AD515A

–6–

REV. A

the total noise by about 40%. Noise currents driving the resistors

also add, but in the AD515A are significant only above 10

11

Ω.

Figure 11. Very High Impedance Instrumentation

Amplifier

Figure 12. Low Drift Integrator and Low Leakage Guarded

Reset

LOW LEVEL CURRENT-TO-VOLTAGE CONVERTERS

Figure 2 shows a standard low level current-to-voltage converter.

To obtain higher sensitivity, it is obvious to simply use a higher

value feedback resistor. However, high value resistors above

10

9

Ω tend to be expensive, large, noisy and unstable. To avoid

this, it may be desirable to use a circuit configuration with

output gain, as in Figure 13. The drawback is that input errors

of offset voltage drift and noise are multiplied by the same gain,

but the precision performance of the AD515A makes the trade-

off easier.

Figure 13. Picoampere to Voltage Converter with Gain

One of the problems with low level leakage current testing or

low level current transducers (such as Clark oxygen sensors) is

finding a way to apply voltage bias to the device while still

grounding the device and the bias source. Figure 14 shows a

technique in which the desired bias is applied at the noninvert-

ing terminal thus forcing that voltage at the inverting terminal.

amplifier converts the floating differential signal to a single-

ended output.

Figure 14. Current-to-Voltage Converters with Grounded

Bias and Sensor

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

TO-99