The
detuned frequency is the result of shorting the
frequency switching component between the antenna
B and VSS. This component value is typically optimized
between one-third to one-half the total L or C value. For
example, three turns between antenna A and antenna
B, and one turn between antenna B and V SS for a t ag
made of a four-turn spiral inductor . If the shorting
element (between antenna B and V SS) is a cap acitor,
the same value of the cap acitor can be chosen for the
element between antenna A and B for simplicity.
The COB for MCRF355 and MCRF450 include two
identical 68 pF cap acitors in series, externally to the
device. The cap acitor C1 is connected between
antenna A and B, and C2 is between antenna B and
The MCRF452 COB does not require external cap aci-
tors since the device has two internal capacitors.
The MCRF452 is the best choice for the COB in a
sense of unit COB production cost. However , for the
highest performance, the MCRF450 is recommended.
Various circuit configurations for each device are
shown in Figure 1.
2002 Microchip Technology Inc.