Datasheet

Data Sheet AD627
Rev. E | Page 21 of 24
INPUT PROTECTION
As shown in the simplified schematic (see Figure 35), both the
inverting and noninverting inputs are clamped to the positive
and negative supplies by ESD diodes. In addition, a 2 k series
resistor on each input provides current limiting in the event of
an overvoltage. These ESD diodes can tolerate a maximum
continuous current of 10 mA. So an overvoltage (that is, the
amount by which the input voltage exceeds the supply voltage)
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.
If the overvoltage is expected to exceed 20 V, use additional
external series current-limiting resistors to keep the diode
current below 10 mA.
RF INTERFERENCE
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 R1 and
Capacitor C1 (and likewise, R2 and C2) form a low-pass RC
filter that has a –3 dB BW equal to
f = 1/(2π(R1 × C1)) (7)
Using the component values shown in Figure 49, this filter has
a –3 dB bandwidth of approximately 8 kHz. Resistor R1 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 C1 and Capacitor C2 must
be 5% mica units, or low cost 20% units can be tested and binned
to provide closely matched devices.
Capacitor C3 is needed to maintain common-mode rejection at
low frequencies. R1/R2 and C1/C2 form a bridge circuit whose
output appears across the input pins of the in-amp. Any mismatch
between C1 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 inputs) and are not applied
differentially. This second low-pass network, R1 + R2 and C3,
has a −3 dB frequency equal to
1/(2π((R1 + R2) × C3)) (8)
+IN
V
OUT
+
V
S
–V
S
C1
1000pF
5%
C2
1000pF
5%
REFERENCE
R
G
0.01µF
0.01µF
C3
0.022µF
0.33µF
0.33µF
R1
20kΩ
1%
R2
20kΩ
1%
AD627
–
IN
00782-047
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 1 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 R1 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.
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 R1 and Resistor R2 can be
common 1% metal film units, but Capacitor C1 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.