Looking to measure current noise in EIS - AD5941

Question

Hi, I'm looking to measure open-circuit current noise for EIS using the AD5941. Can you please confirm if the following methods are correct? 
 My protocol is to run a two-wire setup for a known resistor across SE, RE, and CE. 
 I run an EIS measurement at a set frequency and set RTIA, and then obtain the DFT values of that known resistor. From there, I use the following equation: 
 
 I_{RMS} = (DFT_{MAGNITUDE} * V_{DD}) / ((2^{16} - 1) * R_{TIA})

Is that equation correct for calculating the I_RMS using the DFT? I know that I don't need to measure RCAL since I'm only measuring current, but I wanted to ask to make sure. 
 Lastly, is the theoretical value for the I_RMS equivalent to the AC Voltage RMS divided by the sum of the known resistor and the 100 Ohms in series with SE? 
 Thank you!

kbaudue · Accepted Answer

Hi Akila, 
 I figured out a solution that makes sense to me. Showing this here for anyone else who would want to do so. 
 The method below uses a two-electrode setup with the following switch configuration: 
 sw_cfg . Dswitch = SWD_CE0 ; 
 sw_cfg . Pswitch = SWP_RE0 ; 
 sw_cfg . Nswitch = SWN_SE0 ; 
 sw_cfg . Tswitch = SWT_SE0LOAD | SWT_TRTIA ; 
 AD5940_SWMatrixCfgS ( & sw_cfg ); 
 
 And uses the following filter settings:

adc_filter . ADCSinc3Osr = ADCSINC3OSR_4 ; 
 adc_filter . ADCSinc2Osr = ADCSINC2OSR_1333 ; 
 adc_filter . ADCAvgNum = ADCAVGNUM_2 ; /* Don't care about it. Average function is only used for DFT */ 
 adc_filter . ADCRate = ADCRATE_800KHZ ; /* If ADC clock is 32MHz, then set it to ADCRATE_1P6MHZ. Default is 16MHz, use ADCRATE_800KHZ. */ 
 adc_filter . BpNotch = bTRUE ; /* SINC2+Notch is one block, when bypass notch filter, we can get fresh data from SINC2 filter. */ 
 adc_filter . BpSinc3 = bTRUE ; /* Bypassing SINC3 Filter */ 
 adc_filter . Sinc2NotchEnable = bTRUE ; /* Enable the SINC2+Notch block. You can also use function AD5940_AFECtrlS */ 
 AD5940_ADCFilterCfgS ( & adc_filter );

Here, CE0 and RE0 are tied together to have the same common-mode / bias voltage as SE0. See below: 
 I set the HSTIA common-mode voltage to be at 1.11V using the 1.11V reference ( HsLoopCfg . HsTiaCfg . HstiaBias = HSTIABIAS_1P1 ), and measure the voltage through SE0 through the following switch configuration:

adc_base . ADCMuxP = ADCMUXP_HSTIA_P ; 
 adc_base . ADCMuxN = ADCMUXN_HSTIA_N ; 
 adc_base . ADCPga = ADCPGA_1 ; 
 
 Then I sample it through a similar method using ADCPolling.c by reading 
 
 AD5940_ReadAfeResult(AFEResult_SINC3). Even though SINC2NotchEnable is on, SINC3 is bypassed. By reading SINC3, we actually read the raw ADC codes. 
 
 This gives me VSE0 w.r.t. HSTIA_N, which is 1.11 V as per the datasheet. 
 
 To measure VRE0, I use the ADC using the following MUX configuration:

adc_base . ADCMuxP = ADCMUXP_VRE0 ; 
 adc_base . ADCMuxN = ADCMUXN_VREF1P1 ; 
 adc_base . ADCPga = ADCPGA_1 ; 
 
 This is valid because RE0 and CE0 are tied together and set at the same baseline voltage as the HSTIA common-mode, which is also the 1.11V reference. 
 
 Then I sample it again using a similar method as ADCPolling.c, also reading the SINC3 filter result. This gives me VRE0 w.r.t. 1.11V, also equivalent to HSTIA_N. 
 
 Thus, we achieve VOCP = VSE0 - VRE0. I've confirmed this differential measurement with a DMM at the SE0 and RE0/CE0 (tied together pins) and got the similar values between the DMM and the calculated VOCP using this method. 
 
 This method utilizes the HSTIA to measure input current noise through SE0, which is what makes sense to me for measuring input current noise. 
 
 Hope this helps! please confirm if this is a valid method! 
 
 Thank you for your help, Akila!)

Akila · Answer

Hi, 
 Yes. To ensure that no excitation is given to the load, sw_cfg . Dswitch can be set to SWD_OPEN.

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Looking to measure current noise in EIS - AD5941

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