Common ground issue with low side differential current sensing - Page 1
Forum section: electronics / projects

author: ElectronSurf
author_rank: Regular Contributor
timestamp: December 06, 2020, 03:11:40 pm
content: In below diagram; Do I need to have a common ground between V1 and V2 or not?The op amp is OP07, supply voltage is ±12V and signal voltage doesn't exceed 2.5V.Op amp supply is a center tapped transformer which is not connected to the earth ground.On the breadboard the circuit measures the current with and without a common ground but output of the op amp changes a few mV when disconnecting common ground.It's the third forum that I'm crossposting this question, I've got some answers in those forums too but not anything certain which makes me desperate, because I'm just a hobbyist with little knowledge...

author: Marco
author_rank: Super Contributor
timestamp: December 06, 2020, 04:02:16 pm
content: It doesn't need a common ground, the circuit left of Rs just needs some type of reference so it doesn't float. The difference amplifier isn't designed for a floating input voltage, it will kinda sorta work out, but it's not how it's meant to be used.Just grounding the bottom of V2 will be one way to reference it ... but at that point you shouldn't use a difference amplifier at all, just a plain inverting or non-inverting amplifier, saves you 2 resistors.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: December 06, 2020, 04:17:23 pm
content: Quote from: Marco on December 06, 2020, 04:02:16 pmHow do the multimeters measure the voltage? with an in-amp?Quote from: Marco on December 06, 2020, 04:02:16 pmWhy? just to lower the price by saving two resistors?

author: Marco
author_rank: Super Contributor
timestamp: December 06, 2020, 04:39:54 pm
content: Quote from: ElectronSurf on December 06, 2020, 04:17:23 pmThe multimeter generally floats and uses one of the inputs as its circuit ground.QuoteLess resistors to buy, less resistors to put on the PCB, less resistors which can drift. Less is more.That said, there can be some justification for using a difference amplifier. Itlets you get awaywith not using a bipolar power supply.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: December 06, 2020, 04:57:01 pm
content: Quote from: Marco on December 06, 2020, 04:39:54 pmSee that's the problem, in other forum someone told me that the multimeters don't share a common ground but when I was checking the Dave's multimeter schematic he have a common ground.https://www.eevblog.com/wp-content/plugins/download-attachments/includes/download.php?id=11618So overall you suggest having a common ground is the way to go, I'm very confused to be honest...

author: Marco
author_rank: Super Contributor
timestamp: December 06, 2020, 05:28:54 pm
content: I'm suggesting a common ground AND a single ended amplifier.

author: Vovk_Z
author_rank: Super Contributor
timestamp: December 07, 2020, 03:11:01 am
content: Differential amplifier doesn't need a common ground between two different power suplies (if those power suplies are galvanically isolated).

author: Marco
author_rank: Super Contributor
timestamp: December 07, 2020, 06:06:29 am
content: An instrumentation amplifier is also a differential amplifier, if the input floats it will float right out of the input range.The difference amplifier has low resistance enough input it will kinda reference a floating input and keep it in input range, but what's the point? Easier to just connect one of the inputs to circuit ground and go single ended.

author: David Hess
author_rank: Super Contributor
timestamp: December 07, 2020, 07:13:25 pm
content: Quote from: Marco on December 06, 2020, 04:02:16 pmIf it has some kind of reference so that it does not float, then in practice is has a common ground whether two "grounds" are explicitly connected or not.With that in mind, what happens in this case if there is no connection? See below.Quote from: ElectronSurf on December 06, 2020, 03:11:40 pmIf V1 is truly floating, then ground on the right charges to the common mode voltage at the inputs and that becomes the reference between the two circuits. This can work however it can also present a safety hazard if the common mode voltage is high compared to Earth ground so it is discouraged. See below about how multimeters work.The above is why high voltage differential oscilloscope probes should *never* be used with an isolated oscilloscope input. If they are, then the BNC shield side of the isolated oscilloscope input will charge to the common mode voltage at the input to the differential probe, which can easily be kilovolts or 10s of kilovolts if that is the sort of measurement being made, which is both hazardous and likely to damage the isolated input. For safety reasons, high voltage differential probes have three inputs with the third being the common reference at their BNC shield output.Quote from: ElectronSurf on December 06, 2020, 04:17:23 pmMultimeters, with the exception of differential multimeters, make a singled ended measurement with common referenced to one terminal, usually the negative terminal. The measurement circuits are galvanically isolated from everything else which allows the multimeter's common connection to charge to the input voltage. This asymmetry between the two input terminals is reflected in the specifications which list a "common mode" input capacitance which is the capacitance from the negative terminal to Earth ground, often between 100 and 200 picofarads.This asymmetry in the input impedance of the two inputs can cause problems with some types of high impedance measurements so a multimeter with true differential inputs would be required. Or for instance connecting the ground lead to a sensitive circuit node could cause unintended operation with its effective 100 to 200 picofarads of capacitance to Earth ground.For a handheld multimeter, the entire unit is contained and insulated allowing the common internal connection to charge to the voltage applied ot the negative or common terminal. Bench multimeters use an internal isolation barrier for power and usually data to the measurement circuits so the negative "common" terminal is floating.You can verify the above on a handheld multimeter. Hold it in one hand and measure line voltage with *one lead* to the common terminal and then the volts terminal. The volts input will show a few volts, measured to the multimeter chassis which is capacitively coupled to your hand and then Earth, while the common input will show almost nothing because it is connected to internal common and the chassis so there is nothing to measure.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: December 08, 2020, 07:33:15 am
content: Thanks to everyone who contributed in this topic, I found an article about this very subject."You may have read something like: a differential voltage is floating, in that it has no reference to ground. This definition is partially correct, but misleading. The term ‘floating’ should not be used to describe differential input measurement. The differential potentials must have an indirect or direct reference to the input measurement return lead. This is because no differential input potential can truly float, nor is it good practice to allow an instrument’s inputs to float. In fact, no meaningful measurement can even be made if either of the differential input potentials is floating."Part 1Part 2