Output load problem

Discussion in 'The Projects Forum' started by rachelgl, May 8, 2012.

  1. rachelgl

    Thread Starter New Member

    Apr 5, 2011

    I have attached a picture of my circuit. This is a control circuit for a MEMS capacitor but the operation isn't hugely important. The issue I am having is in measuring the output voltage. I have modelled a scope probe as being a 10MΩ resistance to ground. The problem is the output resistance of my circuit is quite high so 10MΩ is not enough and the current is just all sinking through this rather than going to my load capacitor CL1. Can anyone recommend a way for me to accurately measure the output? One idea I had was to place an opamp buffer at the output but I'm hoping to get other suggestions too.

    Thanks for your time

  2. #12


    Nov 30, 2010
    The output resistance of THAT circuit is not high. It's just 2 diodes in series with 24 volts.

    However, I don't know what a MEMS capacitor is and I don't know why you have an "ABM" connected to ground.
  3. wayneh


    Sep 9, 2010
    It appears to be "micro-electromechanical systems (MEMS) technology [applied] to variable capacitors". Tunable capacitor.
  4. mcasale


    Jul 18, 2011
    I don't know what the MEMS capacitor is, or the ABM (anti-ballistic missile?).

    The idea of a buffer amp should work if it has a very low bias current at the input. What measurement resolution do you need, 1uV or 1mV or what? There will always be some leakage current. You may need a guard around the measurement node. You also need to generate a DC supply to drive the op amp - like 5VDC maybe. A device like the LMP2011 uses a single supply and has a VERY high impedance.