AD627
THEORY OF OPERATION
The AD627 is a true instrumentation amplifier, built using two
feedback loops. Its general properties are similar to those of the
classic two-op-amp instrumentation amplifier configuration but
internally the details are somewhat different. The AD627 uses a
modified current feedback scheme, which, coupled with interstage
feedforward frequency compensation, results in a much better
common-mode rejection ratio (CMRRꢀ at frequencies above
dc (notably the line frequency of 50 Hz to 60 Hzꢀ than might
otherwise be expected of a low power instrumentation amplifier.
The inverting terminal gain of A± (±.25ꢀ times the gain of A2
(−4ꢀ makes the gain from the inverting and noninverting
terminals equal.
The differential mode gain is equal to ± + R4/R3, nominally 5,
and is factory trimmed to 0.0±% final accuracy. Adding an
external gain setting resistor (RGꢀ increases the gain by an
amount equal to (R4 + R±ꢀ/RG. The output voltage of the
AD627 is given by
V
OUT = [VIN(+ꢀ – VIN(−ꢀ] × (5 + 200 kΩ/RGꢀ + VREF
(±ꢀ
In Figure 35, A± completes a feedback loop that, in conjunction
with V± and R5, forces a constant collector current in Q±. Assume
that the gain-setting resistor (RGꢀ is not present. Resistors R2
and R± complete the loop and force the output of A± to be equal
to the voltage on the inverting terminal with a gain of nearly
±.25. A2 completes a nearly identical feedback loop that forces
a current in Q2 that is nearly identical to that in Q±; A2 also
provides the output voltage. When both loops are balanced, the
gain from the noninverting terminal to VOUT is equal to 5,
whereas the gain from the output of A± to VOUT is equal to −4.
Laser trims are performed on R± through R4 to ensure that
their values are as close as possible to the absolute values in the
gain equation. This ensures low gain error and high common-
mode rejection at all practical gains.
EXTERNAL GAIN RESISTOR
R1
100kΩ
R
R4
100kΩ
G
REF
–IN
R2
25kΩ
R3
25kΩ
+V
+V
S
S
2kΩ
2kΩ
+IN
Q1
Q2
–V
–V
S
S
A1
A2
OUTPUT
R5
200kΩ
R6
200kΩ
V1
0.1V
Figure 3±. Simplified Schematic
Rev. D | Page 14 of 24
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