I am attempting to use a uCurrent Gold as referenced by several SE posts such as measuring microampere current without noise with a mains-grounded oscilloscope, to observe the small current into an inverting opamp input, for the first stage of a close reproduction of David Ashton's Pulsing Led for Competition.
My setup is diagrammed below, and I reason this is not the correct setup. The oscilloscope I'm using is a mains-grounded scope, so the connection to the V- jack on the uCurrent is shorted to the I- jack, and that represents a path to mains ground.
Referring to the diagram below, the points I marked as P1 and P2 (purple color) are normally shorted, but to insert the uCurrent, I've opened them up and inserted in the current inputs to the uCurrent. The ground wires for CH1 and CH2 of the scope, along with the connection between V- and I- on the uCurrent, result in shorting points P2 and P3 to ground, thus shorting the capacitor to ground, which is not intended.
Basically, I've concluded that the dashed green lines in my diagram below are all at equipotential, which is an incorrect test setup.
And, I reason that, had this circuit used a large separate mains-grounded (not isolated) power supply, versus what it uses which is a CR2032 battery, I would run the risk of damaging my scope (reference EEVblog #279 - How NOT To Blow Up Your Oscilloscope!).
Is my assessment correct? If so, are all of the following approaches viable? :
- Buy a differential probe (EEVblog #279 offset 1214s), or
- Use two different scope channels, one for the uCurrent V+ and a different one for the uCurrent V-, leaving all grounds at the same equipotential, and avoid hooking any of the scope's grounds at point P2, and using the scopes math function to subtract the two signals, effectively resulting in a voltage differential being plotted (only viable if I have a > 2 channel scope), or,
- Just stop using CH2 on the scope to view the voltage on top of capacitor, as that takes out the short at point P3 (but I don't want to do do that as I want to see how the small current going into the inverting op-amp input changes (or doesn't change much at all) along with the voltage on the capacitor).
See also uCurrent with an oscilloscope for a video of someone else struggling with what seems to be a similar issue.
Here is the diagram showing the circuit under test, the latest uCurrent Gold schematic, and the front diagram of the Rigol scope I'm using with connections (high resolution image at imgur):