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Incorrect voltage alignment (square wave voltammetry example)

Category: Hardware
Product Number: ADUCM355

Hello!

 I'd appreciate some help with an issue I'm encountering.

I'm running the square wave voltammetry example on the EVAL-ADucm355 and trying to match the results to a standard Lab potentiostat (PalmSens4). Using a 1 kOhm resistor star I'm able to achieve a close enough match (although with slightly different parameters).
I've performed a SWV test with 5 mV steps and 50 mV amplitude and 4 Hz frequency from -0.1 V to 0.7 V.

Here is the code snippet I used:
pRampCfg->RampStartVolt = -79.0f; 
pRampCfg->RampPeakVolt = +721.0f; 
pRampCfg->VzeroStart = 1300.0f; 
pRampCfg->VzeroPeak = 1300.0f; 
pRampCfg->Frequency = 4;
pRampCfg->SqrWvAmplitude = 50; 
pRampCfg->LPTIARtiaSel = LPTIARTIA_1K; 

 pRampCfg->SqrWvRampIncrement = 5;

pRampCfg->SampleDelay = 90;

My aim is to plot the measured current Vs. real time voltage extracted from the REG_AFE_LPDACDAT0 register, in the following way: (Not completely sure this is the best approach) 

V = ((AD5940_ReadReg(REG_AFE_LPDACDAT0)&0xfff)*DAC12BITVOLT_1LSB - (AppSWVCfg.VzeroPeak - 200.0f))/1e3f; 

And is here is a visualization of the results comparison:

And when zooming in - 

  

The problem begins when I try to perform the same test on a Ferro/Ferricyanide solution (In this case I'm attempting to perform a DPV test by decreasing the reverse current from the forward current).  I've managed to achieve similar graphs only when using a voltage range of -320 - 420 mV (rather than the alleged -100 - 700 mV or the previously aligned range of -79-721 mV) - 

pRampCfg->RampStartVolt = -380.0f; 
pRampCfg->RampPeakVolt = +420.0f;

 

Obviously the chemical reaction observed by the Lab potentiostat is captured by the ADuCM355 with the modified voltage range.

When trying to use the original range (-79-721 mV) the results aren't near as similar- 

My question would be - Why is it that with the resistor star the results relatively match the gold standard (allegedly pointing out that the applied voltage is as we intended) but with the FF solution the same results are obtained with a completely different voltage range.

Thanks in advance :) 

Parents
  • Hi,

     May I know if the excitation voltage with range (-79-721 mV) is monitored and is the same for both resistor star and Ferro/Ferricyanide solution?

    If so, isn't it that Ferro/Ferricyanide solution and resistor star are expected to have different impedances as hence different current pattern?

    Regarding the skipping noises,

    Are you using the measured current values and subtracting one from another?

    Kindly note that the SWV code doesn't measure current continuously.

    It uses sequences and wakeup timers so that device periodically sleeps after each measurement and is woken up by the timer.

Reply
  • Hi,

     May I know if the excitation voltage with range (-79-721 mV) is monitored and is the same for both resistor star and Ferro/Ferricyanide solution?

    If so, isn't it that Ferro/Ferricyanide solution and resistor star are expected to have different impedances as hence different current pattern?

    Regarding the skipping noises,

    Are you using the measured current values and subtracting one from another?

    Kindly note that the SWV code doesn't measure current continuously.

    It uses sequences and wakeup timers so that device periodically sleeps after each measurement and is woken up by the timer.

Children
  • Hi Akila,

    Thank you for your kind response.

    At this point, I haven’t monitored the excitation voltage in both cases. My idea was to use the resistor star as a kind of "sanity check" to ensure the ADuCM355 is producing the correct voltage steps. While the resistor star and the Ferro/Ferricyanide (F/F) cell should indeed have different impedances, my question isn't about these differences in pattern. Rather, I’m curious as to why the ADuCM355 seems to produce the same pattern but shifted by ~300 mV in the DPV graphs, even though the resistor star results confirm the expected voltage pattern.

    Here’s a comparison of how the two different voltage ranges affect the resistor star. It’s noticeable that the (-79 to 721 mV) range matches better than the (-380 to 420 mV) range.

    I’m struggling to understand why one range fits better for the resistor star while the other is better for the chemical reaction. What could be causing this voltage shift?

    Regarding the DPV utilization:

    Yes, I am subtracting subsequent current values from one another. Could you please explain the difference between continuous current measurement and discrete current measurement in the context of my question?

    Thank you!

  • Hi,

       If the excitation voltage generated from ADuCM355 is same in both cases (resistor star and F/F chemical), and AFE configuration is same in both cases,

    then current behavior in shifted range is likely solely due the nature of the load.

    Regarding the intermittent jumps in differential current measurement, I wanted to clarify if the current samples obtained in above "UART data" graph is subtracted one sample from another or some other measurement samples were used for this differential plot.

    Because, if it is first case, then since the shape of current plot in "UART data" graph is same on potentiostat and on ADuCM355, shape of differential plot is expected to be same on potentistat and ADuCM355.