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

Hi Mark,

Thank you again for answering my questions!

Finally I understood (more or less) what does the ADuCM350 chip and what is the main reason for our problem - the geometry of our setup!

As I said the target is a wrist held device which could detect the heart beat in the bio-impedance measurements. The electrodes are small (4x4mm) and the distance between the (centers of) electrodes  is also small (15 mm  square).

Due to the small distance between the electrodes the resistance is also small (in the range of 100 Ohm or lower) and the resolution, in number of ADC steps, in far too low.

I understood why, when I had connected electrodes in pairs, I got a higher resistance / magnitude! When using 2 electrodes the Raccess is not excluded thus the magnitude value included also 2xRaccess.

Last two days I spent thinking how to solve the problem.  

There are almost no options:

1) Decrease RCAL from 1kOhm to 100 Ohm to have it in the range of the Zunknown?

2) Increase the gain of  the AD8226 OpAmp to fit better to the ADC input range? But there are some  bandwidth limitation so the gain and/or the excitation frequency has to be limited? Use two OpAmps?

3) Decrease somehow the Raccess to increase the voltage thus the current on Zuknown?

4) Use 4-wires not Bio-Isolated ?? I do not see why it might be better in our case but just to mention it

What would be your approach? Is there a "magical solution"?

Hi Mark,

Thank you again for answering my questions!

Finally I understood (more or less) what does the ADuCM350 chip and what is the main reason for our problem - the geometry of our setup!

As I said the target is a wrist held device which could detect the heart beat in the bio-impedance measurements. The electrodes are small (4x4mm) and the distance between the (centers of) electrodes  is also small (15 mm  square).

Due to the small distance between the electrodes the resistance is also small (in the range of 100 Ohm or lower) and the resolution, in number of ADC steps, in far too low.

I understood why, when I had connected electrodes in pairs, I got a higher resistance / magnitude! When using 2 electrodes the Raccess is not excluded thus the magnitude value included also 2xRaccess.

Last two days I spent thinking how to solve the problem.  

There are almost no options:

1) Decrease RCAL from 1kOhm to 100 Ohm to have it in the range of the Zunknown?

2) Increase the gain of  the AD8226 OpAmp to fit better to the ADC input range? But there are some  bandwidth limitation so the gain and/or the excitation frequency has to be limited? Use two OpAmps?

3) Decrease somehow the Raccess to increase the voltage thus the current on Zuknown?

4) Use 4-wires not Bio-Isolated ?? I do not see why it might be better in our case but just to mention it

What would be your approach? Is there a "magical solution"?