AD620B AD [Analog Devices], AD620B Datasheet - Page 10

• Manufacturer Part Number: AD620B
• Description: Low Cost, Low Power Instrumentation Amplifier
• Manufacturer: AD [Analog Devices]

AD620
10V p-p
INPUT
Figure 31b. Gain Nonlinearity, G = 100, R
(100 V = 10 ppm)
Figure 31c. Gain Nonlinearity, G = 1000, R
(1 mV = 100 ppm)
*ALL RESISTORS 1% TOLERANCE
100k
11k
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 32. Settling Time Test Circuit
1k
G=1000
49.9
100
G=100
499
G=10
5.49k
G=1
10k *
1
2
8
3
AD620
1k
10T
–V
4
S
+V
7
S
10k
L
5
L
= 10 k
= 10 k
6
V
OUT
–10–
THEORY OF OPERATION
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 construc-
tion 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 33), yet offer 10×
lower Input Bias Current thanks to Superβeta processing. Feed-
back through the Q1-A1-R1 loop and the Q2-A2-R2 loop main-
tains constant collector current of the input devices Q1, Q2
thereby impressing the input voltage across the external gain
setting resistor R
inputs to the A1/A2 outputs given by G = (R1 + R2)/R
The unity-gain subtracter A3 removes any common-mode sig-
nal, yielding a single-ended output referred to the REF pin
potential.
The value of R
preamp stage. As R
ductance 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, C2 and the preamp transconductance) increases with pro-
grammed gain, thus optimizing frequency response. (c) The
input voltage noise is reduced to a value of 9 nV/ Hz, deter-
mined mainly by the collector current and base resistance of the
input devices.
The internal gain resistors, R1 and R2, are trimmed to an abso-
lute value of 24.7 k , allowing the gain to be programmed
accurately with a single external resistor.
– IN
400
R3
Figure 33. Simplified Schematic of AD620
I1
Q1
G
20 A
G
also determines the transconductance of the
C1
SENSE
. This creates a differential gain from the
GAIN
G
The gain equation is then
A1
is reduced for larger gains, the transcon-
R1
R
–V
G
V
G
R
B
S
R2
G
SENSE
GAIN
A2
49.4 k
so that
20 A
C2
49.4 k
R
G 1
G
Q2
I2
10k
1
10k
400
R4
10k
A3
10k
+IN
G
OUTPUT
REF
REV. E
+ 1.