Looking to measure current noise in EIS - AD5941

Hi,

Since you are measuring only current,

if using EIS example code in GitHub,

1) you may remove below sections:

- /* RLOAD Measurement */

-   /* RCAL Measurement */ 

inside measure sequence (AppIMPSeqMeasureGen()).

2) To reduce complexity, you may not enable DFT block in measure sequence:

AD5940_AFECtrlS(AFECTRL_ADCCNV|AFECTRL_DFT, bTRUE);  /* Start ADC convert and DFT */

3) You may just do data process as below. It returns current input from SE. (load includes the 100ohm resistor at SE)

static AD5940Err AppIMPDataProcess(int32_t * const pData, uint32_t *pDataCount)
{
uint32_t i, datacount;
datacount = *pDataCount;
float *pOut = (float *)pData;
for(i=0;i<datacount;i++)
{
pData[i] &= 0xffff;
pOut[i] = AppAMPCalcCurrent(pData[i]);
}
return AD5940ERR_OK;
}

Hi,

Since you are measuring only current,

if using EIS example code in GitHub,

1) you may remove below sections:

- /* RLOAD Measurement */

-   /* RCAL Measurement */ 

inside measure sequence (AppIMPSeqMeasureGen()).

2) To reduce complexity, you may not enable DFT block in measure sequence:

AD5940_AFECtrlS(AFECTRL_ADCCNV|AFECTRL_DFT, bTRUE);  /* Start ADC convert and DFT */

3) You may just do data process as below. It returns current input from SE. (load includes the 100ohm resistor at SE)

static AD5940Err AppIMPDataProcess(int32_t * const pData, uint32_t *pDataCount)
{
uint32_t i, datacount;
datacount = *pDataCount;
float *pOut = (float *)pData;
for(i=0;i<datacount;i++)
{
pData[i] &= 0xffff;
pOut[i] = AppAMPCalcCurrent(pData[i]);
}
return AD5940ERR_OK;
}

Hi Akila, 

Is this still accounting for the AC amplitude? If I understand correctly, I use the following method: 

1. Apply AC signal at set frequency to two-electrode configuration

2. Poll the ADC register and NOT the DFT one

3. Use the Current and Voltage code from the Amperometry example 

I also have a few questions about the amperometry example: ad5940-examples/examples/AD5940_Amperometric/Amperometric.c at master · analogdevicesinc/ad5940-examples (github.com)

AppAMPCalcVoltage uses a k-factor: 1.835/1.82 - what is the origin of these values?

It also uses a 1.8V Ref Voltage, what pin do I measure that from? 

The RTIA magnitude it uses, in the EIS case, is this just the RTIA I set for that measurement?

From the current measurements, how would I obtain RMS from these values since I'm using an AC signal instead of the DC one used in Amperometry? 

Thank you!

Is there a followup for this? Thank you!

Hi,

1) k-factor is required because factory calibration was done at 1.835V.

2) 1.8V Ref Voltage can be measured from VREF_1V82 pin.

3) Could you please clarify? In EIS code, RTIA value is not used for impedance calculation. It uses ratiometric calculation. ( Rz = (1/DftRzRload - 1/DftRload)*DftRcal*RCAL)

4) You may use the statistics block inside AD5940 to calculate mean or variance of measured data.

     Kindly refer to AD5940_StatisticCfgS(&stat_cfg) function in AD5940.c.

     For using statistics block, fifo source must to set to FIFOSRC_VAR or FIFOSRC_MEAN.

AppAMPCalcCurrent(), to convert to voltage and current, uses the RTIA in its function. 

I am asking if this is the same RTIA I set when I measure the current from the ADC?

Hi,

Kindly refer to this: (+) Convert ADC Code to current in Amperometric measurement - Q&A - Precision ADCs - EngineerZone (analog.com)

Hi Akila, I notice the link refers to the LPRTIA. Since I'm using the HS Loop for EIS, i wanted to quantify the current using the HS Loop. Is the process the same? Is there a calibration function for the HSRTIA or can I simply use the actual gain value of the RTIA (i.e. HSRTIA_40K = 40,000 Ohms) when calculating the current. 

I currently have it set to apply a sinusoid voltage across a two-electrode configuration and I poll the ADC (i.e. ADCCNV) results through the SINC3 filter (i.e. AFE_ReadResult(AFERESULT_SINC3) since that is recommended to always be enabled. Or should I poll the SINC2 Notch as per the Amperometric code if I want to measure current noise?

Hi,

  You may use AD5940_HSRtiaCal() function defined in AD5940.c

Sinc3 filter is used to reduce noise.

Hence, to measure current noise, you may bypass filters as below:

adc_filter.BpNotch = bTRUE; 
adc_filter.BpSinc3 = bTRUE; 
adc_filter.Sinc2NotchEnable = bTRUE

Hi Akila, 

Thanks for the response! 

Will polling the SINC3 filter after I bypass it give me the results directly from the ADC? I can't find an AFERESULT_ADC similar to AFERESULT_SINC3 and AFERESULT_SINC2NOTCH. 

Also, do you have any recommendations for the filter settings for the calibration? For reference, here are the frequencies I'm looking to measure noise at (using 10 mV sinusoid excitation with a 1.1V bias):

1. 200 kHz

2. 100 kHz

3. 10 kHz

4. 1000 Hz

5. 100 Hz

6. 10 Hz

7. 1 Hz

8. 0.1 Hz

Thank you!

Hi Akila, 

Sorry about the inconvenience, but I wanted to ask about the open-circuit noise (measuring current without an electrochemical cell). This means applying a 0V signal and then reading the results through the ADC via the SE0 pin. 

What settings need to be enabled / disabled such that I can achieve this 0V signal across SE0, RE0, and CE0? I ask because I know that the HSDAC has a standard 1.1V bias when not using the LPDAC, so I'm not too sure how to go about this. 

Hi,

Yes. After bypassing Sinc3 filter, calling AD5940_ReadAfeResult() reads directly from ADCDAT register.

You may refer to AD5940_GetFreqParameters() function in defined in AD5940.c for calibration filter settings w.r.t. frequency.