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Dynamic optimization of RTIA & PGA gain in EIS measurement

Category: Software
Product Number: AD5940

Dear team AD,

I am trying to get an automated impedance measurement system (EIS) based on 2-wire electrode using the evaluation board. 

I understand that while I am measuring a Zunknwon with large or small impedance, the ADC output would be somewhat too large or too small and some adjustment is required (RTIA or PGA).

This is easy if I already know the impedance of the Zunknown. 

But when dealing with an unknown impedance, we need an automated system that can suggest an accurate result by making multiple trials. 

1) For instance, if I made an initial measurement and if the ADC voltage is too small, there would be a significant error. 

In the following trial, the EVAL should increase the RTIA.

Repeat until the ADC voltage is in the reasonable range and suggest the impedance value to user. 

2) In the opposite case, if the initial measurement is saturated, the impedance result would be wrong. 

In the following trial, the EVAL would reduce the RTIA. 

Repeat until the ADC voltage is in the reasonable range and suggest the impedance value to user. 

In order to do so, I need to get the voltage magnitude of the AC signal. (ADCed value at somepoint)

I am a noob in signal processing and I can only see that I have couple of options. 

Can you please provide an insight on how I can automate this process? 

Is there a way the magnitude of the AC (voltage just before the ADC) can be obtained?

Thanks!

  • Hi,  .

    Can your team please help look into this query.

    Regards,
    Jo

  • Dear Team ADI, 

    Is there any suggestion regarding the OP?

    Thanks!

  • Hi,

         You may refer to Github example "AD5940_Impedance_Adjustable_with_frequency" as a starting point.

         For impedance measurement, "AD5940_Impedance_Adjustable_with_frequency" example in Github allows automatic updating of filter settings including RTIA after each measurement, depending on frequency of excitation Sine wave. (you may refer to AppIMPCheckFreq() function in this example).

    For EIS example, though similar code flow can be used for automatic RTIA update, it is slightly more complicated because EIS does three acquisitions for each sample measurement as listed below:

    It does 

    1) Sensor + Rload Measurement

    2) RLOAD Measurement

    3) RCAL Measurement

    for arriving at on sample measurement output.

    Hence AppIMPCheckFreq() update must be done for each of the acquisitions above

    unlike impedance example above where AppIMPCheckFreq() is called only once after each sample measurement output assuming that RCAL is chosen close to expected load impedance.