AD549 datasheet(14/18 Pages) AD

14 / 18 page

AD549

Data Sheet

Rev. K | Page 14 of 18

The test apparatus is calibrated without a device under test

present. After power is turned on, a 5 minute stabilization

period is required. First, VERR1 and VERR2 are measured. These

voltages are the errors caused by the offset voltages and leakage

currents of the I-to-V converters.

VERR1 = 10 (VOSA – IBA × RSa)

VERR2 = 10 (VOSB – IBB × RSb)

VOS

–

AD549

9.01kΩ

1kΩ

RSa

1010Ω

VERR1/VA

VERR2/VB

VOUT

–

CAL/TEST

20pF

0.1µF

–

AD549

9.01kΩ

1kΩ

RSb

1010Ω

20pF

0.1µF

1kΩ

9.01kΩ

DEVICE

UNDER

TEST

I (+)

I (–)

GUARD

Figure 40. Sample and Difference Circuit for Measuring

Electrometer Leakage Currents

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 testing in addition to pro-

viding the I-to-V conversion. The offset error of the device

under testing appears as a common-mode signal and does not

affect the test measurement. As a result, only the leakage

current of the device under testing is measured.

VA – VERR1 = 10[RSa × IB(+)]

VX – VERR2 = 10[RSb × IB(–)]

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 I-to-V converters or

changes in the ambient environment. Laboratory results 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 low input current and low input offset voltage of the AD549

make it an excellent choice for very sensitive photodiode preamps

(see Figure 41). The photodiode develops a signal current, IS,

equal to

IS = R × P

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

and R is the photodiode responsivity in amps/watt. RF converts

the signal current to an output voltage

VOUT = RF × IS

AD549

–VS

10pF

109Ω

10kΩ

1µF

VOUT

–

Figure 41. Photodiode Preamp

The dc error sources and an equivalent circuit for a small area

(0.2 mm square) photodiode are indicated in Figure 42.

VOUT

–

VOS

–

IS–

109Ω

20pF

109Ω

10pF

Figure 42. Photodiode Preamp DC Error Sources

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