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high side current amplifiers were built in integrators?

Discussion in 'Electronic Design' started by Michael, Apr 26, 2007.

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  1. Michael

    Michael Guest

    Hi there - I'm trying to monitor current going to a motor. The current
    going to it is in a square wave with a (fairly) constant magnitude,
    and a variable duty cycle. Right now I'm using a small shunt resistor
    combined with a high side current amplifier (Maxim MAX4372, amplifies
    voltage 100x) to monitor this current. But really, I mostly just have
    to watch for the rising and falling edges of this pulse, instead of
    the actual magnitude of the pulse. What would be really nice would be
    if I could just read in a single analog voltage that would be a sum of
    the current that has gone through the device since I last read it. I
    will be reading it at a constant frequency, about 75Hz. I was thinking
    that putting an op-amp integrator on the output of the high side
    current amplifier would make it work pretty well. Combine that with a
    fet on the capacitor that could discharge it after ever reading - and
    I think I'd have a decent setup. However - I'm really running out of
    space on my PCB - so three op-amps (I have three of these motors on
    the PCB) and three FETs would be really, really difficult to find
    space for. It might be doable, but I just wanted to see if anybody had
    any ideas for other options.

    More specs about the board:
    motors consume about 2-400ma average. Duty cycle of current pulses
    ranges from about 0-100%. Everything is powered by 5V, except the
    motors which are 6V. An onboard microcontroller is running the show -
    and would be the one reading in the analog voltages with it's 10b ADC.

    Any suggestions? Perhaps are there integrating high side current
    amplifiers? I haven't been able to find anything.


  2. On a sunny day (26 Apr 2007 07:46:46 -0700) it happened Michael

    I would personally see if I could get a trigger on a PIC comparator
    input, say 20mV should be possible.
    then use a PIC with more then one comparator and do the timing and math
    in software, serial out of whatever.
    Or use a PIC with AD and input mux.
    One or 2 chip solution for all three motors.

    I know people here have demanded that when recommending a PIC one should
    also publish the asm code...
    But I ain't going to do that for nopes.

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