INA117

SBOS154A

10

Figure 8 shows very high input impedance buffer used to

measure low leakage currents. Here, the buffer op amp is

powered with an isolated, split-voltage power supply. Using

an isolated power supply allows full

±200V common-mode

input range.

NOISE PERFORMANCE

The noise performance of the INA117 is dominated by the

internal resistor network. The thermal or Johnson noise of

these resistors produces approximately 550nV/

√Hz noise.

The internal op amp contributes virtually no excess noise at

frequencies above 100Hz.

Many applications may be satisfied with less than the full

200kHz bandwidth of the INA117. In these cases, the noise

can be reduced with a low-pass filter on the output. The two-

pole filter shown in Figure 9 limits bandwidth to 1kHz and

reduces noise by more than 15:1. Since the INA117 has a

1/f noise corner frequency of approximately 100Hz, a cutoff

frequency below 100Hz will not further reduce noise.

380k

Ω

380k

Ω

380k

Ω

21.1k

Ω

20k

Ω

2

3

81

5

e

6

OPA111

INA117

D

1k

Ω

100k

Ω

9k

Ω

Device

Under

Test

100M

Ω

±200V max

I

L

(1V/nA)

+15V

+15V

Com

–15V

PWS725

Isolated DC/DC Converter

*D

base-collector junction (emitter open).

FIGURE 9. Output Filter for Noise Reduction.

BUTTERWORTH

LOW-PASS

OUTPUT NOISE

(mVp-p)

200kHz

1.8

No Filter

100kHz

1.1

11k

Ω

11.3k

Ω

100pF

200pF

10kHz

0.35

11k

Ω

11.3k

Ω

1nF

2nF

1kHz

0.11

11k

Ω

11.3k

Ω

10nF

20nF

0.05

11k

Ω

11.3k

Ω

0.1

µF

0.2

µF

NOTE: (1) Since the INA117 has a 1/f noise corner frequency of approximately 100Hz,

bandwidth reduction below this frequency will not significantly reduce noise.

380k

Ω

380k

Ω

380k

Ω

20k

Ω

21.1k

Ω

47

2

3

81

5

V

6

V–

V+

V

3

V

2

R

2

11.3k

Ω

C

2

0.02µF

OPA27

2-Pole Butterworth

Low-Pass Filter

C

1

0.01µF

R

1

11.0k

Ω

See Application Bulletin AB-017 for other filters.

FIGURE 8. Leakage Current Measurement Circuit.