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 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
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
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
R
4
V
OUT
=
(
V
−
V
)
1
+
IN1
IN2
R
3
for
R1 = R4
and
R2 = R3.
Input resistance is high, thus permitting the signal source to
have an unbalanced output impedance.
R
G
(OPTIONAL)
R
G
(OPTIONAL)
R1
R1
49.9k
49.9k
+V
S
+V
S
0.1 F
0.1 F
2
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 =
The desired circuit gain.
without resistor R
G
, R2 and R3 = 49.9 kΩ/G–1.
R2
R2
R3
R3
R4
R4
49.9k
49.9k
1/2
1/2
Table I. Operating Gains of Amplifiers A1 and A2 and
Practical 1% Resistor Values for the Circuit of Figure 6
–
–
+
+
8
8
V
IN1
V
IN1
R
P
*
R
P
*
1k
1k
R
P
*
R
P
*
1k
1k
A1
A1
1/2
1/2
1
1
5
5
+
–
–
+
AD706
AD706
A2
A2
4
4
7
7
Circuit Gain
OUTPUT
OUTPUT
0.1 F
0.1 F
Gain of A1 Gain of A2 R2, R3
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
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
3
3
AD706
AD706
6
6
V
IN2
V
IN2
V
OUT
= (V
IN1
– V
IN2
) (1+ R4 ) + ( 2R4 )
V
OUT
= (V
IN1
– V
IN2
) (1+ R4 ) + ( 2R4 )
R3
R
G
R
G
FOR R1 = R4, R2 = R3
R3
FOR R1 = R4, R2 = R3
–V
S
–V
S
* OPTIONAL INPUT PROTECTION RESISTOR FOR GAINS GREATER
*OPTIONAL
INPUT PROTECTION RESISTOR FOR GAINS GREATER
THAN 100
100
INPUT VOLTAGES EXCEEDING THE SUPPLY VOLTAGE.
THAN
OR
OR INPUT VOLTAGES EXCEEDING THE SUPPLY VOLTAGE.
1.10
1.33
1.50
2.00
10.1
101.0
1001
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
REV.
F
–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 ]
