Low Current Ammeter Circuit
This 7-range ammeter measures currents between a few pA to 100 nA without using precision resistors with very high values. The circuit is set up around a current mirror T1a-T1b. The input current is mirrored in this transistor pair, and the current through T1s is greater than the input current by a factor set with S1. Meter M1 is a 100 uA fsd type for displaying the measured value.
The effective series voltage drop at the input terminals of the instrument is only 500 pV because the voltage across the inputs of A1 is forced to virtually nought. The accuracy of the ammeter depends mainly on the components used.
Depending on the required precision, certain components may be replaced by types with a better specification. The Type LF411 opamp used in the A1 position, for example, can be replaced with the Type OP-41 to achieve a ten-fold reduction in the input bias current, and hence an improvement in the final accuracy of the instrument.
Transistor pair T1a- T1b may be replaced by a Type MAT-O2, and the voltage reference set up with Ts—T4 by a Type LM3l3. These high-quality parts should ensure an accuracy of 1% over most of the range. The meter is calibrated in the pA range. Preset P1 is adjusted for full scale detection of M1 at an input current of 1nA.
When it is intended to make a printed circuit board for the x pico ammeter, it should be borne in mind that two 2.5 cm ( long, parallel running, copper tracks spaced 1.25 mm and etched on a high quality epoxy glass carrier represent a leakage resistance of about 100 GSE
This corresponds to a leakage current of 150 pA at a voltage difference of 15 V. Evidently, the PCB for the present ammeter should be thoroughly cleaned to rule out leakage current through residual moisture or resin.
Also note that the insulation of standard test leads is likely to make reliable measuring of currents smaller i than 1 pA impossible. The only e way to overcome this difficulty is to use dry air or PTFE 1 (Teflon).
Source: PMI Linear and Conversion Applications Handbook.