AD627

–15–

REV. A

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 of Figure 44 provides good RFI

suppression without reducing performance within the in amp’s

passband. 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/(2

π R1C1). Using the component values shown, this

filter has a –3 dB bandwidth of approximately 8 kHz. Resistors

R1 and R2 were selected to be large enough to isolate the circuit’s

input from the capacitors, but not large enough to significantly

increase the circuit’s noise. To preserve common-mode rejec-

tion in the amplifier’s pass band, capacitors C1 and C2 need to

be 5% mica units, or low cost 20% units can be tested and

“binned” to provide closely matched devices.

REFERENCE

0.33 F

0.01 F

0.01 F

0.33 F

AD627

C1

1000pF

5%

R1

20k

1%

R2

20k

1%

C3

0.022 F

C2

1000pF

5%

LOCATE C1–C3 AS CLOSE TO

THE INPUT PINS AS POSSIBLE

+IN

–IN

Figure 44. Circuit to Attenuate RF Interference

Capacitor C3 is needed to maintain common-mode rejection at

the low frequencies. R1/R2 and C1/C2 form a bridge circuit

whose output appears across the in amp’s input pins. Any mis-

match between C1 and C2 will unbalance the bridge and reduce

common-mode rejection. C3 insures 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/(2

π (R1 + R2) (C3)).

Using a C3 value of 0.022

µF as shown, the –3 dB signal BW of

this circuit is approximately 200 Hz. The typical dc offset shift

over frequency will be less than 1 mV and the circuit’s RF signal

rejection will be better than 57 dB. The 3 dB signal bandwidth

of this circuit may be increased by reducing the value of resistors

R1 and R2. The performance is similar to that using 20 k

Ω

resistors, except that the circuitry preceding the in amp must

drive a lower impedance load.

The circuit of Figure 44 should be built using a PC board with a

ground plane on both sides. All component leads should be as

short as possible. Resistors R1 and R2 can be common 1%

metal film units but capacitors C1 and C2 need to be

±5%

tolerance devices to avoid degrading the circuit’s common-

mode rejection. Either the traditional 5% silver mica units or

Panasonic

±2% PPS film capacitors are recommended.

APPLICATIONS CIRCUITS

A Classic Bridge Circuit

Figure 45 shows the AD627 configured to amplify the signal

from a classic resistive bridge. This circuit will work in either

dual or single supply mode. Typically the bridge will be excited

by the same voltage as is used to power the in amp. Connecting

the bottom of the bridge to the negative supply of the in amp (usu-

ally either 0, –5 V, –12 V or –15 V), sets up an input common

mode voltage that is optimally located midway between the

supply voltages. It is also appropriate to set the voltage on the

REF pin to midway between the supplies, especially if the input

signal will be bipolar. However the voltage on the REF pin can

be varied to suit the application. A good example of this is when

± V

output range divided by the input range.

AD627

200k

GAIN-5

0.1 F

0.1 F

Figure 45. A Classic Bridge Circuit

AGND DGND

AD7892-2

ADC

12

DGND

PROCESSOR

0.1 F

0.1 F

0.1 F

POWER SUPPLY

+5V

GND

AD627

Figure 43. Optimal Ground Practice in a Single Supply Environment