Need Help with a signal circuit

Discussion in 'The Projects Forum' started by 737NUT, Mar 28, 2015.

  1. 737NUT

    Thread Starter New Member

    Mar 28, 2015
    2
    0
    Hi all,
    I need to design a circuit that does the following, I need to drive an instrument that has an op amp on it's signal input. 2 wires, one is ground the other is the input which has to go from -550uA to +550uA I would like to use a PWM output from a microcontroller to to send this varying signal to the input of this gauge. I know i'm going to have to use an op-amp driven from the PWM output of microcontroller but not sure how to make it swing from neg to pos or vice versa and at that low of current. Is this at all possible via PWM?
    Thanks
    Rob
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    What supply voltages are available? Is the instrument powered from the same supplies as the microcontroller, or are they separable?

    Of the two inputs to the instrument, can its "ground" input be tied to a voltage like 2.5V? Presumably, the instrument input is differential, and has some common-mode input range?
     
  3. 737NUT

    Thread Starter New Member

    Mar 28, 2015
    2
    0
    The instrument uses 26Vac and 28Vdc But the signal needs to be around 3Vdc to 3.5Vdc at +/-550uA I currently have this set-up, I am using a Teensy which is arduino based usb interface. 3.3V See the pic attached for the rough hand drawn schematic. I need about a 1Vdc more each way to get full travel Right now Im only getting -1.49Vdc one way and +2.0V the other direction using the circuit shown and PWM from the Teensy. I don't have an internal wiring schematic for the gauge, just a block diagram of the internals. I will post it as well. In the block diagram, towards the bottom J2 pins 25 and 26 26 is always gnd and 25 is the signal input to the op amp inside the gauge.
     
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