struction, but the advantages are sometimes well worth the

effort of using point-to-point up-in-the-air wiring. See

Figure

9.

BIAS CURRENT TESTING

The test method of

Figure 10 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

DS011227-13

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

All other pins connected to PC board.)

FIGURE 9. Air Wiring

DS011227-14

FIGURE 10. Simple Input Bias Current Test Circuit

Photodiode Current-toVoltage Converter

DS011227-15

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

DS011227-16

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

limit dictated by minimum current requirement of LM385.)

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