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Isolation Amplifier

Philip Mak
Prodigy 10 points

I have an application where I am reading a voltage across a shunt resistor. Currently, I am able to take good voltage readings when supplying DC power through the shunt but when I try using an AC power supply, I get skewed data. I believe putting an isolation amplifier will solve the problem. The set up is a variac (connected to 208V building power) supplying power to a resistance heater and I am using a shunt resistor to calculate the current so I can then find the power of the heater. I am hoping to get about 5 kW of power from the resistance heater with a resistance of ~10 ohms. Additionally, the shunt being used is .001 Ohms. I would need a gain of 1, I'm just trying to isolate from the common mode of the variac. Would any of the isolation amplifiers sold by TI work for this particular application? Any additional information would be much appreciated. 

  • 0
    Intellectual 410 points
    5 kW @ 10 ohms is 22A. At those current levels, wouldn't a current transformer or hall-effect current sensor be far simpler? Allegro Microsystems is one supplier.

    These days, analog isolation amplifiers are rare. Since you need a floating supply for the input stage anyway, it's common to float and entire ADC circuit and isolate the digital output. If you need more accuracy than a magnetic current sensor can give you, that's the usual technique.

  • 0 in reply to Cypher Punks
    Guru 21975 points

    Philip,

    I agree with Cypher that this situation would seem well-suited to a current transformer or other type of current sensor that encircles measured current conductor. This provides inherent isolation for safety which is, to my thinking, the real value of an isolation amplifier in your measurement approach. Still, measurement with a current shunt seems to be the wrong approach at this current and voltage level.

    This sounds as if it may be a laboratory measurement situation. There are many types of commercial clamp-on current meters that might be an easy alternative.

    Regards, Bruce.