AD706_02 Datasheet, PDF (7/12 Pages) Analog Devices – Dual Picoampere Input Current Bipolar Op Amp

10k⍀
10k⍀
V_IN
SQUARE
WAVE
INPUT
+V_S
–₈
1/2
AD706
+
4
–V_S
+
0.1␮F
R_L
2.5k⍀
0.1µF
V_OUT
C_L
Figure 23a. Unity Gain Inverter Connection
AD706
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:
V_OUT
=
(V_IN
#1
-
V_IN
#2
)
ÊËÁ₁₊
R4ˆ
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
R2
R3
R4
49.9k⍀
+V_S
49.9k⍀
V_IN#1
V_IN#2
R_P*
1k⍀
R_P*
1k⍀
0.1␮F
2_–⁸
A1
1
3+
1/2
AD706
1/2
AD706
5–
A2
7
6+ ₄
OUTPUT
–V_S
V_OUT = (V_IN#1 – V_IN#2)
FOR R1 = R4, R2 = R3
(1+ ^R4 ) + ( ^2R4 )
R3
R_G
0.1␮F
*OPTIONAL INPUT PROTECTION RESISTOR FOR GAINS GREATER
THAN 100 OR INPUT VOLTAGES EXCEEDING THE SUPPLY VOLTAGE.
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
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 kW
R2
=
R3
=
49.9 kW
0.9 G -1
R_G
=
99.8 kW
0.06 G
where G = Desired Circuit Gain
Table I provides practical 1% resistance values. (Note that
without resistor R_G, R2 and R3 = 49.9 kW/G–1.)
Table I. Operating Gains of Amplifiers A1 and A2 and
Practical 1% Resistor Values for the Circuit of Figure 24
Circuit Gain
1.10
1.33
1.50
2.00
10.1
101.0
1001
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 kW
150 kW
100 kW
49.9 kW
5.49 kW
499 W
49.9 W
R1, R4
49.9 kW
49.9 kW
49.9 kW
49.9 kW
49.9 kW
49.9 kW
49.9 kW
For a much more comprehensive discussion of in-amp applica-
tions, refer to the Instrumentation Amplifier Applications Guide—
available free from Analog Devices, Inc.
REV. D
–7–