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LMP8640: 25% gain error in simple application circuit

Eli Hughes38
Prodigy 70 points
Part Number: LMP8640


I have a simple application of the LMP8640MK-T (Gain of 20).   (Snippet below).     I have a a sensor resistor of 50mOhm w/ a kelvin connection.

I notived that the apparent gain of the circuit is roughly 25 instead of 20.     I Ran a constant current through the resistor and measureed the voltage across the sensor resistor with a meter and the output of the sense amp:

Current (Amps) Vrb (millivolts) (Fluke 117) Across R6 Cout (millivolts) (Fluke 117) Vout to ground Gain
1 51 1198 23.5
0.75 38 898 23.6
0.5 25 608 24.3
0.25 12 301 25.1

Any ideas?

  • 0
    Prodigy 70 points

    You can close this case.  We realize this is a layout error.   Pins 2 and 4 should be swapped to get a kelvin connection.  We modified the board and the circuit function as expected. 

     

  • 0
    TI__Mastermind 27715 points

    Hello Eli,

    The differential input voltages you report are pretty close to ideal for your input current range with a shunt of 50m, so I would advise looking for the issue at the output.

    I see that you've got a second-order RC filter and a trace to some other circuitry connected to the LMP8640 output. Let's eliminate those circuit interactions as variables while we debug. Can you try removing R9, then measuring the LMP8640 output directly at pin 1?

    Best regards,

    Ian Williams
    Applications Manager
    Current Sensing

  • 0 in reply to Eli Hughes38
    TI__Mastermind 27715 points

    Good catch, I missed that on first inspection in your layout snippet. Glad you're up and running!

    Best regards,

    Ian Williams