AD627
Capacitor C3 is needed to maintain common-mode rejection at
low frequencies. R±/R2 and C±/C2 form a bridge circuit whose
output appears across the input pins of the in-amp. Any mismatch
between C± and C2 unbalances the bridge and reduces common-
mode rejection. C3 ensures that any RF signals are common
mode (the same on both in-amp inputsꢀ and are not applied
differentially. This second low-pass network, R± + R2 and C3,
has a −3 dB frequency equal to
INPUT PROTECTION
As shown in the simplified schematic (see Figure 35ꢀ, both the
inverting and noninverting inputs are clamped to the positive
and negative supplies by ESD diodes. In addition, a 2 kΩ series
resistor on each input provides current limiting in the event of
an overvoltage. These ESD diodes can tolerate a maximum
continuous current of ±0 mA. So an overvoltage (that is, the
amount by which the input voltage exceeds the supply voltageꢀ
of ±20 V can be tolerated. This is true for all gains, and for
power on and off. This last case is particularly important
because the signal source and amplifier can be powered
separately.
±/(2π((R± + R2ꢀ × C3ꢀꢀ
(1ꢀ
+V
S
0.33µF
0.01µF
C1
1000pF
5%
R1
20kΩ
1%
If the overvoltage is expected to exceed 20 V, use additional
external series current-limiting resistors to keep the diode
current below ±0 mA.
+IN
–IN
R2
20kΩ
1%
C3
R
V
G
AD627
OUT
0.022µF
RF INTERFERENCE
REFERENCE
0.01µF
C2
1000pF
5%
All instrumentation amplifiers can rectify high frequency out-
of-band signals. Once rectified, these signals appear as dc offset
errors at the output. The circuit in Figure 49 provides good RFI
suppression without reducing performance within the pass
band of the instrumentation amplifier. Resistor R± and
Capacitor C± (and likewise, R2 and C2ꢀ form a low-pass RC
filter that has a –3 dB BW equal to
0.33µF
–V
S
Figure 49. Circuit to Attenuate RF Interference
Using a C3 value of 0.022 μF, as shown in Figure 49, the −3 dB
signal bandwidth of this circuit is approximately 200 Hz. The
typical dc offset shift over frequency is less than ± mV and the
RF signal rejection of the circuit is better than 57 dB. To increase
the 3 dB signal bandwidth of this circuit, reduce the value of
Resistor R± and Resistor R2. The performance is similar to that
when using 20 kΩ resistors, except that the circuitry preceding
the in-amp must drive a lower impedance load.
f = ±/(2π(R1 × C1ꢀꢀ
(7ꢀ
Using the component values shown in Figure 49, this filter has
a –3 dB bandwidth of approximately 1 kHz. Resistor R± and
Resistor R2 were selected to be large enough to isolate the circuit
input from the capacitors but not large enough to significantly
increase circuit noise. To preserve common-mode rejection in
the amplifier pass band, Capacitor C± and Capacitor C2 must
be 5% mica units, or low cost 20% units can be tested and binned
to provide closely matched devices.
When building a circuit like that shown in Figure 49, use a PC
board with a ground plane on both sides. Make all component
leads as short as possible. Resistor R± and Resistor R2 can be
common ±% metal film units, but Capacitor C± and Capacitor C2
must be ±5% tolerance devices to avoid degrading the common-
mode rejection of the circuit. Either the traditional 5% silver mica
units or Panasonic ±2% PPS film capacitors are recommended.
Rev. D | Page 21 of 24
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