AD705

REV. B

–7–

A High Performance Differential Amplifier Circuit

Figure 25 shows a high input impedance, differential amplifier

circuit that features a high common-mode voltage, and which

operates at low power. Table I details its performance with

changes in gain. To optimize the common-mode rejection of

this circuit at low frequencies and dc, apply a 1 volt, 1 Hz sine

wave to both inputs. Measuring the output with an oscilloscope,

adjust trimming potentiometer R6 for minimum output. For the

best CMR at higher frequencies, capacitor C2 should be replaced

with a 1.5 pF to 20 pF trimmer capacitor.

Both the IC socket and any standoffs at the op amp’s input ter-

minals should be made of Teflon* to maintain low input current

drift over temperature.

*Teflon is a registered trademark of E.I. DuPont, Co.

6

4

0.1µF

7

0.1µF

2

3

AD705

R2

10M

Ω

C1

5pF

R3

200k

Ω

R5

*

R4

*

DC CMR

ADJUST

R6

500k

Ω

C2

5pF

R2'

10M

Ω

R1'

100M

Ω

R1

100M

Ω

SOURCE

GND

CIRCUIT GAIN, G = –

(1+

)

R2+R3

R1

R5

R4

COMMON MODE INPUT RANGE =

RESISTORS R1 AND R1', R2 AND

R2' ARE VICTOREEN MOX-200

1/4 WATT, 1% METAL OXIDE.

WARNING: POTENTIAL DANGER FROM HIGH SOURCE VOLTAGE.

THIS DIFFERENTIAL AMPLIFIER DOES NOT PROVIDE GALVANIC

ISOLATION. INPUT SOURCE MUST BE REFERRED TO THE SAME

GROUND CONNECTION AS THIS AMPLIFIER.

Figure 25. A High Performance Differentials

Amplifier Circuit

Table I. Typical Performance of Differential Amplifier

Circuit Operating at Various Gains

Circuit

R4

R5

Trimmed

RTI Average Circuit

Gain

( )( )

DC CMR

Drift TC

Bandwidth

(dB)

( V/ C)

–3 dB

1

1.13 k

Ω 10 kΩ≥85

30

4.4 kHz

10

100

Ω

9.76 k

Ω≥85

30

2.8 kHz

100

10.2

Ω

10 k

Ω≥85

30

930 Hz

100

90

10

0%

5µs

20mV

Figure 22d. Unity Gain Inverter Small Signal

Pulse Response C, = 1000 pF

6

4

0.1µF

7

0.1µF

2

3

AD705

SQUARE WAVE

INPUT

10k

Ω

5k

Ω

10pF

*

5

NEARLY IDENTICAL

FOR CAPACITANCE

VALUES OF 0 TO 100pF

4.1nF

Figure 23a. Follower Connected

in Feed-Forward Mode

100

90

10

0%

5V

5µs

5V

INPUT

OUTPUT

Figure 23b. Follower Feed-Forward

Pulse Response

6

5

8

4

1

2

3

7

AD705

OVERCOMPENSATION

CAPACITOR

0.1µF

20k

Ω

0.1µF

Figure 24. Offset Null and Overcompensation

Connections