AD706
10k
+V
S
+
10k
V
IN
0.1 F
V
OUT
R
L
2.5k
–
8
1/2
AD706
+
SQUARE
WAVE
INPUT
4
C
L
0.1µF
–V
S
Figure 23a. Unity Gain Inverter Connection
Figure 23b. Unity Gain Inverter Large
Signal Pulse Response, C
L
= 1,000 pF
Figure 23c. Unity Gain Inverter Small
Signal Pulse Response, C
L
= 100 pF
Figure 23d. Unity Gain Inverter Small
Signal Pulse Response, C
L
= 1000 pF
Figure 24 shows an in-amp circuit that has the obvious advan-
tage 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:
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:
R4
V
OUT
=
(V
IN
#1
−
V
IN
#2
)
1+
R3
for R1 = R4 and R2 = R3
Input resistance is high, thus permitting the signal source to
have an unbalanced output impedance.
R
G
(OPTIONAL)
R1
49.9k
+V
S
0.1 F
2
R1= R4
=
49.9
kΩ
49.9
kΩ
R2
=
R3
=
0.9
G
−1
99.8
kΩ
R
G
=
0.06
G
where G = Desired Circuit Gain
R2
R3
R4
49.9k
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 24
–
+
8
1/2
AD706
1
5
R
P
*
3
A1
1/2
–
A2
7
Circuit Gain
OUTPUT
0.1 F
Gain of A1 Gain of A2
11.00
4.01
3.00
2.00
1.11
1.01
1.001
1.10
1.33
1.50
2.00
10.10
101.0
1001
R2, R3
499 kΩ
150 kΩ
100 kΩ
49.9 kΩ
5.49 kΩ
499
Ω
49.9
Ω
R1, R4
49.9 kΩ
49.9 kΩ
49.9 kΩ
49.9 kΩ
49.9 kΩ
49.9 kΩ
49.9 kΩ
V
IN#1
1k
R
P
*
AD706
6
+
–V
S
4
V
IN#2
1k
V
OUT
= (V
IN#1
– V
IN#2
) (1+ R4 ) + ( 2R4 )
R3
R
G
FOR R1 = R4, R2 = R3
*OPTIONAL INPUT PROTECTION RESISTOR FOR GAINS GREATER
THAN 100 OR INPUT VOLTAGES EXCEEDING THE SUPPLY VOLTAGE.
1.10
1.33
1.50
2.00
10.1
101.0
1001
Figure 24. A 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
REV. C
–7–
For a much more comprehensive discussion of in-amp applica-
tions, refer to the
Instrumentation Amplifier Applications Guide—
available free from Analog Devices, Inc.
AD [ ANALOG DEVICES ]
