AD546

REV. A

–11–

tacting the device under test. Rigid Teflon coaxial cable is used

to make connections to all high impedance nodes. The use of

rigid coax affords immunity to error induced by mechanical vi-

bration and provides an outer conductor for shielding. The en-

tire circuit is enclosed in a grounded metal box.

The test apparatus is calibrated without a device under test

present. A five minute stabilization period after the power is

These voltages are the errors caused by offset voltages and leak-

age currents of the current to voltage converters.

× RSa)

Once measured, these errors are subtracted from the readings

taken with a device under test present. Amplifier B closes the

feedback loop to the device under test, in addition to providing

current to voltage conversion. The offset error of the device un-

der test appears as a common-mode signal and does not affect

the test measurement. As a result, only the leakage current of

the device under test is measured.

Although a series of devices can be tested after only one calibra-

tion measurement, calibration should be updated periodically to

compensate for any thermal drift of the current-to-voltage con-

verters or changes in the ambient environment. Laboratory re-

sults have shown that repeatable measurements within 10 fA can

be realized when this apparatus is properly implemented. These

results are achieved in part by the design of the circuit, which

eliminates relays and other parasitic leakage paths in the high

impedance signal lines, and in part by the inherent cancellation

of errors through the calibration and measurement procedure.

PHOTODIODE INTERFACE

The AD546’s 1 pA current and low input offset voltage make it

a good choice for very sensitive photodiode preamps (Figure

× P

where P is light power incident on the diode’s surface in watts

the signal current to an output voltage:

× I

S

Figure 39. Photodiode Preamp

DC error sources and an equivalent circuit for a small area

(0.2 mm square) photodiode are indicated in Figure 40.

proportional to the feedback resistance:

× R

F

The op amp’s input voltage offset will cause an error current

amplifier’s output equal to:

Given typical values of photodiode shunt resistance (on the or-

der of 10

9

Ω), R

perature, as photodiode shunt resistance typically drops by a

factor of two for every 10

°C rise in temperature. An op amp

with low offset voltage and low drift helps maintain accuracy.

Figure 40. Photodiode Preamp DC Error Sources

Photodiode Preamp Noise

Noise limits the signal resolution obtainable with the preamp.

The output voltage noise divided by the feedback resistance is

the minimum current signal that can be detected. This mini-

mum detectable current divided by the responsivity of the pho-

todiode represents the lowest light power that can be detected

by the preamp.

Noise sources associated with the photodiode, amplifier, and

feedback resistance are shown in Figure 41; Figure 42 is the

voltage spectral density versus frequency plot of each of the

noise source’s contribution to the output voltage noise (circuit

parameters in Figure 40 are assumed). Each noise source’s rms

contribution to the total output voltage noise is obtained by in-

tegrating the square of its spectral density function over fre-

quency. The rms value of the output voltage noise is the square

root of the sum of all contributions. Minimizing the total area

under these curves will optimize the preamplifier’s resolution for

a given bandwidth.

Figure 41. Photodiode Preamp Noise Sources