Using 2 or 4 electrodes for 4-Wire, Bio-Isolated Impedance?

Question

I beg an excuse for my stupid question. 
 It is about 4-Wire, Bio-Isolated Impedance Measurement Applications (described in AN-1302). 
 We want to build into a wrist held device this type of measurement. The SW is mostly ready but the HW is still open because is not clear (for stupid me) how to use and how to place these 4 electrodes 
 Let's have look at the following picture: 
 
 There are 2 opinions: 
 1) The 4 wires are used to eliminate the influence on the voltage drop in wires on the measurement results. Then UNKNOWN Z is the human body. That means for a wrist held device that only 2 (two) electrodes are required, and because it we would like to use Bio-Isolated Impedance the external capacitors (Ciso) and the OpAmp AD8226 are required. It is also unclear about Raccess resistors - should they be or not. 
 2) The 4 wires are connected to 4 (four) electrodes in contact with human skin. In this case the positioning of electrodes might be important. Before posting this question I did some Googling. It is obvious, I could not find the answer, but I saw implementations where the electrodes are in one line and the excitation electrodes are at the both ends while the measurement electrodes are placed in between. 
 In our setup the 4 electrodes are in the corners of a square with a side approx 12mm. 
 I have tried to calibrate the sensor using different resistors and 4 wires connected to 2 electrodes. This was working perfect. I tried to connect myself instead of UNKNOWN Z and got a magnitude between 20-40kOhm. 
 Then I attached the board with 4 electrodes on it to my skin and the magnitude in this case was in the range or below 1kOhm. 
 Now I am puzzled what to do? What is the right way to use the ADuMC350 for 4-wire Bio-Isolated Impedance Measurement Application??? 
 We spent a lot of time trying to find the correct answer and now we are completely out of time, i.e. as usual - having an answer is more than urgent! Help!!! 
 Best Regards, 
 Sergey

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mark.ramos · Answer

Hi, 
 This Ezone link will help answer your questions above: Bioimpedance Measurement Using ADuCM350 
 1) Raccess Resistor represents the impedance of your cable/wire connection to Zunknown. It doesn't have to be installed on board. 
 2) I did not quite get the question here, but I'd say that the 1K resistor connected on your skin is correct. You can follow the instructions on the link above for Bioimpedance Measurement. There is a user code and document attached on the link on using the BIO3Z board 
 -Mark

mark.ramos · Answer

Hi, 
 On your first question: "What would be your advice – repeat the whole sequence (including toggling of the AFE switch) or set the switch and keep toggling the ADC mux?", you can do two sequences, one is for the calibration routine which you'll only call once and the other is for the measurement routine where you'll toggle between adc channels. 
 The 4-wire Bio-Isolated Impedance is designed to connect E1 to E2, and E3 to E4, more like the one's you did on your scenario 5. E1 and E4 will measure the current through Zunknown using the TIA while E2 and E3 will measure the voltage across Zunknown. I'm not sure what will happen on your scenario 4 where leads are separated from each other. 
 -Mark

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Using 2 or 4 electrodes for 4-Wire, Bio-Isolated Impedance?