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AD706AR-REEL

Dual Picoampere Input Current Bipolar Op Amp

Analog Devices 

10k⍀

10k⍀

VIN

+VS

–8

1/2

AD706

+4

+

0.1␮F

RL

2.5k⍀

VOUT

CL

SQUARE

WAVE

INPUT

0.1µF

–VS

Figure 5a. Unity Gain Inverter Connection

AD706

Figure 5b. Unity Gain Inverter Large

Signal Pulse Response, CL = 1,000 pF

Figure 5c. Unity Gain Inverter Small

Signal Pulse Response, CL = 100 pF

Figure 5d. Unity Gain Inverter Small

Signal Pulse Response, CL = 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 RG) is

VOUT

= (VIN1

− VIN2

)





1

+

R4 

R3

for R1 = R4 and R2 = R3.

Input resistance is high, thus permitting the signal source to

have an unbalanced output impedance.

R1

49.9k⍀

VIN1

VIN2

RP*

1k⍀

RP*

1k⍀

RG (OPTIONAL)

R2

R3

+VS

0.1␮F

2– 8

A1

1

3+

1/2

AD706

R4

49.9k⍀

1/2

AD706

5–

A2

7

6+ 4

OUTPUT

–VS

VOUT = (VIN1 – VIN2) (1+

FOR R1 = R4, R2 = R3

R4

R3

)

+

(

2R4

RG

)

0.1␮F

*OPTIONAL INPUT PROTECTION RESISTOR FOR GAINS GREATER

THAN 100 OR INPUT VOLTAGES EXCEEDING THE SUPPLY VOLTAGE.

Figure 6. Two Op Amp Instrumentation Amplifier

Furthermore, the circuit gain may be fine trimmed using an

optional trim resistor, RG. 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, RG, can be calculated using

R1= R4 = 49.9 kΩ

R2 = R3 = 49.9 kΩ

0.9 G −1

RG

=

99.8 kΩ

0.06 G

where G = The desired circuit gain.

Table I provides practical 1% resistance values. Note that

without resistor RG, 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

1.10

1.33

1.50

2.00

10.1

101.0

1001

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

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. E

–7–

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