Three electrode measurement for AD5933?

Hi Oscar,

Silly question... are you interested in magnitude?, phase? or both?

Depending on your interest,  I can recommend one circuit or another...

Regards,

Miguel

Hi everybody,

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,

Oscar

Hi Miguel,

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.

Thanks!,

Oscar

Hi Oscar,

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?

Regards,

Miguel

Hi Miguel,

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!

Oscar

Hi,

Im sorry but I don't know exactly what you did, I need more details.

What is the RFB value used?

Could you provide a schematic about the load connected? Or sensor in the measurements..

Regards,

Miguel

I'm currently using this AFE:

The first stage takes the AD5933 200mV peak-peak signal to 0.18Vdc, 28mV p-p, the second stage is the buffer like you have above with the three electrodes (I'm not using a transistor for current), the third stage is for some gain, and the fourth stage moves the DC bias back to Vdd/2 for the AD5933 internal op amp. The gain is 1 for the internal op amp. 

I've tested the circuit on the scope and verified the frequencies, DC and AC voltages, and ensured that the signal wasn't clipping. I'm sweeping from 100 kHz - 1 Hz, the PGA gain is 1, my calibration resistor is 747 ohms (interested range is roughly 200~2000 ohms), and I'm interested in the Nyquist plot (real and imaginary values). I've measured different values of "unknown" resistors just fine. I'm expecting graphs like the one below, where as more of the biomarker is added, the semi-circle should be getting larger. 

Example from online:

Here's an example of data I collected. The black lines are the attempt at circle curve fitting for the value of interest.

Removed the last two data points (1.5 Hz and 1 Hz) so it's easier to see

The circle shape starts out alright but then gets kinda wild. 

This is the professional EIS device's results. They aren't the best as a whole (the circles should be getting larger as more biomarker is added), but the individual curves are nicely shaped.

Do you have any further suggestions for making the AD5933 work like a potentiostat or work in a three electrode configuration? 

The RFB is 20k ohms. I'm using the AFE below and in practice the Reference and Counter are not shorted together, but connected to electrodes in the biomarker solution like the examples Oscar gave above. 

Edit:

The red boxes which I tried to add in Paint are multiplexers and the red thing between them in the calibration resistor.

For measuring the impedance, I start at 100 kHz and measure the calibration resistor and then the biomarker solution, then I manually step the frequency down to 63 kHz and measure the calibration resistor and then the biomarker solution, and I keep decreasing the frequency until 1 Hz. Then at the end of the sweep, the code calculates the gain factor and the actual real and imaginary values for the solution.

AD5933 is not quite suitable for measurements under 10 KHz - there are artifacts from the way the part performs "DFT". 100 Hz is achievable by working through some cumbersome math, but single Hz range will require reducing frequency of the external clock. 

I'm using a 16 MHz - 4.1 kHz programmable clock for the AD5933 MCLK and I use the 4.1 kHz for the 1 Hz and I have to slow the I2C clock.