AD628
THEORY OF OPERATION
The AD628 is a high common-mode voltage difference
amplifier, combined with a user-configurable output amplifier
(see Figure 28 and Figure 29). Differential mode voltages in
excess of 120 V are accurately scaled by a precision 11:1 voltage
divider at the input. A reference voltage input is available to the
user at Pin 3 (VREF). The output common-mode voltage of the
difference amplifier is the same as the voltage applied to the
reference pin. If the uncommitted amplifier is configured for
gain, connect Pin 3 to one end of the external gain resistor to
establish the output common-mode voltage at Pin 5 (OUT).
The uncommitted amplifier is a high open-loop gain, low offset,
low drift op amp, with its noninverting input connected to the
internal 10 kΩ resistor. Both inputs are accessible to the user.
Careful layout design has resulted in exceptional common-
mode rejection at higher frequencies. The inputs are connected
to Pin 1 (+IN) and Pin 8 (−IN), which are adjacent to the power
pins, Pin 2 (−VS) and Pin 7 (+VS). Because the power pins are at
ac ground, input impedance balance and, therefore, common-
mode rejection are preserved at higher frequencies.
R
G
The output of the difference amplifier is internally connected
to a 10 kΩ resistor trimmed to better than 0.1% absolute
accuracy. The resistor is connected to the noninverting input of
the output amplifier and is accessible at Pin 4 (CFILT). A
capacitor can be connected to implement a low-pass filter, a
resistor can be connected to further reduce the output voltage,
or a clamp circuit can be connected to limit the output swing.
100kΩ
100kΩ
10kΩ
G = +0.1
–IN
+IN
–IN
+IN
OUT
A2
–IN
10kΩ
A1
+IN
10kΩ
V
C
FILT
REF
Figure 28. Simplified Schematic
C
FILT
+V
S
AD628
100kΩ
10kΩ
–IN
+IN
G = +0.1
–IN
10kΩ
A1
+IN
OUT
A2
+IN
–IN
100kΩ
10kΩ
–V
V
REF
R
S
G
R
EXT3
R
R
EXT1
REFERENCE
VOLTAGE
EXT2
Figure 29. Circuit Connections
Rev. F | Page 14 of 20
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