# Current sensing on positive or negative end [closed]

On a flying drone (hexacopter) I want to measure the current consumption using ACS758. This sensor uses Hall Effect and the load circuit is isolated. Sensor outputs analog voltage referenced to common ground (Batt-). I use MCU (STM32F405) with ADC, which is powered from a buck converter (22.2V to 3.3V). All electronics use same reference ground (Batt-). Frame is also grounded.

## closed as unclear what you're asking by Dmitry Grigoryev, Daniel Grillo, Voltage Spike, ThreePhaseEel, uint128_tDec 22 '16 at 1:21

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• neither is better than the other. do whichever is easiest. – Jasen Dec 20 '16 at 6:10
• The second option lets you measure the current sense voltage relative to ground which might be slightly simpler. – Ken Shirriff Dec 20 '16 at 6:17
• @Ken Shirriff: How do you know where GND is? Maybe the positive terminal of the battery is GND. – Curd Dec 20 '16 at 8:56
• Suppose you have a MCU-based measurement system (i.e. ADC) and this system is supplied from BATT then this means that they both share the same GND (i.e. BATT-). So, since the measurement system and LOAD has the same reference point (i.e. GND = BATT-) then low-side measurement is the cheapest and the easiest way. If measurement system is isolated (from LOAD or BATT) then you can go for either low- or high-side (just like an ammeter - doesn't matter where you placed the ammeter in). – Rohat Kılıç Dec 20 '16 at 9:21
• Define "better". There will be different answers for different applications. – Dave Tweed Dec 20 '16 at 12:36

## 3 Answers

Since the ACS758 uses a Hall sensor to completely isolate the sense circuit from the output circuit it really doesn't matter where in the current loop you put it. Do whatever is most convenient.

There is no fundamental difference.

There are practical differences however. Generally, most people like everything connected to ground, I know I do. That then leaves a choice for which circuit elements are not so connected.

We almost always connect the PSU to ground. Often, the load is ground connected. In that case, you cannot use the second configuration, and have to use the first. Any load switches, controllers and current sensors all need to be on the positive side.

If we have the luxury of a floating load however, then we have the option of using a ground connected current sensor.

It all depends which components you can get, feel most comfortable using, or what your existing system configuration forces or allows you to do.

• I do not understand you :( – Roman Simonyan Dec 20 '16 at 7:39
• TL;DR there is no fundamental difference, do whichever you feel like, you can make either work, 'better' is meaningless. – Neil_UK Dec 20 '16 at 8:07

This is highly situational in real cases.

In your particular example, it may not matter, however I've seldom run into a real application that simple.

Usually the input power is grounded, there are multiple supplies or other loads sharing the ground, the battery terminal is not easily accessible etc. etc. Most typically you need a current monitor signal that is ground-referenced (meaning, say with a 12V battery, (-) side grounded (say 10-14.5V in) and a derived 5V supply you would much prefer a signal that goes from 0-2.5V for 0 to full scale current to a signal that goes from whatever the input voltage is down to that minus 2.5V.

In theory it doesn't matter- current is the same anywhere in a loop. In practical applications, it usually matters quite a bit, and often the high side (the side not grounded) is the one you'll have to use and the complexity will be relatively high as a result.