AD622

Rev. D | Page 12 of 16

Figure 18 shows an example where the differential filter

frequency is approximately 400 Hz, and the common-mode

filter frequency is approximately 40 kHz. With this differential

filter in place and operating at gain of 1000, the typical dc offset

shift over a frequency range of 1 Hz to 20 MHz is less than 1.5 μV

RTI, and the RF signal rejection of the circuit is better than

71 dB. At a gain of 100, the dc offset shift is well below 1 mV

RTI, and RF rejection is greater than 70 dB.

The input resistors should be selected to be high enough to

enough not to influence system noise. Mismatch between

should be high precision types such as NPO/COG ceramics.

GROUND RETURNS FOR INPUT BIAS CURRENTS

Input bias currents are those currents necessary to bias the

input transistors of an amplifier. There must be a direct return

path for these currents; therefore, when amplifying floating

input sources such as transformers or ac-coupled sources, there

must be a dc path from each input to ground as shown in

Figure 19, Figure 20, and Figure 21. Refer to the Designer’s

Guide to Instrumentation Amplifiers (free from Analog Devices,

Inc.) for more information regarding in-amp applications.

AD622

2

1

8

3

4

5

6

7

LOAD

–IN

+IN

REF

TO POWER

SUPPLY

GROUND

Figure 19. Ground Returns for Bias Currents with Transformer Coupled Inputs

AD622

2

1

8

3

4

5

6

7

LOAD

–IN

+IN

REF

TO POWER

SUPPLY

GROUND

Figure 20. Ground Returns for Bias Currents with Thermocouple Inputs

AD622

2

1

8

3

4

5

6

7

LOAD

–IN

+IN

REF

TO POWER

SUPPLY

GROUND

100kΩ

100kΩ

Figure 21. Ground Returns for Bias Currents with AC-Coupled Inputs