AD704

Rev. E | Page 11 of 16

THEORY OF OPERATION

–VIN

+VIN

OUTPUT

+VS

0.1µF

–VS

0.1µF

Ct

C2

C1

R3

6.34k

Ω

RG

GAIN TRIM

(500k

Ω POT)

R4

47.5k

Ω

R5

2.4k

Ω

R1

6.34k

Ω

R2

49.9k

Ω

R6

1M

Ω

R10, 2M

Ω

C5, 0.01µF

R7

1M

Ω

R8

1M

Ω

1/4

AD704

1/4

AD704

1/4

AD704

C4

C3

R11, 2M

Ω

C6, 0.01µF

R9

1M

Ω

1/4

AD704

DC

CMRR

TRIM

(5k

Ω POT)

OPTIONAL

AC CMRR TRIM

OPTIONAL BALANCE RESISTOR NETWORKS

CAN BE REPLACED WITH A SHORT.

NOTES

1. INSTRUMENTATION AMPLIFIER GAIN = 1 +

+

(FOR R1 = R3, R2 = R4 + R5).

2. CAPACITORS C2 AND C4 ARE SOUTHERN ELECTRONICS MPCC, POLYCARBONATE, ±5%, 50V.

3. ALL RESISTORS METAL FILM, 1%.

R2

R1

2R2

RG

Q1 =

C1

4C2

ω =

1

R6 C1C2

R6 = R7

Q2 =

C3

4C4

ω =

1

R8 C3C4

R8 = R9

Figure 34. Gain-of-10 Instrumentation Amplifier with Post Filtering

The instrumentation amplifier with post filtering (see Figure 34)

combines two applications that benefit greatly from the AD704.

This circuit achieves low power and dc precision over temperature

with a minimum of components.

The instrumentation amplifier circuit offers many performance

benefits, including BiFET level input bias currents, low input

offset voltage drift, and only 1.2 mA quiescent current. It operates

for gains that are G ≥ 2 and, at lower gains, it benefits from no

output amplifier offset and no noise contribution as encountered

in a 3-op-amp design. Good low frequency CMRR is achieved

even without the optional ac CMRR trim (see Figure 35). Table 4

provides resistance values for three common circuit gains. For

other gains, use the following equations:

5

10

5

)

(

π

2

1

06

.

0

k

8

.

99

1

9

.

0

k

9

.

49

k

9

.

49

×

≈

Ω

=

−

Ω

=

=

Ω

=

+

=

R3

C

G

R

of

Value

Max

G

R3

R1

R5

R4

R2

t

G

Table 4. Resistance Values for Various Gains

Circuit

Gain (G)

R1 and R3

RG (Max Value of

Trim Potentiometer)

Bandwidth

(−3 dB), Hz

10

6.34 kΩ

166 kΩ

50 k

100

526 Ω

16.6 kΩ

5k

1000

56.2 Ω

1.66 kΩ

0.5 k

160

140

120

100

80

60

40

20

0

1

10

100

1k

10k

FREQUENCY (Hz)

GAIN = 10, 0.2V p-p COMMON-MODE INPUT

TYPICAL MONOLITHIC IN AMP

WITHOUT CAPACITOR Ct

CIRCUIT TRIMMED

USING CAPACITOR Ct

Figure 35. Common-Mode Rejection vs. Frequency with and Without

Capacitor Ct