How can a differential scope probe work on a floating circuit re. bias currents? - Page 1
Forum section: electronics / beginners

author: tinfever
author_rank: Regular Contributor
timestamp: December 05, 2022, 06:34:29 am
content: When using a high-voltage high-impedance non-isolated oscilloscope probe (like a Micsig DP10013) on a floating circuit (like literally two AA batteries), how do the bias currents flow in a way that the probe still functions?I'd had the understanding that in a single opamp differential amplifier circuit, if the bias current has no way to flow, it won't work. For example, when using an INA180 current sense amp, if the circuit being measured is completely isolated from the supply to the amplifier, it shouldn't work. Pardon my poor drawing, but there would be no way for the current to flow from V1 in to the IN+ terminal, through the internal resistors, to the GND terminal of the amplifier, and make a full circuit back to the negative terminal of V1.I know a differential probe uses two extra opamps to buffer the inputs to have an even high input impedance, but don't even those buffer opamps have bias currents that must flow?Is the input impedance of those buffer opamps so high that it doesn't even matter at that point? Or is the strategic connection of ground between the two input resistors strings doing more than I think? Still, no current should flow in to ground when connecting to a floating input circuit, but I guess maybe connecting to ground there does...something?

author: Terry Bites
author_rank: Super Contributor
timestamp: December 05, 2022, 11:01:30 am
content: In the ral world example the input resistors to gnd (R1, R6 etc) provide the bias path as do the opamp feedback resistors.The sketch of a diff amp has internal paths via it's resistor network. In the absence of an external path from the shunt to a common gnd, the resistors set the CMV too.

author: tinfever
author_rank: Regular Contributor
timestamp: December 09, 2022, 02:50:59 am
content: Thank you. I think I understand it at least a little bit better.It hadn't occurred to me that in the diff amp, the current might flow from the output of the opamp, through the resistor network, and effectively through the circuit connected to the inputs. Thus it actually works fine whether the input circuit is floating or not.I had been reading the TI app note "Grounding Considerations in Current-Sensing Applications", SBOA536 (https://www.ti.com/lit/pdf/sboa536) and in there they state:QuoteThey spend a while on pages 8-10 talking about the need for a return path for the bias currents flowing in to the CSA.Reading it again, I think they must be talking about the specific type of current sense amps where "the common-mode voltage is used to power the front end amplifier when it is sufficiently high, or higher than the power-supply voltage". (Meaning, not the INA180 I had pictured in the original post)

author: bdunham7
author_rank: Super Contributor
timestamp: December 09, 2022, 03:48:13 am
content: Quote from: tinfever on December 05, 2022, 06:34:29 amYes, that is actually the whole show right there. A differential probe doesn't actually have an isolated differential input as you are thinking of it. It has two inputs with a voltage divider with a given--and often surprisingly low--impedance to ground. That is why there are always two input op-amps. Even if your circuit was truly floating, it isn't anymore after you connect your 'differential' probe. You can use a floating or isolated scope, but then whatever is connected to the 'ground' tap on the differential probe will float to somewhere near middle of your input voltage. If you actually try this it generally won't work as well as you might hope due to parasitic capacitive coupling to all sorts of things.