AD627ARZ-R7 datasheet(21/24 Pages) AD

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AD627

Rev. D | 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 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 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

VOUT

+VS

–VS

1000pF

REFERENCE

0.01µF

0.022µF

0.33µF

20kΩ

AD627

–IN

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.

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