BIAS CURRENT TESTING

The test method of

Figure 7 is appropriate for bench-testing

bias current with reasonable accuracy. To understand its op-

eration, first close switch S2 momentarily. When S2 is

opened, then

A suitable capacitor for C2 would be a 5 pF or 10 pF silver

mica, NPO ceramic, or air-dielectric. When determining the

magnitude of I

must be taken into account. Switch S2 should be left shorted

most of the time, or else the dielectric absorption of the ca-

pacitor C2 could cause errors.

Similarly, if S1 is shorted momentarily (while leaving S2

shorted)

where C

DS010547-24

(Input pins are lifted out of PC board and soldered directly to components.

All other pins connected to PC board.)

FIGURE 6. Air Wiring

DS010547-25

FIGURE 7. Simple Input Bias Current Test Circuit

Photodiode Current-to-Voltage Converter

DS010547-17

Note: A 5V bias on the photodiode can cut its capacitance by a factor of 2

or 3, leading to improved response and lower noise. However, this bias on

the photodiode will cause photodiode leakage (also known as its dark

current).

Micropower Current Source

DS010547-18

Note: (Upper limit of output range dictated by input common-mode range;

lower limit dictated by minimum current requirement of LM385.)

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