This is a take-off on Mr. Marian’s “Simple Micro Ampere Meter Circuit” that utilizes active circuitry and an analog meter to make sensitive DC current measurements. A reader subsequently asked if it could measure AC microamperes and that is what spawned the idea.
While the schematic is similar, there are notable differences—the voltage drop is now 100mV rather than 1V, the sensitivity is 10nA full scale rather than 100nA and it now measures AC or DC. A precision rectifier rectifies the AC voltage so that a DC meter may be used.
OP Amp Selection
I used a TL074 quad JFET op amp only because that is what was available—there are many suitable candidates—the main requirement is for it to have very low input bias current, and that is why I selected a JFET version. A CA3130 CMOS op amp would work just as well—perhaps better because it has offset null capability.
The sensitive op amp input is protected against dangerous transients via the series resistor R9 and the anti-parallel diodes across the input.
By putting the meter rectifier inside the feedback loop, the voltage drop of the diodes becomes inconsequential due to the high open loop gain of the op amp (Avol = 200,000) that easily corrects this error.
I used my venerable Simpson 260 multimeter on the 0.5mA scale. In this case, R8 = 0.1V /0.5mA = 200Ω. Any serious experimenter should have an analog multimeter in his bag of tricks. I have always had best luck with Simpson.
All is not lost if you do not have an analog meter—a properly applied DVM can do this also, but this circuit is designed around the analog meter.
10nA Full Scale
Actually, this is wishful thinking because the sensitivity is too great for my set-up. As a result, the meter reads half-scale (5nA AC noise) without anything connected. The 100nA setting is much more reasonable—Mr. Marian obviously knew what he was doing by limiting his circuit to 100nA FS. 10nA can still be measured, but the meter would be reading 10% deflection on the 100nA range. All would work better if packaged inside a shielded enclosure.
On the 10nA range, it is so sensitive that AC current caused by body capacitance at a distance of 30mm from the input lead was easily visible—something like putting your hand near a high impedance oscilloscope probe and watching the AC pickup on the trace.
To eliminate ground loops, battery operation is essential. The primary to secondary capacitance of the power supply power transformer can drives this circuit crazy. I cheated and used my line-powered power supply and I believe that this is what caused so much residual noise on the 10nA range.
One caution about battery operation—the short-circuit current of even a tiny 9V battery may be as high as 5A and active circuitry is very unforgiving if connected incorrectly—do not let the smoke escape because the smoke is what makes it work!
One Important Application
Measuring the ground leakage on electronic equipment is required for UL /CSA /VDE … UL limits such current to 5mA to prevent shock hazard. Actually, 5mA is high in my estimation as it may be easily felt.
RMS /Average of a sine wave
If measuring sine wave current, remember that this is an average reading meter and that the RMS value of a sine wave is a factor of 1.1 higher than the average.
For the future
I’m sure that readers are wondering what other neat circuits are already on the Protoboard. Stay tuned because there are many interesting circuits in the pipeline.