AD622
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
isolate the sensor from the C
C
and C
D
capacitors but low
enough not to influence system noise. Mismatch between
R × C
C
at the positive input and R × C
C
at the negative input
degrades the CMRR of the AD622. Therefore, the C
C
capacitors
should be high precision types such as NPO/COG ceramics.
The tolerance of the C
D
capacitor is less critical.
+V
S
–IN
2
1
7
R
G
8
AD622
5
4
3
6
V
OUT
LOAD
+IN
REF
00777-018
–V
S
TO POWER
SUPPLY
GROUND
Figure 19. Ground Returns for Bias Currents with Transformer Coupled Inputs
+V
S
–IN
2
1
7
R
G
8
AD622
5
4
3
6
V
OUT
LOAD
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
(free from Analog Devices,
Inc.) for more information regarding in-amp applications.
+IN
REF
00777-019
00777-020
–V
S
TO POWER
SUPPLY
GROUND
Figure 20. Ground Returns for Bias Currents with Thermocouple Inputs
+V
S
–IN
2
1
7
R
G
8
AD622
5
4
3
6
V
OUT
LOAD
+IN
100kΩ
100kΩ
REF
–V
S
TO POWER
SUPPLY
GROUND
Figure 21. Ground Returns for Bias Currents with AC-Coupled Inputs
Rev. D | Page 12 of 16
AD [ ANALOG DEVICES ]
