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TLV342A: Low-side Current Sense incorrect output

Julian Gentry
Prodigy 10 points
Part Number: TLV342A

I have a solar panel analysis device (university project) that measures voltage of one panel and the current of the whole string of connected panels in a given array.

The only hiccup I'm having in the whole design is that the current sense part of the circuit is giving a constant output instead of changing as the solar panel voltage changes.

The resistor in question is a 1 mOhm, 4 W package, and the amplifier is set up as a non-inverting amplifier of gain 85. 

I've got a power supply attached that is outputting 8 V at roughly 0.1 A. This design should hold up to 10 A, but this is just a test scenario. As I increase the voltage, the current should decrease, and therefore the output of the amplifier should decrease proportionally. It instead stays the same, at roughly 120 mV. This corresponds to a current of 1.4 A, which isn't correct.

I was thinking that maybe the Offset voltage was too high.

Any ideas?

P.S. In the photo, the highlighted area in red is the amp, and the other highlighted area is the sense resistor.

  • 0
    TI__Guru**** 159731 points

    Hello Julian,

    Offset voltage will create an output DC offset that is 100k/1.2k+1 times greater than the input offset.

    However, the voltage should change with sense current at a rate of 84.3mV per ampere. In other words the gain (dV/dI) is not affected by input offset voltage.

    The GND node for R7 must be kelvin connection to R9, otherwise the gain will not be correct.

    Check pin 7 output at two different current levels. 

    10A at 1mOhm is just 10mV and offset can be +/-1.25mV at 25C, so that could be a +/-12.5% full scale error. Use a lower offset op amp or high sense resistance.

    10A * 10A * 0.001 ohm = 100mV, well under 4W