Pico ammeter

Care must be taken to eliminate any stray currents from flowing into the current summing node. This can be accomplished by forcing all points surrounding the input to the same potential as the input. In this case the potential of the input is at virtual ground, or OV. Therefore, the case of the device is grounded to intercept any stray leakage currents that may otherwise exist between the ±15 V input terminals and the inverting input summing junctions. Feedback capacitance should be kept to a minimum in order to maximize the response time of the circuit to step function input currents.
Pico ammeter - schematic

The time constant of the circuit is approximately the produce of the feedback capacitance C^ times the feedback resistor R^. For instance, the time constant of the circuit is I sec if C^, = lpF. Thus, it takes approximately 5 sec (5 time constants) for the circuit to stabilize to within 1% of its final output voltage after a step function of input current has been applied. of less than 0.2 to 0.3 pF can be achieved with proper circuit layout.

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