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Isolated current sense instrumentation

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eecook:
Hey Guys,

Following JS's advice, I'll go for the single-ended version. I've been playing with NL5 and decided to go with the circuit depicted below. Some background first...
The goal of the design is to measure AC mains load currents with the following spec:
Current range: 2.5mA to 1A.
Freq Range: 50Hz to 2kHz (to capture harmonic content).
Signal conditioning to be fed to an ADC with 2.5V full-scale and a sampling rate of 40ksps.

Some of the relevant parts are, the current transformer (PN: FIS155NL) and the op-amp (PN: MCP6L1T-E/OT).

I would love to read some thoughts.
 
This is the schematic:


This is the frequency response (from I1 to VX):


David Hess:
That looks workable but isn't R2 awfully low in value?  I get 12.4 millivolts with a 1 amp input and the transformer datasheet shows that values up to 87.5 ohms total secondary resistance allowing an output up to 175 millivolts total should be acceptable for 50 Hz operation.

eecook:

--- Quote from: David Hess on June 12, 2018, 10:44:10 pm ---That looks workable but isn't R2 awfully low in value?  I get 12.4 millivolts with a 1 amp input and the transformer datasheet shows that values up to 87.5 ohms total secondary resistance allowing an output up to 175 millivolts total should be acceptable for 50 Hz operation.

--- End quote ---
It is horribly low, the problem is the magnetizing inductance of the current transformer. If I increment R2, I lose flatness (and increase phase distortion) in the frequency response.

JS:
The R loading the transformer should be low, as it work as the shunt multiplied by the transformer ratio, if too high the burden voltage is high and the core could saturate and loose precision. Current transformers are rated for that resistor, so datasheet should express the maximum value or power of that reaistor. The lower the better for the transformer. It also depends on frequency, as higher frequency would allow for higher resistance/coupled power.

The topology seems rigt and the bode plot as well, I cant see the values as I can only download a low q pict right now.

JS

David Hess:
I am just saying that it is awfully low based on the transformer specifications which I checked for operation down to even 50 Hz.  Of course there is lots of leeway given the operational amplifier's low input noise and AC coupled operation and maybe you wanted to operate down below 5 Hz.

If you are looking for operation down to the lowest possible frequency, then no load resistor is required at all.  Instead use the transformer to drive the summing node of an inverting amplifier so the load resistance becomes "zero" and only the winding resistance remains.  The feedback resistor for the inverting amplifier then sets the gain in volts/amp or transimpedance.

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