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ISO124: ISO124U

Frank Bidwell
Prodigy 110 points
Part Number: ISO124

The specification for the ISO124U calls out an input impedance of 200 kOhm typical.  We are using a current sense amplifier to drive this part and the gain is set by the output impedance of the amplifier to ground so the input impedance of the ISO124 is affecting our current sense accuracy. 

What is the range for the input impedance and how much does it change over temperature? 

During our ALT tests we are running some thermal shock from -20C to +50C over 5 minutes and see some strange stuff going on with our current sense. 

If the ISO124 input impedance changes over temperature we may need to compensate for that in our gain settings.

  • 0
    TI__Guru 55062 points

    Hi Frank,

    ISO124U is a fairly old device so my resources are limited. However I would expect a variation over process for input impedance in the neighborhood of +/-15%, with 3-10ppm/C of drift. If you provide the schematic and plots you've recorded I'd be happy to take a closer look. 

    Just confirming that ALT = Altitude tests? 

    Is this for a new design or an existing?

  • 0 in reply to Alexander Smith
    Prodigy 110 points

    ALT = Accelerated Life Test where we thermally cycle the unit and do a constant 5G vibration and a -20C to +50C thermal shock (rise / fall time for temperature is 5 minutes) to simulate aging of the system.

    Here is the portion of the circuit that we are using. The LT6105 current sense amplifier has a gain set by the input resistance / output resistance ( R46 /R193 + R159).  The input impedance of the ISO 124 is in parallel with R193 and R159 so it is effecting the current sense gain.

  • 0 in reply to Frank Bidwell
    Prodigy 110 points

    I see my snap shot for the circuit did not appear in the response.  So I made it into a JPEG and attached it to this response.

  • 0 in reply to Frank Bidwell
    TI__Guru 55062 points

    Hi Frank,

    Thank you for clarifying and providing additional detail. 

    Depending on your desired level of accuracy and timeline I see a couple possibilities for solutions. 

    A gain calibration could be performed by applying two very precise known voltages to the input of the signal chain, calculating a line of best fit between them and comparing this line of best fit to the ideal. Then the inverse can be applied to the measured value, such that the end result is calibrated for gain error. The benefit of this is that it would work regardless of the variation of input impedance for the ISO124U. This would not account for changes in input impedance over temperature however. 

    Another option is that the Rf/Rg for the INA could be decreased such that the variation of ISO124U input impedance is less dominant. 

    Is this for a new design or an existing? What is the timeline for this project?