Can I measure a floating voltage with an instrumentation amp?

Discussion in 'General Electronics Chat' started by JustWood, Jun 23, 2010.

1. JustWood Thread Starter New Member

Jun 16, 2010
3
0
I am trying to measure the current through a precision 1 ohm resistor.
To do this, I have an INA126's (TI instrumentation amp) inputs connected across the resistor.
I am feeding the output of the amp into the analog input pin of a microcontroller.
I know nothing about the circuit I'm measuring, and I don't know if I can connect the "negative side" of the 1ohm resistor to the same ground as the in amp or the microcontroller.
I am pretty sure the in amp won't be able to measure a floating voltage like that, right?
If not, how can I do this?

Thank you!

2. timrobbins Active Member

Aug 29, 2009
318
16
You have to confirm that either the sense resistor is electrically connected to the measurement circuit, or it is isolated. Then you have to characterise the type of connection - if it is electrically connected then you need to understand whether it is for instance within a bounded range of voltage from your 0V point - if it is isolated then you need to understand if any parasitic current could flow in any way via your measurement circuit.

If you have a good floating circuit connected to the sense resistor then it is fine to connect to the IA - imagine a battery connected to the sense resistor.

Ciao, Tim

3. timrobbins Active Member

Aug 29, 2009
318
16
Oops, that first sentance was a bit clumsy. The reference to 'isolated' is for a floating circuit connected to the sense resistor. The reference to 'electrically connected' is for a circuit that has the sense resistor in it, but also has other electrical connections to the measurement circuit (ie. IA and uP) - possibly through devious paths.

Ciao, Tim

4. JustWood Thread Starter New Member

Jun 16, 2010
3
0
Well the "circuit" I'm trying to measure is just a DC power supply of some sort (It's a metal box you plug into the wall, and out the other side comes a red and black wire carrying a DC voltage). It also generates a frequency, etc (on a separate pair of wires). I do not know if the black DC wire is connected to earth ground.

The only two wires from my circuit touching the other circuit at all should be the + and - terminals of the IA connected to the two sides of the resistor.

My circuit is connected to the wall through a transformer though. So one side of the transformer goes into the wall, and the other side is center tapped providing a +, gnd, and - (which are then rectified, regulated, etc.). As long as my power supply transformer isolates me from the wall, I should be fine, right?

As in I can safely connect the IA "-" terminal to ground (at least through a big resistor?)

5. beenthere Retired Moderator

Apr 20, 2004
15,815
282
You are probably correct in that the DC output is not referenced to earth ground. And that the transformer gives safety isolation from the line.

Why not use a meter to check the DC output, though? That has to be simpler than an instrumentation amp. If you want to get experience with an IA, mterering the DC level is good to do so you know what to expect.

The convention should have the black lead out of the supply as the negative, so the red lead should be positive with respect to the black. This is another useful function for a meter - to eliminate surprises. Anyway, once polarity is established, attaching the black lead to your circuit ground gives a reference, so the red lead's voltage may be applied to any old amp.