AD621 Analog Devices, Inc., AD621 Datasheet - Page 14

• Manufacturer Part Number: AD621
• Description: Low Drift, Low Power Instrumentation Amplifier
• Manufacturer: Analog Devices, Inc.

AD621
Gain Selection
The AD621 has accurate, low temperature coefficient (TC),
gains of 10 and 100 available. The gain of the AD621 is nomi-
nally set at 10; this is easily changed to a gain of 100 by simply
connecting a jumper between Pins 1 and 8.
As shown in Figure 11, the device can be programmed for any
gain between 10 and 100 by connecting a single external resistor
between Pins 1 and 8. Note that adding the external resistor will
degrade both the gain accuracy and gain TC. Since the gain
equation of the AD621 yields:
This can be solved for the nominal value of external resistor for
gains between 10 and 100:
Table III gives practical 1% resistor values for several com-
mon gains.
Desired Recommended
Gain
10
20
50
100
A High Performance Programmable Gain Amplifier
The excellent performance of the AD621 at a gain of 10 makes
it a good choice to team up with the AD526 programmable gain
amplifier (PGA) to yield a differential input PGA with gains of
10, 20, 40, 80, 160. As shown in Figure 12, the low offset of the
AD621 allows total circuit offset to be trimmed using the offset
null of the AD526, with only a negligible increase in total drift
error. The total gain TC will be 9 ppm/°C max, with 2 µV/°C
typical input offset drift. Bandwidth is 600 kHz to gains of 10 to
80, and 350 kHz at G = 160. Settling time is 13 µs to 0.01%
for a 10 V output step for all gains.
Factory trimmed–exact value depends on grade.
1% Resistor Value
∞ (Pins 1 and 8 Open)
4.42 kΩ
698 Ω
0 (Pins 1 and 8 Shorted)
Table III. Practical 1% External Resistor
Values for Gains Between 10 and 100
R
R
X
EXT
G = 1 +
=
(G – 1) 555.555 – 55,000
5,555.5
555.5
9 (R
(R
X
X
(10 – G )
+ 6,111.111)
+ 555.555)
AD621
Gain Error Coefficient (TC)
± 10%
± 10%
Temperature
5 ppm/°C max
≈0.4 (50 ppm/°C
+ Resistor TC)
≈0.4 (50 ppm/°C
+ Resistor TC)
5 ppm/°C max
COMMON-MODE REJECTION
Instrumentation amplifiers like the AD621 offer high CMR
which is a measure of the change in output voltage when both
inputs arc changed by equal amounts. These specifications are
usually given for a full-range input voltage change and a speci-
fied source imbalance.
For optimal CMR, the reference terminal should be tied to a
low impedance point, and differences in capacitance and resis-
tance should be kept to a minimum between the two inputs. In
many applications shielded cables are used to minimize noise,
and for best CMR over frequency the shield should he properly
driven. Figures 13 and 14 show active data guards that are config-
ured to improve ac common-mode rejections by “bootstrapping”
the capacitances of input cable shields, thus minimizing the
capacitance mismatch between the inputs.
INPUTS
+
–
0.1 F
100
100
+
–
AD621
–V
AD648
S
100
G = 10
+V
S
– INPUT
+ INPUT
AD548
0.1 F
–INPUT
+INPUT
100k
100k
–V
S
0.1 F
–
+
1
8
2
3
+V
AD621
–V
+
AD621
–
S
AD526
S
+V
–V
+V
S
–V
S
7
4
2
S
S
0.1 F
20k
REFERENCE
5
REFERENCE
6
OUTPUT
V
V
OUT
OUT