I am currently measuring the current draw of a PCB and have managed to get down into the uA range. I am able to read 8uA when I use my TENMA 72-7780 as a current sensor.
However when I use a 7R5 current sense resistor and two scope probes either side of the resistor, both referenced to the ground connection of the N93CX power supply I get a ~600uA calculated current draw from V = IR, 0V005 = I 7R5 .
I had a look at the input resistance of my probes ( P6060 ) and they are 10MR, which should only give ~0.3uA draw. The power supply has a ~0.5m cable from it to the 7R5 resistor. The scope probes are ground referenced by their clips to the end of the ground cable.
Does anyone have any idea where this 600uA calculation error is coming in?