®

INA105

6

FIGURE 2. Offset Adjustment.

FIGURE 3. Precision Difference Amplifier.

For low source impedance applications, an input stage using OPA27 op

amps will give the best low noise, offset, and temperature drift performance.

At source impedances above about 10k

Ω, the bias current noise of the

OPA27 reacting with the input impedance begins to dominate the noise

performance. For these applications, using the OPA111 or dual OPA2111

FET input op amp will provide lower noise performance. For lower cost use

the OPA121 plastic. To construct an electrometer use the OPA128.

R

1

R

2

GAIN

CMRR

MAX

NOISE AT 1kHz

A

(

Ω)(Ω)

(V/V)

(dB)

I

B

(nV/

OPA27A

50.5

2.5k

100

128

40nA

4

OPA111B

202

10k

100

110

1pA

10

OPA128LM

202

10k

100

118

75fA

38

FIGURE 4. Precision Instrumentation Amplifier.

5

6

3

INA105

2

V

1

1

V

0

0utput

A

2

A

1

R

2

R

2

R

1

–In

V

1

+In

V

FIGURE 5. Current Receiver with Compliance to Rails.

V

3

5

6

3

V

O

INA105

V

Offset Adjustment

Range = ±300µV

2

R

3

R

1

R

2

R

4

V

2

10

Ω

499k

Ω

10

Ω

100k

Ω

+15V

–15V

1

V

3

5

6

3

V

0

INA105BM

V

Gain Error = 0.005%

CMR = 100dB

Nonlinearity = 0.0002%

2

R

3

R

1

R

2

V

2

25k

Ω

+In

–In

25k

Ω

R

4

25k

Ω

25k

Ω

1

V–

5

6

3

INA105

2

100

1%

Ω

1

V

0

0 to 2V

I

IN

0 to 20mA

100

1%

Ω