REV. E
AD706
–7–
10k
+
AD706
+
0.1µF
8
4
V
IN
V
OUT
–
+V
S
10k
C
L
1/2
0.1F
R
L
2.5k
SQUARE
WAVE
INPUT
–V
S
Figure 5a. Unity Gain Inverter Connection
Figure 5b. Unity Gain Inverter Large
Signal Pulse Response, C
L
= 1,000 pF
Figure 5c. Unity Gain Inverter Small
Signal Pulse Response, C
L
= 100 pF
Figure 5d. Unity Gain Inverter Small
Signal Pulse Response, C
L
= 1000 pF
Figure 6 shows an in-amp circuit that has the obvious advantage
of requiring only one AD706, rather than three op amps, with
subsequent savings in cost and power consumption. The transfer
function of this circuit (without R
G
) is
VVV
R
R
OUT IN1 IN2
=− +
()1
4
3
for R1 = R4 and R2 = R3.
Input resistance is high, thus permitting the signal source to
have an unbalanced output impedance.
+V
S
0.1F
1k
+
–
A1
AD706
1/2
R
P
*
1k
49.9k
R2 R3
R4
V
IN1
R
G
(OPTIONAL)
R1
49.9k
A2
+
–
0.1F
AD706
1/2
OUTPUT
*OPTIONAL INPUT PROTECTION RESISTOR FOR GAINS GREATER
THAN 100 OR INPUT VOLTAGES EXCEEDING THE SUPPLY VOLTAGE.
V
OUT
= (V
IN1
– V
IN2
) (1+ ) + ( )
FOR R1 = R4, R2 = R3
R4
R3
2R4
R
G
–V
S
R
P
*
V
IN2
3
2
8
1
5
6
7
4
Figure 6. Two Op Amp Instrumentation Amplifier
Furthermore, the circuit gain may be fine trimmed using an
optional trim resistor, R
G
. Like the three op amp circuit, CMR
increases with gain, once initial trimming is accomplished—but
CMR is still dependent upon the ratio matching of Resistors R1
through R4. Resistor values for this circuit, using the optional
gain resistor, R
G
, can be calculated using
R1= R4 = 49.9 kΩ
R2 = R3 =
49.9 kΩ
0.9 G −1
R
G
=
99.8 kΩ
0.06 G
where G = The desired circuit gain.
Table I provides practical 1% resistance values. Note that
without resistor R
G
, R2 and R3 = 49.9 kΩ/G–1.
Table I. Operating Gains of Amplifiers A1 and A2 and
Practical 1% Resistor Values for the Circuit of Figure 6
Circuit Gain Gain of A1 Gain of A2 R2, R3 R1, R4
1.10 11.00 1.10 499 kΩ 49.9 kΩ
1.33 4.01 1.33 150 kΩ 49.9 kΩ
1.50 3.00 1.50 100 kΩ 49.9 kΩ
2.00 2.00 2.00 49.9 kΩ 49.9 kΩ
10.1 1.11 10.10 5.49 kΩ 49.9 kΩ
101.0 1.01 101.0 499 Ω 49.9 kΩ
1001 1.001 1001 49.9 Ω 49.9 kΩ
For a much more comprehensive discussion of in-amp applica-
tions, refer to the Instrumentation Amplifier Applications Guide—
available free from Analog Devices, Inc.
