LPC661 Datasheet, PDF (12/17 Pages) National Semiconductor (TI) – Low Power CMOS Operational Amplifier

Application Hints (Continued)
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
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
DS011227-14
FIGURE 10. Simple Input Bias Current Test Circuit
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^−, the leakage of the capacitor and socket
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_x is the stray capacitance at the + input.
Typical Single-Supply Applications (V+ = 5.0 V_DC)
Photodiode Current-toVoltage Converter
Micropower Current Source
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).
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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