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Best way of measuring current with shunt

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Twistx77:
Hi,

I'm working right now in a Open Soucer project but I have doubts about the a measuring current circuit. I need to measure the current in the power supply and I want to make a high side measurement. The problem is that I want it to be as cheap as possible.

So what I want to do, is to use a cheap opamp like the LM324 to set up an amplifier differential amplifier, and measure the current in high side. What I don't know if I can connect the LM324 to a voltage greater than its supply. The datasheet seems to point that the maximum voltage in the input must not exceed VDD + 1.5V.  I would be measuring currents up to 1A in a 50mohm shunt which would be 50mV at full range but the voltage I'll be using to supply that current could get up to 16V so the commom mode voltage might be too high.

Can anyone advice me on this? should I use another opamp? should I use low side instead high side measurement? (I'd rather prefer high side)

Thank you!

robrenz:
The other voltage drops in your circuit required to achieve 1amp thru your circuit and shunt is separate from the voltage across the shunt. If you dont exceed 1 amp through the shunt,you wont exceed 50mV across the shunt.

Twistx77:
I know that the voltage in the shunt won't exceed 50mV but the opamp will be using 3.3V in VDD and 0V in VDD- . Since the relative voltage of the shunt to the GND will be 16 V in the worst case, won't be that a problem since the VDD of the opamp is several times below those 16V?

The usual way is to use 2 potential dividers (relative to ground) 1 on each side of the shunt. It does need close-tolerance resistors, but mount them physically close to each other for thermal stability. This way it brings the measured values below the power rail potential.

nctnico:

--- Quote from: Zad on April 22, 2012, 03:34:27 pm ---The usual way is to use 2 potential dividers (relative to ground) 1 on each side of the shunt. It does need close-tolerance resistors, but mount them physically close to each other for thermal stability. This way it brings the measured values below the power rail potential.

--- End quote ---
There are special chips for this purpose if the voltage isn't too high. IIRC Zetex has several of them for a low price.
The problem with the divider resistors is that they need to be matched closely (0.1% resistors) and that the differential signal needs to be amplified a lot. This cannot be achieved with low cost opamps. You'll need a special differential opamp chip.