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I need a reference circuit for 4_wire the impedance measurement using AD5934, range 50 to 250 ohm.

Category: Hardware
Product Number: AD5934

I saw the reference design file CN0349 and CN0217, I think both are 2-wire implementation of the AD5P3x family IC. Can I get the circuit reference for the 4-wire implementation, with impedance range of 50 to 250 ohm. 

Help will be highly appreciated. 

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  • I did test and calibrate using different resistor values. I noted that the measurement accuracy is better near the calibrated resistance value. I think this is not a problem, this I can improve using some tricks.

  • The problem now I face, is, when I use 4 wires and short 2 wires together (T1 with R1 and T2 with R2) see the picture.

    In this case I can get reading. See the pictures as a reference.

    4 wires, 2 wires short together.

    This is the waveform in the saline water.

    But, WHEN I use for wires or electrodes separated from each other, then the behavior changes completely. I can not get any proper reading. please. check the pictures below.

    This is 4 wires, first (white) and last (brown) are T1 and T2. Second (Blue) and  third (black) are R1 and R2 (R stands for receiver)

    This is the acquire waveform,

    Saline water is used in both cases, with conductivity of 7.37 mS/cm.

    Can you help with this.



  • wires or electrodes separated from each other

    I am not sure it is possible to separate the excitation wire Tn from the corresponding sensing wire Rn when in saline - it is going to break the DC working point of your circuit. Roughly speaking, the wires in saline are not galvanically connected to each other due to polarization layer forming at the interface. Very crudely, you can think of it as a capacitor between the wire surface and bulk saline, so the equivalent circuit approximating corresponding behavior would be something like this:

      So, the sensing side of your circuit does not have any DC coupling to the rest of the circuit and your INAMP inputs are hanging at whatever voltage these capacitors are charged to by the input pin currents and leakages. On the excitation side this polarization "capacitance" would break the DC feedback loop for U2, that is why some kind of R11 is necessary.

  • I will add a capacitor on each Rn wires. This might block DC values. 

  • The point is that, once wires are disconnected and put into the saline, the DC voltages are blocked by the equivalent capacitance of the polarization layer between the wire and electrolyte. Adding capacitors will put those in sequence with the polarization layer and will not help with the DC bias of the INAMP - the inputs would need to be DC-biased by some additional circuits. Connecting T1 to R1 and T2 to R2 takes care of the DC bias automatically, why would it be necessary to keep those wires apart?

  • the DC voltages are blocked by the equivalent capacitance of the polarization layer between the wire and electrolyte.

    Got it,

    But, I just went through the following link.

    I did not found any additional circuit. 

  • But, I just went through the following link.

    This link does not mention additional capacitors and nebulous enough showing a rectangle in place of "Body" and providing no information on the electrodes used.

  • In this link adding capacitor is suggested on Rn wires.

    Yes, but it does not explain what these capacitors are there for. It could be that the "body" is kept at some DC potential far different from Vdd/2, could be a requirement to DC-isolate the "body" from the circuit altogether (that could be the reason to also have the two more capacitors in the feedback loop of the OPAMP at the bottom of the diagram) or it could be that some non-polarizable electrodes were used (such as AgAgCl). If you want to follow this link - install all 4 capacitors and connect both INAMP inputs to the output of U3 with two larger resistors, 1MΩ or so. Thing to watch out with these 4 capacitors would be the wait time after Start Frequency command and before starting the measurements - this time should be long enough for the transients caused by these capacitors and resistors to settle and sine wave baseline to become stable.

  • I did solve the problem in simulation. I will test it on the PCB and I will post my results.