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Op-amp circuit help

Discussion in 'Electronic Design' started by Humphrey B. Bear, Jun 29, 2006.

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  1. Hello all, I've been trying without success to design a circuit that has 2
    inputs and one output, such that the output is:- Vout = Vin1 + (Vin1-Vin2):-

    Vin1----|
    Vin2----|

    I'm using a single 5V supply and both input voltages come from op-amp
    voltage-followers, (originally derived from PWM signals). I need the output
    to swing from 0V to as close as possible to the 5V supply.

    The circuit is intended as part of a closed-loop speed control system for a
    DC motor.

    Can anyone help with ideas, links etc.?

    .... Humphrey
     
  2. R1 R2 R1=R2= 10k-ish.
    Vin2---/\/\---+---/\/\---+
    | _ |
    +--|- \ |
    | >---+---> Vout = 2*Vin1 - Vin2
    Vin1-------------|+_/
    Look for an opamp described as rail-rail input/output.
     
  3. Too easy. Looks good to me. Thanks, Tony.
    Do you think I'd strike any problems if I used 1M resistors instead of 10K,
    so that I could do away with the voltage-followers and take the inputs
    directly from the PWM filter capacitors? (Vin1 is the requested motor speed,
    a 20kHz PWM signal, and Vin2 is the motor speed feedback signal, PWM with a
    frequency ranging from 100Hz at 60RPM to 13.33kHz at 8000RPM, derived from a
    100 pulse-per-revolution, 2", optical encoder disk Vin1 is smoothed with a
    1uF cap and Vin2 is smoothed by a 10uF cap)

    .... Humphrey
     
  4. Yes you could do. You could even probably reduce
    the circuit to just one filter. But if this is a
    first go then retaining the followers allows you
    to look at Vin1 and Vin2 separately.... useful for
    diagnostics.
     
  5. Good one. Thanks again for the help, Tony.
     
  6. Ban

    Ban Guest

    Hi Humphrey,
    I think with your idea it will be very difficult to control the motor speed,
    because of long time constants. If your motor is a DC motor, you could
    actually use the fact that speed is proportional to applied voltage, if you
    correct for armature resistance. So just a current sensor in form of a
    resistor is needed instead of this encoder.
    http://www.elecdesign.com/Articles/ArticleID/6199/6199.html
    http://www.elecdesign.com/Articles/ArticleID/6240/6240.html
    These articles will give you some ideas I hope.
     
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