Accurately Measure Nanoampere And Picoampere Currents

Measuring current is always a nuisance because you have to break the circuit to put the measuring device in series with the circuit. That problem never goes away. Still, any high-end digital multimeter can accurately measure currents down into the microampere and high-nanoampere range. But the process gets tricky as the current levels fall into the low-nanoampere and picoampere ranges.

At these levels, noise becomes a real problem. Also, the internal impedances of the measuring instrument or circuit become extremely critical to ensure an accurate measurement. By following a few key guidelines, though, designers can achieve accurate low-current measurements with moderately priced test equipment.The Shunt Ammeter Approach

Typical digital multimeters (DMMs) make low-current measurements by passing the current to be measured (I_IN) through a precision resistor (R_S) known as a shunt and then measuring the voltage across it (Fig. 1). The voltage produced across it by the current to be measured is called the burden voltage (V_burden). The burden voltage is an error source since it subtracts from the applied voltage. This causes the current flow to be less than the actual value, thereby producing an error.

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