Datasheet

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.
380kΩ 380kΩ
380kΩ
21.1kΩ
20kΩ
2
3
815
e
O
= I
L
x 10
9
6
OPA111
INA117
D
1,2
*
1kΩ
100kΩ
9kΩ
Device
Under
Test
100MΩ
±200V max
I
L
(1V/nA)
+15V
+15V
Com
–15V
PWS725
Isolated DC/DC Converter
*D
1
and D
2
are each a 2N3904 transistor
base-collector junction (emitter open).
FIGURE 9. Output Filter for Noise Reduction.
BUTTERWORTH
LOW-PASS OUTPUT NOISE
f
–3dB
(mVp-p) R
1
R
2
C
1
C
2
200kHz 1.8 No Filter
100kHz 1.1 11kΩ 11.3kΩ 100pF 200pF
10kHz 0.35 11kΩ 11.3kΩ 1nF 2nF
1kHz 0.11 11kΩ 11.3kΩ 10nF 20nF
≤100Hz
(1)
0.05 11kΩ 11.3kΩ 0.1µF 0.2µF
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.
380kΩ 380kΩ
380kΩ
20kΩ21.1kΩ
47
2
3
815
V
O
= V
2
– V
3
6
V–
V+
V
3
V
2
R
2
11.3kΩ
C
2
0.02µF
OPA27
2-Pole Butterworth
Low-Pass Filter
C
1
0.01µF
R
1
11.0kΩ
See Application Bulletin AB-017 for other filters.
FIGURE 8. Leakage Current Measurement Circuit.