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Isolated current sense instrumentation
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eecook:
Hi All,

I am designing a current sense circuit for educational (my own education) purposes and can't seem to move forward in one minor aspect. I have gone through many application notes and online info but the information seems contradictory to me.
In particular the circuit I want to design is depicted in the following image:



A current transformer with a termination resistor generates a voltage proportional to the current on the primary. This goes through CM and DM filtering and into the InAmp.
The question is, do I need R4 and R5 (in red) to provide a current path to bias the InAmp input stage?

I have read repeatedly that indeed that is what I should be doing, the canonical example being thermocouple instrumentation. On the other hand however, I stumbled upon this (http://www.ti.com/lit/ug/tiducy7/tiducy7.pdf ) appnote, where all the measurements seem to neglect this fact (see Figure 18 for instance).

Of course I am going to build it and test it. I just want to make sure I understand what is going on behind the scenes.

Thanks,
Dave:
You need to set some DC bias on the input pins, otherwise nothing is keeping the potentials on the inputs from floating around, likely out of the common mode input range of the amplifier (because of high impedance inputs).
The application note is completely irrelevant here, because that circuit DOES have a DC bias due to the very fact that you're measuring voltages on resistors, which are referenced to the ground of the circuit and not an isolated source like a transformer.
David Hess:

--- Quote from: eecook on May 23, 2018, 02:49:25 pm ---The question is, do I need R4 and R5 (in red) to provide a current path to bias the InAmp input stage?

I have read repeatedly that indeed that is what I should be doing, the canonical example being thermocouple instrumentation. On the other hand however, I stumbled upon this (http://www.ti.com/lit/ug/tiducy7/tiducy7.pdf ) appnote, where all the measurements seem to neglect this fact (see Figure 18 for instance).
--- End quote ---

The transformer provides galvanic isolation unlike figure 18 so a return path to ground for the amplifier's bias current is necessary.

An instrumentation amplifier and balanced filtering is not needed with the output of a current transformer.  The secondary of the current transformer is galvanically isolated so one side may be referenced to ground (or any other reference) and single ended circuits used.
JS:
You don't need them, as you don't need CM filter. Just tie one end of the transformer to your circuit's ground and the other to the input of the opamp with a proper shunt resistor, note the power disippation of the shunt as it depends on the measured current and the transformer ratio. NM (your DM) filtering could be still used, the transformer would already be filtering something but as it's a transformer working in current mode, heavily loaded, it might filter very little.

Then the DC bias would be provided throw the secondary of your transformer, this could be a problem if you are trying to measure very low currents with a tini tiny transformer and a high DC bias opamp, but for most cases it won't.

Note that the amp in your figure is configured as a differential amplifier block, not an operational amplifier, you will need to add some resistors and probably compensation for your opamp to work as a gain block.

JS
eecook:

--- Quote from: Dave on May 23, 2018, 05:21:34 pm ---The application note is completely irrelevant here, because that circuit DOES have a DC bias due to the very fact that you're measuring voltages on resistors, which are referenced to the ground of the circuit and not an isolated source like a transformer.

--- End quote ---
Thank you for the input Dave. The application note has GND for the power electronics and AGND for control and instrumentation. I was assuming that this two grounds were not connected. So I guess that judging by the fact that the circuit in the app note presumably works and sharing grounds is the only way in which the InAmp gets properly biased, that's how you conclude the two grounds are electrically connected. Is this an accurate assessment?
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