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LMP8358: Differential input ?

AcHmed Muhamed
Prodigy 70 points
Part Number: LMP8358
Other Parts Discussed in Thread: ADC141S626

Why is the LMP8358 limited to 100mV differential input? I would like to use it to 150mV or even 200mV. I see the absoulute max input is limited to the rails. I am suppling the INAMP with 5V and -0.3V.  I'm using it to sense current loside (0 to 200mV ) above circuit ground.

  • 0
    Guru 140200 points
    Hi Achmed,

    why not using a smaller shunt resistance or a voltage divider at the input?

    Kai
  • 0
    TI__Guru 60570 points

    The differential input of LMP8358 is limited to max +/-100mV by the circuit topology of the input stage.  As you apply larger and larger differential voltage above 100mV voltage, the part will be forced to operate deeper in the non-linear region, resulting in a gradual increase of the output error. The absolute maximum rated differential voltage of Vsupply has nothing to do with the linear operation of LMP8358 - the part stops working properly long before such point but will not be damaged until the differential input voltage is greater than Vsupply.

  • 0 in reply to Marek Lis
    Prodigy 70 points
    How significant is the error increase and would it be consistent. Is it just off-set that increses or do other parameters like gain linearity also degrade? Could it be compensated for in software is what I'm asking over the dynamic range I'm after .
  • 0 in reply to AcHmed Muhamed
    TI__Guru 60570 points
    A short answer is no. Once the input stage operates in a non-linear region, all bets are off. Not only offset and drift gets worse but also gain error, noise and bandwidth gets out of specs. You may NOT compensate for it in software because of the lot to lot process variation.
  • 0 in reply to kai klaas69
    Prodigy 70 points
    Well I'm after as high a resolution as I can get preferably 1mA . The actual typical measurments will be in the 1 -15A range but I want to capture start and stall currents of motors as well.

    I'm using a precision 5mOhm kelvin shunt and a 14b ADC (ADC141S626) with a 2.048Vref (250uV per step). So for example at 1mA through the 5mOhm shunt you get a drop of 5uV gained 50x is 250uV so 1mA resolution (calibarate out offest error and gain errors and average readings to reduce noise component). I was planning on using the part to adjust gain up and down as requireid to stay within the ADC input range. The current is bidirectional as well.
  • 0 in reply to Marek Lis
    Prodigy 70 points
    Thank you Marek I will think of something else.