board at all, but bend it up in the air and use only air as an

insulator. Air is an excellent insulator. In this case you may

have to forego some of the advantages of PC board

construction, but the advantages are sometimes well worth

the

effort

of

using

point-to-point

up-in-the-air

wiring.

See

Figure 6.

BIAS CURRENT TESTING

The test method of

Figure 7 is appropriate for bench-testing

bias current with reasonable accuracy. To understand its

operation, 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

capacitor C2 could cause errors.

Similarly, if S1 is shorted momentarily (while leaving S2

shorted)

where C

DS010548-24

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

All other pins connected to PC board.)

FIGURE 6. Air Wiring

DS010548-25

FIGURE 7. Simple Input Bias Current Test Circuit

Photodiode Current-to-Voltage Converter

DS010548-17

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

DS010548-18

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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