Datasheet

AD621
REV. B
–12–
AD548
TO
REF
C
R
TO
V
OUT1
+
+
10
AD621
DAC
0 TO 10V
+
V
DIFF
0.5V
INPUT A:
10V CM
V
COM
10V
+
+
INPUT B:
1V
OFFSET
V
DIFF
+ V
OFFSET
(1.25V + 1V)
V
OUT1
G = 10
+
10
AD621
V
OUT2
TOTAL GAIN = 100
10k
10k
OPTIONAL
USE THIS IN PLACE OF THE DAC FOR ZERO SUPPRESSION FUNCTION.
Figure 6. Suppressing a Large Common-Mode or Offset Voltage in Order to Measure a Small Differential Signal
(V
S
=
±
15 V)
The AD621, as well as many other monolithic instrumentation
amplifiers, is based on the “three op amp” in amp circuit (Fig-
ure 7) amplifier. Since the input amplifiers (A1 and A2) have a
common-mode gain of unity and a differential gain equal to the
set gain of the overall in amp, the voltages V1 and V2 are defined
by the equations
V
1
= V
CM
+ G × V
DIFF
/2
V
2
= V
CM
G × V
DIFF
/2
The common-mode voltage will drive the outputs of amplifiers
A1 and A2 to the differential-signal voltage, multiplied by the
gain, spreads them apart. For a 10 V common-mode 0.1 V
differential input, V1 would be at 10.5 V and V2 at 9.5 V.
A1
A3
+
10k
10k
10k
10k
A2
+
20k
20k
+
V1
V2
INPUT AMPLIFIER
DIFFERENTIAL GAIN = 10
COMMON MODE GAIN = 1
OUTPUT AMPLIFIER
DIFFERENTIAL GAIN = 1
COMMON MODE GAIN = 1/1000
4.44k
Figure 7. Typical Three Op Amp Instrumentation
Amplifier, Differential Gain = 10
The AD621’s input amplifiers can provide output voltage within
2.5 V of the supplies. To avoid saturation of the input amplifier
the input voltage must therefore obey the equations:
V
CM
+ G × V
DIFF
/2
(Upper Supply – 2.5 V)
V
CM
– G × V
DIFF
/2
(Lower Supply + 2.5 V)
Figure 8 shows the trade-off between common-mode and
differential-mode input for ±15 V supplies and G = 10.
By cascading with use of the optional AD621, the circuit of
Figure 6 will provide ±1 V of zero suppression at gains of 10
and 100 (at V
OUT1
and V
OUT2
respectively) with maximum TCs
of ±4 ppm/°C and ±8 ppm/°C, respectively. Therefore, depend-
ing on the magnitude of the differential input signal, either
V
OUT1
or V
OUT2
may be used as the output.
V
CM
Volts
1.2
0.2
0102
V
DIFF
Volts
48
1.0
0.8
0.6
0.4
6
0
12
Figure 8. Trade-Off Between V
CM
and V
DIFF
Range (V
S
=
±
15 V, G = 10), for Reference Pin at Ground