AD515 Analog Devices, AD515 Datasheet - Page 6

• Manufacturer Part Number: AD515
• Description: Monolithic Precision/ Low Power FET-Input Electrometer Op Amp
• Manufacturer: Analog Devices

the total noise by about 40%. Noise currents driving the resistors
also add, but in the AD515A are significant only above 10
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
AD515A
9
tend to be expensive, large, noisy and unstable. To avoid
Dimensions shown in inches and (mm).
OUTLINE DIMENSIONS
11
.
TO-99
–6–
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.
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.
The current is sensed by R
amplifier converts the floating differential signal to a single-
ended output.
Figure 14. Current-to-Voltage Converters with Grounded
Bias and Sensor
Figure 13. Picoampere to Voltage Converter with Gain
F
, and the AD524 instrumentation
REV. A