we're using the AD5933 for bioelectrical electrode impedance measurements. Usually, the electrodes have to be immersed in a saline solution for measuring, and a 2 point measurement (Electrode to Reference electrode) adds unwanted potential offsets to the measurement. For a more accurate measurement, a technique using 3 electrodes is employed:
(Fig. 4 from this link)
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Is there any way to connect the AD5933 in such an arrangement to measure impedance? Do you have any additional suggestions for performing such a measurement?
Thanks and Kindest Regards,
Silly question... are you interested in magnitude?, phase? or both?
Depending on your interest, I can recommend one circuit or another...
I add some extra info I have researched to see if somebody could help us. Here's one schematic of what the lab equipment does to measure electrode impedance. There is a stimulation voltage Vin that generates a current i, which is measured at a TIA. However, Vin is not directly applied to the electrode, but to a control loop that ensures that Vref=Vin. Therefore, Vin is between Vref and Virtual GND (working electrode). The impedance contribution of the counter electrode is thus eliminated.
(Source: E760 - Lectures on Electrodes I - Martin Schüttler . University College London 2009)
Do you think such arrangement is feasible with the AD5933?
Cheers and Kind Regards,
One for the magnitude is already wonderful, though we need both of them. Is the mag/phase distorted in some of your ideas? If possible, tell me one that respects both values.
The AD5933 does not excite by current... so, you will need to modified slightly the AFE.
As a preliminary analysis, I'll recommend this circuit,
What do you think?
ok, right now we're trying basically your circuit, without the transistor (too simple, I know), hoping that the OpAmp supplies whatever required current for the sensor to produce the desired VREF.
Since we're working with low stimulation currents (around 100 uA), might be that we don't need the extra driving.
If it doesn't work, we're going to put the transistor.
Thanks and kind regards!