Bioelectric Impedance Monitor Project

Discussion in 'The Projects Forum' started by Cannonball2134, Jul 19, 2012.

  1. Cannonball2134

    Thread Starter New Member

    May 9, 2011
    Hello Everyone,

    I am trying to create a bioelectric impedance monitor, to measure skin to electrode impedance, and perhaps later body compositional analysis.

    I have read quite a few papers on this topic, so i understand the principals of how it should work, but im having problems designing the circuit. I thought the simplest way would be to use the AD5933 (impedance converter).

    To use this I have to add an analogue front end to the AD5933. I believe this includes the removal of the DC offset using a high pass filter, adding a voltage controlled current source (VCCS) such as a Howland VCCS, and inject the output current into one electrode, which is passed across the skin(impedance) resulting in a voltage change. The AD5933 then expects a current as its input, so i thought i would add another Howland VCCS and add the the original bias that was removed.

    My background is in computer science, and i have a reasonable level of self taught electronics experience, however there are still gaps in my knowledge, so any help would be greatly appreciated.

    Based on the above description i have created the circuit you can see attached to this post.

    So here are my questions:

    1. Is this circuit suitable for bioelectric impedance monitoring? Can you suggest improvements? or better circuits?
    2. How do i figure out the DC bias and hence the high pass filter cutoff frequency?
    3. How do i add the same DC bias at the end?
  2. wayneh


    Sep 9, 2010
    Something's not right. R3 and R5 are indistinguishable and yet they appear in different places in the formulae.
  3. Cannonball2134

    Thread Starter New Member

    May 9, 2011
    Hi wayneh,

    I got the howland current source circuit with equations from here. Are you saying the equations are incorrect?