High voltages reading, differential amplifier and Vcm (AD629, INA148)

Discussion in 'General Electronics Chat' started by foxOnTheRun, Jan 9, 2012.

  1. foxOnTheRun

    Thread Starter New Member

    Jun 8, 2010
    5
    0
    I'm puzzled by this application


    [​IMG]



    I would like to be able to measure voltages across sections of this circuit, it has a separate common, and my probe points will sense voltage drop across various segments of it (just like I would use a multimeter).

    I simplyfied it by putting 3 resistors to simulate voltage drops (R1 might be mosfets drop, R2 motor drop, R3 a shunt resistor) and a medium 200V Dc voltage source (far high from opam supply). I think I need a diff. opamp with a high Common Mode Voltage but:

    _ I think I'm pretty ok if I should measure voltage drop across a shunt resistor (R3), I can connect +V and -V of the opamp across the shunt resistor and I get a Vout = (+V) - (-V) referred to the uP board ground (then I can amplify it and so on).

    _ what I miss (somewhat) is how can I read the drop across R2? its voltage range span well outside the opamp power supply rails (this problem is no more a common voltage problem), can I use a normal resistive partitioner to reduce the amplitude of the signal, how would I build the input network (or not)?

    Can you point me application notes? until now I can only find examples of reading a shunt resistor, or (ina148 datasheet) they cheat, and use a separate power supply rails to supply the input shaping opamp (but again, they read a simple shunt).

    Thank you all,
    James.
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,022
    3,236
    You could measure the motor voltage with a standard op amp connected in a standard differential configuration and a gain of 0.05 to get 10V out for 200V across R2. For example in this circuit you could make R1 and R2 equal to 1MΩ and Rf and Rg equal to 50kΩ (all 1% tolerance).

    Note: Make sure the 1MΩ resistors are rated for more than 200V. If not you can use two 500kΩ resistors in series.
     
  3. foxOnTheRun

    Thread Starter New Member

    Jun 8, 2010
    5
    0
    I ran a simulation on this circuit: it seems to work fine ;)


    [​IMG]


    My doubt was where I had to connect the input divider network and the opamp pins without frying it; with this configuration I can read whatever voltage inside the circuit (inside the common mode range of the opamp) just like with a multimeter without isolation issues.

    Looks good to you too?

    About your solution: if it works with a normal opamp, then I think I misunderstood the commond mode range feature of some special amplifier :\
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,022
    3,236
    That circuit should work ok. You are reducing the voltage across the motor to about 5V by the R4-R5 divider so there is no danger to the op amp unless you somehow lost the ground connection of R3. You can protect against that by adding a 10V zener between point b and ground.

    Be aware that the power dissipated in the R5 is 1/2W so it should be a 1W resistor for reliability. You could increase the value of R4 and R5 to reduce this dissipation but that will also increase the error in reading the voltage due to the amp input impedance.

    The common-mode voltage range of an amp is just the voltage it can tolerate at both inputs while still operating normally.
     
  5. foxOnTheRun

    Thread Starter New Member

    Jun 8, 2010
    5
    0
    Ok, tnx for your tips :)
     
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