Pseudo Isolated DC Voltage Sensing

Discussion in 'The Projects Forum' started by themindflayer, Oct 14, 2011.

  1. themindflayer

    Thread Starter Member

    Oct 29, 2010
    I have used this circuit to sense 220VAC RMS with an accuracy of 1 Volt with pseudo Isolation. The AC ground and DC grounds were 'isolated' except through the internals IC.

    Right now I want to measure DC voltage, I am assuming that since there was no common ground for AC and DC previously and yet I was able to get a nice sine wave on the output I could use this circuit with a 48V DC source.

    I tried to simulate this to verify but the results are rather strange / am unable to do it.

    The DC Voltage sensing works fine as in the attachment, however if I increase the R to say 10000MEG (or remove it to create isolated sources) the output remains high for a DC sweep of V2 from 1 to 60.

    My question being :

    a) Will I be able to simulate with reliable accuracy or should I bread board and check (currently equipment is N/A)
    b) Will this work in theory?
    c) Any modifications suggestions?

  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    Like any amplifier, the LM358 requires bias current. The datasheet shows that this is not of the very lowest (45nA typ).

    Your circuit is not balanced for bias currents: in particular there is no resistance from the non-inverting input to amplifier ground. 45nA is not a large current, but multiplying it by 10000MΩ gives a hopelessly big voltage (450V). In practice, this voltage is not available in the circuit, so the amplifier simply fails to work.
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    If you do correct the DC balance of the two inputs as far as the resistance of the paths for bias current is concerned, your next problem is going to be the effect of input bias current offset (the bias currents of the two inputs tend not to be identical).

    Input voltage offset is another issue: Neither of these problems is so bad with AC, because a bit of DC drift may be ignored when measuring AC unless it gets so bad that the amplifier limits.

    Simulation should help, but a fair amount can be done with data-sheet values and a bit of arithmetic. This may show you for instance that your present amplifier does not have good enough performance.

    What you are trying to do is actually quite a demanding amplifier application. If the required resistance division is too big relative to the acceptable offset it may even be impracticable. You may be able to do it though with a higher precision device, possibly a dedicated instrumentation amplifier.
  4. ifixit

    Well-Known Member

    Nov 20, 2008
    Hi themindflayer,

    Add a 91K resistor(R9) from the LM358 pin 3 to ground as Adjuster already susjested. This will configure your differential amplifier circuit correctly. Use 1% resistors if possible. The op amp output should now be battery/59.