Datasheet

AD627 Data Sheet
Rev. E | Page 14 of 24
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 (CMRR) at frequencies above
dc (notably the line frequency of 50 Hz to 60 Hz) than might
otherwise be expected of a low power instrumentation amplifier.
In Figure 35, A1 completes a feedback loop that, in conjunction
with V1 and R5, forces a constant collector current in Q1. Assume
that the gain-setting resistor (R
G
) is not present. Resistors R2
and R1 complete the loop and force the output of A1 to be equal
to the voltage on the inverting terminal with a gain of nearly
1.25. A2 completes a nearly identical feedback loop that forces
a current in Q2 that is nearly identical to that in Q1; A2 also
provides the output voltage. When both loops are balanced, the
gain from the noninverting terminal to V
OUT
is equal to 5,
whereas the gain from the output of A1 to V
OUT
is equal to −4.
The inverting terminal gain of A1 (1.25) times the gain of A2
(−4) makes the gain from the inverting and noninverting
terminals equal.
The differential mode gain is equal to 1 + R4/R3, nominally 5,
and is factory trimmed to 0.01% final accuracy. Adding an
external gain setting resistor (R
G
) increases the gain by an
amount equal to (R4 + R1)/R
G
. The output voltage of the
AD627 is given by
V
OUT
= [V
IN
(+) – V
IN
(−)] × (5 + 200 kΩ/R
G
) + V
REF
(1)
Laser trims are performed on R1 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.
R
G
EXTERNAL GAIN RESISTOR
REF
–IN
+IN
+V
S
–V
S
–V
S
–V
S
+V
S
R1
100kΩ
R2
25kΩ
R5
200kΩ
R6
200kΩ
R3
25kΩ
2kΩ 2kΩ
0.1VV1
A2
A1
Q1 Q2
OUTPUT
R4
100kΩ
00782-033
Figure 35. Simplified Schematic