Data Sheet AD622
Rev. E | Page 9 of 16
THEORY OF OPERATION
The AD622 is a monolithic instrumentation amplifier based on
a modification of the classic three op amp approach. Absolute
value trimming allows the user to program gain accurately (to
0.5% at G = 1000) with only one resistor. Monolithic construction
and laser wafer trimming allow the tight matching and tracking
of circuit components, thus insuring AD622 performance.
Input Transistor Q1 and Input Transistor Q2 provide a single
differential-pair bipolar input for high precision (see Figure 16).
Feedback through the Q1-A1-R1 loop and the Q2-A2-R2 loop
maintains constant collector current of the Q1 and Q2 input
devices, thereby impressing the input voltage across External
Gain-Setting Resistor R
G
. This creates a differential gain from the
inputs to the A1 and A2 outputs given by G = (R1 + R2)/R
G
+ 1.
Unity-Gain Subtractor A3 removes any common-mode signal,
yielding a single-ended output referred to the REF pin potential.
00777-022
V
B
–V
S
A1 A2
A
3
C2
R
G
R1 R2
GAIN
SENSE
GAIN
SENSE
10kΩ
10kΩ
I2
I1
10kΩ
REF
10kΩ
+
IN
– IN
R4
400Ω
OUTPUT
C1
Q2
Q1
R3
400Ω
+V
S
+V
S
+V
S
20µA20µA
Figure 16. Simplified Schematic of the AD622
The value of R
G
also determines the transconductance of the
preamp stage. As R
G
is reduced for larger gains, the trans-
conductance increases asymptotically to that of the input
transistors. This has the following three important advantages:
• Open-loop gain is boosted for increasing programmed
gain, thus reducing gain-related errors.
• The gain-bandwidth product (determined by C1, C2, and
the preamp transconductance) increases with programmed
gain, thus optimizing frequency response.
• The input voltage noise is reduced to a value of 12 nV/√Hz,
determined mainly by the collector current and base
resistance of the input devices.
The internal gain resistors, R1 and R2, are trimmed to an
absolute value of 25.25 kΩ, allowing the gain to be programmed
accurately with a single external resistor.
MAKE vs. BUY: A TYPICAL APPLICATION ERROR
BUDGET
The AD622 offers cost and performance advantages over
discrete two op amp instrumentation amplifier designs along
with smaller size and fewer components. In a typical application
shown in Figure 17, a gain of 10 is required to receive and
amplify a 0 to 20 mA signal from the AD694 current transmitter.
The current is converted to a voltage in a 50 Ω shunt. In
applications where transmission is over long distances, line
impedance can be significant so that differential voltage
measurement is essential. Where there is no connection
between the ground returns of transmitter and receiver, there
must be a dc path from each input to ground, implemented in
this case using two 1 kΩ resistors. The error budget detailed in
Table 5 shows how to calculate the effect of various error
sources on circuit accuracy.
AD694
0 TO 20mA
TRANSMITTER
R
L2
10Ω
R
L2
10Ω
0 TO 20mA
50Ω
0 TO 20mA CURRENT LOOP
WITH 50Ω SHUNT IMPEDANCE
R
G
5.62kΩ
1kΩ
1kΩ
REF
AD622
AD622 MONOLITHIC INSTRUMENTATION
AMPLIFIER, G = 9.986
HOMEBREW IN-AMP, G = 10
1kΩ
1kΩ
1/2
LT1013
1/2
LT1013
9kΩ*
1kΩ* 1kΩ* 9kΩ*
–
+
V
IN
*0.1% RESISTOR MATCH, 50ppm/°C TRACKING
00777-016
Figure 17. Make vs. Buy
