Datasheet

AD620
Rev. H | Page 12 of 20
THEORY OF OPERATION
V
B
–V
S
A1 A2
A3
C2
R
G
R1 R2
GAIN
SENSE
GAIN
SENSE
10kΩ
10kΩ
I2
I1
10kΩ
REF
10kΩ
+IN
– IN
R4
400Ω
OUTPUT
C1
Q2
Q1
00775-0-038
R3
400Ω
+V
S
+V
S
+
V
S
20µA20µA
Figure 36. Simplified Schematic of AD620
The AD620 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 0.15% at G = 100) with only one resistor. Monolithic
construction and laser wafer trimming allow the tight matching
and tracking of circuit components, thus ensuring the high level
of performance inherent in this circuit.
The input transistors Q1 and Q2 provide a single differential-
pair bipolar input for high precision (Figure 36), yet offer 10×
lower input bias current thanks to Superϐeta processing.
Feedback through the Q1-A1-R1 loop and the Q2-A2-R2 loop
maintains constant collector current of the input devices Q1
and Q2, thereby impressing the input voltage across the external
gain setting resistor R
G
. This creates a differential gain from the
inputs to the A1/A2 outputs given by G = (R1 + R2)/R
G
+ 1. The
unity-gain subtractor, A3, removes any common-mode signal,
yielding a single-ended output referred to the REF pin potential.
The value of R
G
also determines the transconductance of the
preamp stage. As R
G
is reduced for larger gains, the
transconductance increases asymptotically to that of the input
transistors. This has three important advantages: (a) Open-loop
gain is boosted for increasing programmed gain, thus reducing
gain related errors. (b) The gain-bandwidth product
(determined by C1 and C2 and the preamp transconductance)
increases with programmed gain, thus optimizing frequency
response. (c) The input voltage noise is reduced to a value of
9 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 24.7 kΩ, allowing the gain to be programmed
accurately with a single external resistor.
The gain equation is then
1
4.49
+
Ω
=
G
R
k
G
1
4.49
−
Ω
=
G
k
R
G
Make vs. Buy: a Typical Bridge Application Error Budget
The AD620 offers improved performance over “homebrew”
three op amp IA designs, along with smaller size, fewer
components, and 10× lower supply current. In the typical
application, shown in Figure 37, a gain of 100 is required to
amplify a bridge output of 20 mV full-scale over the industrial
temperature range of −40°C to +85°C. Table 4 shows how to
calculate the effect various error sources have on circuit
accuracy.