AD5941 optimising for low frequency Impedance measurements

Question

I have been running some experiments to compare the AD5941 development board to a PalmSens4 potentiostat for EIS applications. 
 The AD5941 is running the stock impedance example code and performing a frequency sweep between 1 and 10kHz. We get really promising results in the 300-10,000 Hz range However we struggle to get good data at lower frequencies. 
 For these experiments a Randals circuit with a 560 Ohm resistor connected in series to (10 K ohm in parallel with 33nF capacitor) is used as the load.

Viewing the raw data we can see that both the phase and impedance measurements appear noisy/unstable at the lower frequency range. 
 
 Is there any advice on how to modify the Impedance example code to better perform at lower frequencies? I noticed that when taking measurements with the PalmSens lower frequencies took significantly longer to produce a valid reading. Whereas the AD5941 Impedance example has a fixed length no matter the frequency. So maybe it needs a wider sample window? 
 
 Any advice would be appreciated, 
 Thanks

Akila · Accepted Answer

Hi,

For frequencies less than 5Hz, you may choose HSRTIA (feedback resistor ) to be 40K Ohms to avoid saturation.

You may refer to AppIMPCheckFreq() function in the below code. This function updates ADC/filter parameters w.r.t. excitation frequency.

/*!
 *****************************************************************************
 @file:    Impedance.c
 @author:  Neo Xu
 @brief:   standard 4-wire or 2-wire impedance measurement sequences.
 -----------------------------------------------------------------------------

Copyright (c) 2017-2019 Analog Devices, Inc. All Rights Reserved.

This software is proprietary to Analog Devices, Inc. and its licensors.
By using this software you agree to the terms of the associated
Analog Devices Software License Agreement.
 
*****************************************************************************/
#include "ad5940.h"
#include <stdio.h>
#include "string.h"
#include "math.h"
#include "Impedance.h"

/* Default LPDAC resolution(2.5V internal reference). */
#define DAC12BITVOLT_1LSB   (2200.0f/4095)  //mV
#define DAC6BITVOLT_1LSB    (DAC12BITVOLT_1LSB*64)  //mV

/* 
  Application configuration structure. Specified by user from template.
  The variables are usable in this whole application.
  It includes basic configuration for sequencer generator and application related parameters
*/
AppIMPCfg_Type AppIMPCfg = 
{
  .bParaChanged = bFALSE,
  .SeqStartAddr = 0,
  .MaxSeqLen = 0,
  
  .SeqStartAddrCal = 0,
  .MaxSeqLenCal = 0,

  .ImpODR = 0.0000000000000001,           /* 20.0 Hz*/
  .NumOfData = -1,
  .SysClkFreq = 16000000.0,
  .WuptClkFreq = 32000.0,
  .AdcClkFreq = 16000000.0,
  .RcalVal = 10000.0,

  .DswitchSel = SWD_CE0,
  .PswitchSel = SWP_CE0,
  .NswitchSel = SWN_AIN1,
  .TswitchSel = SWT_AIN1,

  .PwrMod = AFEPWR_HP,

  .HstiaRtiaSel = HSTIARTIA_5K,
  .ExcitBufGain = EXCITBUFGAIN_0P25,//EXCITBUFGAIN_2,
  .HsDacGain = HSDACGAIN_0P2,//HSDACGAIN_1,
  .HsDacUpdateRate = 7,
  .DacVoltPP = 800.0,
  .BiasVolt = -0.0f,

  .SinFreq = 100000.0, /* 1000Hz */

  .DftNum = DFTNUM_16384,
  .DftSrc = DFTSRC_SINC3,
  .HanWinEn = bTRUE,

  .AdcPgaGain = ADCPGA_4,//ADCPGA_1,
  .ADCSinc3Osr = ADCSINC3OSR_2,
  .ADCSinc2Osr = ADCSINC2OSR_22,

  .ADCAvgNum = ADCAVGNUM_16,

  .SweepCfg.SweepEn = bTRUE,
  .SweepCfg.SweepStart = 1000,
  .SweepCfg.SweepStop = 100000.0,
  .SweepCfg.SweepPoints = 101,
  .SweepCfg.SweepLog = bFALSE,
  .SweepCfg.SweepIndex = 0,

  .FifoThresh = 4,
  .IMPInited = bFALSE,
  .StopRequired = bFALSE,
};

/**
   This function is provided for upper controllers that want to change 
   application parameters specially for user defined parameters.
*/
int32_t AppIMPGetCfg(void *pCfg)
{
  if(pCfg)
  {
    *(AppIMPCfg_Type**)pCfg = &AppIMPCfg;
    return AD5940ERR_OK;
  }
  return AD5940ERR_PARA;
}

int32_t AppIMPCtrl(uint32_t Command, void *pPara)
{
  
  switch (Command)
  {
    case IMPCTRL_START:
    {
      WUPTCfg_Type wupt_cfg;

      if(AD5940_WakeUp(10) > 10)  /* Wakeup AFE by read register, read 10 times at most */
        return AD5940ERR_WAKEUP;  /* Wakeup Failed */
      if(AppIMPCfg.IMPInited == bFALSE)
        return AD5940ERR_APPERROR;
      /* Start it */
      wupt_cfg.WuptEn = bTRUE;
      wupt_cfg.WuptEndSeq = WUPTENDSEQ_A;
      wupt_cfg.WuptOrder[0] = SEQID_0;
      wupt_cfg.SeqxSleepTime[SEQID_0] = 4;
      wupt_cfg.SeqxWakeupTime[SEQID_0] = (uint32_t)(AppIMPCfg.WuptClkFreq/AppIMPCfg.ImpODR)-4;
      AD5940_WUPTCfg(&wupt_cfg);
      
      AppIMPCfg.FifoDataCount = 0;  /* restart */
      break;
    }
    case IMPCTRL_STOPNOW:
    {
      if(AD5940_WakeUp(10) > 10)  /* Wakeup AFE by read register, read 10 times at most */
        return AD5940ERR_WAKEUP;  /* Wakeup Failed */
      /* Start Wupt right now */
      AD5940_WUPTCtrl(bFALSE);
      /* There is chance this operation will fail because sequencer could put AFE back 
        to hibernate mode just after waking up. Use STOPSYNC is better. */
      AD5940_WUPTCtrl(bFALSE);
      break;
    }
    case IMPCTRL_STOPSYNC:
    {
      AppIMPCfg.StopRequired = bTRUE;
      break;
    }
    case IMPCTRL_GETFREQ:
      {
        if(pPara == 0)
          return AD5940ERR_PARA;
        if(AppIMPCfg.SweepCfg.SweepEn == bTRUE)
          *(float*)pPara = AppIMPCfg.FreqofData;
        else
          *(float*)pPara = AppIMPCfg.SinFreq;
      }
    break;
    case IMPCTRL_SHUTDOWN:
    {
      AppIMPCtrl(IMPCTRL_STOPNOW, 0);  /* Stop the measurement if it's running. */
      /* Turn off LPloop related blocks which are not controlled automatically by hibernate operation */
      AFERefCfg_Type aferef_cfg;
      LPLoopCfg_Type lp_loop;
      memset(&aferef_cfg, 0, sizeof(aferef_cfg));
      AD5940_REFCfgS(&aferef_cfg);
      memset(&lp_loop, 0, sizeof(lp_loop));
      AD5940_LPLoopCfgS(&lp_loop);
      AD5940_EnterSleepS();  /* Enter Hibernate */
    }
    break;
    default:
    break;
  }
  return AD5940ERR_OK;
}

/* generated code snnipet */
float AppIMPGetCurrFreq(void)
{
  if(AppIMPCfg.SweepCfg.SweepEn == bTRUE)
    return AppIMPCfg.FreqofData;
  else
    return AppIMPCfg.SinFreq;
}

/* Application initialization */
static AD5940Err AppIMPSeqCfgGen(void)
{
  AD5940Err error = AD5940ERR_OK;
  const uint32_t *pSeqCmd;
  uint32_t SeqLen;
  AFERefCfg_Type aferef_cfg;
  HSLoopCfg_Type HsLoopCfg;
  DSPCfg_Type dsp_cfg;
  float sin_freq;

  /* Start sequence generator here */
  AD5940_SEQGenCtrl(bTRUE);
  
  AD5940_AFECtrlS(AFECTRL_ALL, bFALSE);  /* Init all to disable state */

  aferef_cfg.HpBandgapEn = bTRUE;
  aferef_cfg.Hp1V1BuffEn = bTRUE;
  aferef_cfg.Hp1V8BuffEn = bTRUE;
  aferef_cfg.Disc1V1Cap = bFALSE;
  aferef_cfg.Disc1V8Cap = bFALSE;
  aferef_cfg.Hp1V8ThemBuff = bFALSE;
  aferef_cfg.Hp1V8Ilimit = bFALSE;
  aferef_cfg.Lp1V1BuffEn = bFALSE;
  aferef_cfg.Lp1V8BuffEn = bFALSE;
  /* LP reference control - turn off them to save power*/
  if(AppIMPCfg.BiasVolt != 0.0f)    /* With bias voltage */
  {
    aferef_cfg.LpBandgapEn = bTRUE;
    aferef_cfg.LpRefBufEn = bTRUE;
  }
  else
  {
    aferef_cfg.LpBandgapEn = bFALSE;
    aferef_cfg.LpRefBufEn = bFALSE;
  }
  aferef_cfg.LpRefBoostEn = bFALSE;
  AD5940_REFCfgS(&aferef_cfg);	
  HsLoopCfg.HsDacCfg.ExcitBufGain = AppIMPCfg.ExcitBufGain;
  HsLoopCfg.HsDacCfg.HsDacGain = AppIMPCfg.HsDacGain;
  HsLoopCfg.HsDacCfg.HsDacUpdateRate = AppIMPCfg.HsDacUpdateRate;

  HsLoopCfg.HsTiaCfg.DiodeClose = bFALSE;
	if(AppIMPCfg.BiasVolt != 0.0f)    /* With bias voltage */
		HsLoopCfg.HsTiaCfg.HstiaBias = HSTIABIAS_VZERO0;
	else
		HsLoopCfg.HsTiaCfg.HstiaBias = HSTIABIAS_1P1;
  HsLoopCfg.HsTiaCfg.HstiaCtia = 31; /* 31pF + 2pF */
  HsLoopCfg.HsTiaCfg.HstiaDeRload = HSTIADERLOAD_OPEN;
  HsLoopCfg.HsTiaCfg.HstiaDeRtia = HSTIADERTIA_OPEN;
  HsLoopCfg.HsTiaCfg.HstiaRtiaSel = AppIMPCfg.HstiaRtiaSel;

  HsLoopCfg.SWMatCfg.Dswitch = AppIMPCfg.DswitchSel;
  HsLoopCfg.SWMatCfg.Pswitch = AppIMPCfg.PswitchSel;
  HsLoopCfg.SWMatCfg.Nswitch = AppIMPCfg.NswitchSel;
  HsLoopCfg.SWMatCfg.Tswitch = SWT_TRTIA|AppIMPCfg.TswitchSel;

  HsLoopCfg.WgCfg.WgType = WGTYPE_SIN;
  HsLoopCfg.WgCfg.GainCalEn = bTRUE;
  HsLoopCfg.WgCfg.OffsetCalEn = bTRUE;
  if(AppIMPCfg.SweepCfg.SweepEn == bTRUE)
  {
    AppIMPCfg.FreqofData = AppIMPCfg.SweepCfg.SweepStart;
    AppIMPCfg.SweepCurrFreq = AppIMPCfg.SweepCfg.SweepStart;
    AD5940_SweepNext(&AppIMPCfg.SweepCfg, &AppIMPCfg.SweepNextFreq);
    sin_freq = AppIMPCfg.SweepCurrFreq;
  }
  else
  {
    sin_freq = AppIMPCfg.SinFreq;
    AppIMPCfg.FreqofData = sin_freq;
  }
  HsLoopCfg.WgCfg.SinCfg.SinFreqWord = AD5940_WGFreqWordCal(sin_freq, AppIMPCfg.SysClkFreq);
  HsLoopCfg.WgCfg.SinCfg.SinAmplitudeWord = (uint32_t)(AppIMPCfg.DacVoltPP/800.0f*2047 + 0.5f);
  HsLoopCfg.WgCfg.SinCfg.SinOffsetWord = 0;
  HsLoopCfg.WgCfg.SinCfg.SinPhaseWord = 0;
  AD5940_HSLoopCfgS(&HsLoopCfg);
  if(AppIMPCfg.BiasVolt != 0.0f)    /* With bias voltage */
  {
    LPDACCfg_Type lpdac_cfg;
    
    lpdac_cfg.LpdacSel = LPDAC0;
    lpdac_cfg.LpDacVbiasMux = LPDACVBIAS_12BIT; /* Use Vbias to tuning BiasVolt. */
    lpdac_cfg.LpDacVzeroMux = LPDACVZERO_6BIT;  /* Vbias-Vzero = BiasVolt */
    lpdac_cfg.DacData6Bit = 0x40>>1;            /* Set Vzero to middle scale. */
    if(AppIMPCfg.BiasVolt<-1100.0f) AppIMPCfg.BiasVolt = -1100.0f + DAC12BITVOLT_1LSB;
    if(AppIMPCfg.BiasVolt> 1100.0f) AppIMPCfg.BiasVolt = 1100.0f - DAC12BITVOLT_1LSB;
    lpdac_cfg.DacData12Bit = (uint32_t)((AppIMPCfg.BiasVolt + 1100.0f)/DAC12BITVOLT_1LSB);
    lpdac_cfg.DataRst = bFALSE;      /* Do not reset data register */
    lpdac_cfg.LpDacSW = LPDACSW_VBIAS2LPPA|LPDACSW_VBIAS2PIN|LPDACSW_VZERO2LPTIA|LPDACSW_VZERO2PIN|LPDACSW_VZERO2HSTIA;
    lpdac_cfg.LpDacRef = LPDACREF_2P5;
    lpdac_cfg.LpDacSrc = LPDACSRC_MMR;      /* Use MMR data, we use LPDAC to generate bias voltage for LPTIA - the Vzero */
    lpdac_cfg.PowerEn = bTRUE;              /* Power up LPDAC */
    AD5940_LPDACCfgS(&lpdac_cfg);
  }
  dsp_cfg.ADCBaseCfg.ADCMuxN = ADCMUXN_HSTIA_N;
  dsp_cfg.ADCBaseCfg.ADCMuxP = ADCMUXP_HSTIA_P;
  dsp_cfg.ADCBaseCfg.ADCPga = AppIMPCfg.AdcPgaGain;
  
  memset(&dsp_cfg.ADCDigCompCfg, 0, sizeof(dsp_cfg.ADCDigCompCfg));
  
  dsp_cfg.ADCFilterCfg.ADCAvgNum = AppIMPCfg.ADCAvgNum;
  dsp_cfg.ADCFilterCfg.ADCRate = ADCRATE_800KHZ;	/* Tell filter block clock rate of ADC*/
  dsp_cfg.ADCFilterCfg.ADCSinc2Osr = AppIMPCfg.ADCSinc2Osr;
  dsp_cfg.ADCFilterCfg.ADCSinc3Osr = AppIMPCfg.ADCSinc3Osr;
  dsp_cfg.ADCFilterCfg.BpNotch = bTRUE;
  dsp_cfg.ADCFilterCfg.BpSinc3 = bFALSE;
  dsp_cfg.ADCFilterCfg.Sinc2NotchEnable = bTRUE;
  dsp_cfg.DftCfg.DftNum = AppIMPCfg.DftNum;
  dsp_cfg.DftCfg.DftSrc = AppIMPCfg.DftSrc;
  dsp_cfg.DftCfg.HanWinEn = AppIMPCfg.HanWinEn;
  
  memset(&dsp_cfg.StatCfg, 0, sizeof(dsp_cfg.StatCfg));
  AD5940_DSPCfgS(&dsp_cfg);
    
  /* Enable all of them. They are automatically turned off during hibernate mode to save power */
  if(AppIMPCfg.BiasVolt == 0.0f)
    AD5940_AFECtrlS(AFECTRL_HSTIAPWR|AFECTRL_INAMPPWR|AFECTRL_EXTBUFPWR|\
                AFECTRL_WG|AFECTRL_DACREFPWR|AFECTRL_HSDACPWR|\
                AFECTRL_SINC2NOTCH, bTRUE);
  else
    AD5940_AFECtrlS(AFECTRL_HSTIAPWR|AFECTRL_INAMPPWR|AFECTRL_EXTBUFPWR|\
                AFECTRL_WG|AFECTRL_DACREFPWR|AFECTRL_HSDACPWR|\
                AFECTRL_SINC2NOTCH|AFECTRL_DCBUFPWR, bTRUE);
    /* Sequence end. */
  AD5940_SEQGenInsert(SEQ_STOP()); /* Add one extra command to disable sequencer for initialization sequence because we only want it to run one time. */

  /* Stop here */
  error = AD5940_SEQGenFetchSeq(&pSeqCmd, &SeqLen);
  AD5940_SEQGenCtrl(bFALSE); /* Stop sequencer generator */
  if(error == AD5940ERR_OK)
  {
    AppIMPCfg.InitSeqInfo.SeqId = SEQID_1;
    AppIMPCfg.InitSeqInfo.SeqRamAddr = AppIMPCfg.SeqStartAddr;
    AppIMPCfg.InitSeqInfo.pSeqCmd = pSeqCmd;
    AppIMPCfg.InitSeqInfo.SeqLen = SeqLen;
    /* Write command to SRAM */
    AD5940_SEQCmdWrite(AppIMPCfg.InitSeqInfo.SeqRamAddr, pSeqCmd, SeqLen);
  }
  else
    return error; /* Error */
  return AD5940ERR_OK;
}


static AD5940Err AppIMPSeqMeasureGen(void)
{
  AD5940Err error = AD5940ERR_OK;
  const uint32_t *pSeqCmd;
  uint32_t SeqLen;
  
  uint32_t WaitClks;
  SWMatrixCfg_Type sw_cfg;
  ClksCalInfo_Type clks_cal;

  clks_cal.DataType = DATATYPE_DFT;
  clks_cal.DftSrc = AppIMPCfg.DftSrc;
  clks_cal.DataCount = 1L<<(AppIMPCfg.DftNum+2); /* 2^(DFTNUMBER+2) */
  clks_cal.ADCSinc2Osr = AppIMPCfg.ADCSinc2Osr;
  clks_cal.ADCSinc3Osr = AppIMPCfg.ADCSinc3Osr;
  clks_cal.ADCAvgNum = AppIMPCfg.ADCAvgNum;
  clks_cal.RatioSys2AdcClk = AppIMPCfg.SysClkFreq/AppIMPCfg.AdcClkFreq;
  AD5940_ClksCalculate(&clks_cal, &WaitClks);

  AD5940_SEQGenCtrl(bTRUE);
  AD5940_SEQGpioCtrlS(AGPIO_Pin2); /* Set GPIO1, clear others that under control */
  AD5940_SEQGenInsert(SEQ_WAIT(16*250));  /* @todo wait 250us? */
  sw_cfg.Dswitch = SWD_RCAL0;
  sw_cfg.Pswitch = SWP_RCAL0;
  sw_cfg.Nswitch = SWN_RCAL1;
  sw_cfg.Tswitch = SWT_RCAL1|SWT_TRTIA;
  AD5940_SWMatrixCfgS(&sw_cfg);
	
	AD5940_AFECtrlS(AFECTRL_HSTIAPWR|AFECTRL_INAMPPWR|AFECTRL_EXTBUFPWR|\
                AFECTRL_WG|AFECTRL_DACREFPWR|AFECTRL_HSDACPWR|\
                AFECTRL_SINC2NOTCH, bTRUE);
  AD5940_AFECtrlS(AFECTRL_WG|AFECTRL_ADCPWR, bTRUE);  /* Enable Waveform generator */
  //delay for signal settling DFT_WAIT
  AD5940_SEQGenInsert(SEQ_WAIT(16*10));
  AD5940_AFECtrlS(AFECTRL_ADCCNV|AFECTRL_DFT, bTRUE);  /* Start ADC convert and DFT */
	
	AD5940_SEQGenFetchSeq(NULL, &AppIMPCfg.SeqWaitAddr[0]); /* Record the start address of the next command. */

  AD5940_SEQGenInsert(SEQ_WAIT(WaitClks/2));
   AD5940_SEQGenInsert(SEQ_WAIT(WaitClks/2));
 
  //wait for first data ready
  AD5940_AFECtrlS(AFECTRL_ADCPWR|AFECTRL_ADCCNV|AFECTRL_DFT|AFECTRL_WG, bFALSE);  /* Stop ADC convert and DFT */

  /* Configure matrix for external Rz */
  sw_cfg.Dswitch = AppIMPCfg.DswitchSel;
  sw_cfg.Pswitch = AppIMPCfg.PswitchSel;
  sw_cfg.Nswitch = AppIMPCfg.NswitchSel;
  sw_cfg.Tswitch = SWT_TRTIA|AppIMPCfg.TswitchSel;
  AD5940_SWMatrixCfgS(&sw_cfg);
  AD5940_AFECtrlS(AFECTRL_ADCPWR|AFECTRL_WG, bTRUE);  /* Enable Waveform generator */
  AD5940_SEQGenInsert(SEQ_WAIT(16*10));  //delay for signal settling DFT_WAIT
  AD5940_AFECtrlS(AFECTRL_ADCCNV|AFECTRL_DFT, bTRUE);  /* Start ADC convert and DFT */
	
	AD5940_SEQGenFetchSeq(NULL, &AppIMPCfg.SeqWaitAddr[1]); /* Record the start address of next command */
       
  AD5940_SEQGenInsert(SEQ_WAIT(WaitClks/2));
   AD5940_SEQGenInsert(SEQ_WAIT(WaitClks/2));

  AD5940_AFECtrlS(AFECTRL_ADCCNV|AFECTRL_DFT|AFECTRL_WG|AFECTRL_ADCPWR, bFALSE);  /* Stop ADC convert and DFT */
    AD5940_AFECtrlS(AFECTRL_HSTIAPWR|AFECTRL_INAMPPWR|AFECTRL_EXTBUFPWR|\
                AFECTRL_WG|AFECTRL_DACREFPWR|AFECTRL_HSDACPWR|\
                AFECTRL_SINC2NOTCH, bFALSE);
  AD5940_SEQGpioCtrlS(0); /* Clr GPIO1 */

  AD5940_EnterSleepS();/* Goto hibernate */

  /* Sequence end. */
  error = AD5940_SEQGenFetchSeq(&pSeqCmd, &SeqLen);
  AD5940_SEQGenCtrl(bFALSE); /* Stop sequencer generator */

  if(error == AD5940ERR_OK)
  {
    AppIMPCfg.MeasureSeqInfo.SeqId = SEQID_0;
    AppIMPCfg.MeasureSeqInfo.SeqRamAddr = AppIMPCfg.InitSeqInfo.SeqRamAddr + AppIMPCfg.InitSeqInfo.SeqLen ;
    AppIMPCfg.MeasureSeqInfo.pSeqCmd = pSeqCmd;
    AppIMPCfg.MeasureSeqInfo.SeqLen = SeqLen;
    /* Write command to SRAM */
    AD5940_SEQCmdWrite(AppIMPCfg.MeasureSeqInfo.SeqRamAddr, pSeqCmd, SeqLen);
  }
  else
    return error; /* Error */
  return AD5940ERR_OK;
}

/* Depending on frequency of Sin wave set optimum filter settings */
AD5940Err AppIMPCheckFreq(float freq)
{
  ADCFilterCfg_Type filter_cfg;
  DFTCfg_Type dft_cfg;
  HSDACCfg_Type hsdac_cfg;
  uint32_t WaitClks;
  ClksCalInfo_Type clks_cal;
  FreqParams_Type freq_params;
  uint32_t SeqCmdBuff[32];
  uint32_t SRAMAddr = 0;;
  /* Step 1: Check Frequency */
  freq_params = AD5940_GetFreqParameters(freq);
  
       if(freq < 0.51)
	{
            /* Update HSDAC update rate */
    hsdac_cfg.ExcitBufGain =EXCITBUFGAIN_2;// AppIMPCfg.ExcitBufGain;
    hsdac_cfg.HsDacGain = HSDACGAIN_1;//AppIMPCfg.HsDacGain;
     hsdac_cfg.HsDacUpdateRate = 0x1B;
    AD5940_HSDacCfgS(&hsdac_cfg);
    AD5940_HSRTIACfgS(HSTIARTIA_40K);
    
    /*Update ADC rate */
    filter_cfg.ADCRate = ADCRATE_800KHZ;
    AppIMPCfg.AdcClkFreq = 16e6;
    
    /* Change clock to 16MHz oscillator */
    AD5940_HPModeEn(bFALSE);
	}
        else if(freq < 5 )
	{
       /* Update HSDAC update rate */
    hsdac_cfg.ExcitBufGain =EXCITBUFGAIN_2;// AppIMPCfg.ExcitBufGain;
    hsdac_cfg.HsDacGain = HSDACGAIN_1;//AppIMPCfg.HsDacGain;
    hsdac_cfg.HsDacUpdateRate = 0x1B;
    AD5940_HSDacCfgS(&hsdac_cfg);
    AD5940_HSRTIACfgS(HSTIARTIA_40K);
    
    /*Update ADC rate */
    filter_cfg.ADCRate = ADCRATE_800KHZ;
    AppIMPCfg.AdcClkFreq = 16e6;
    
    /* Change clock to 16MHz oscillator */
    AD5940_HPModeEn(bFALSE);
    
	}else if(freq < 450)
	{
       /* Update HSDAC update rate */
    hsdac_cfg.ExcitBufGain =AppIMPCfg.ExcitBufGain;
    hsdac_cfg.HsDacGain = AppIMPCfg.HsDacGain;  
    
     hsdac_cfg.HsDacUpdateRate = 0x1B;
    AD5940_HSDacCfgS(&hsdac_cfg);
    AD5940_HSRTIACfgS(HSTIARTIA_5K);
    
    /*Update ADC rate */
    filter_cfg.ADCRate = ADCRATE_800KHZ;
    AppIMPCfg.AdcClkFreq = 16e6;
    
    /* Change clock to 16MHz oscillator */
    AD5940_HPModeEn(bFALSE);
	}
       else if(freq<80000)
       {
           /* Update HSDAC update rate */
    hsdac_cfg.ExcitBufGain =AppIMPCfg.ExcitBufGain;
    hsdac_cfg.HsDacGain = AppIMPCfg.HsDacGain;
    hsdac_cfg.HsDacUpdateRate = 0x1B;
    AD5940_HSDacCfgS(&hsdac_cfg);
    AD5940_HSRTIACfgS(HSTIARTIA_5K);
    
    /*Update ADC rate */
    filter_cfg.ADCRate = ADCRATE_800KHZ;
    AppIMPCfg.AdcClkFreq = 16e6;
    
    /* Change clock to 16MHz oscillator */
    AD5940_HPModeEn(bFALSE);
       }
        /* High power mode */
	if(freq >= 80000)
	{
		  /* Update HSDAC update rate */
    hsdac_cfg.ExcitBufGain =AppIMPCfg.ExcitBufGain;
    hsdac_cfg.HsDacGain = AppIMPCfg.HsDacGain;
    hsdac_cfg.HsDacUpdateRate = 0x07;
    AD5940_HSDacCfgS(&hsdac_cfg);
    AD5940_HSRTIACfgS(HSTIARTIA_5K);
    
    /*Update ADC rate */
    filter_cfg.ADCRate = ADCRATE_1P6MHZ;
    AppIMPCfg.AdcClkFreq = 32e6;
    
    /* Change clock to 32MHz oscillator */
    AD5940_HPModeEn(bTRUE);
	}
  
  /* Step 2: Adjust ADCFILTERCON and DFTCON to set optimumn SINC3, SINC2 and DFTNUM settings  */
  filter_cfg.ADCAvgNum = ADCAVGNUM_16;  /* Don't care because it's disabled */ 
  filter_cfg.ADCSinc2Osr = freq_params.ADCSinc2Osr;
  filter_cfg.ADCSinc3Osr = freq_params.ADCSinc3Osr;
  filter_cfg.BpSinc3 = bFALSE;
  filter_cfg.BpNotch = bTRUE;
  filter_cfg.Sinc2NotchEnable = bTRUE;
  dft_cfg.DftNum = freq_params.DftNum;
  dft_cfg.DftSrc = freq_params.DftSrc;
  dft_cfg.HanWinEn = AppIMPCfg.HanWinEn;
  AD5940_ADCFilterCfgS(&filter_cfg);
  AD5940_DFTCfgS(&dft_cfg);
  
  /* Step 3: Calculate clocks needed to get result to FIFO and update sequencer wait command */
  clks_cal.DataType = DATATYPE_DFT;
  clks_cal.DftSrc = freq_params.DftSrc;
  clks_cal.DataCount = 1L<<(freq_params.DftNum+2); /* 2^(DFTNUMBER+2) */
  clks_cal.ADCSinc2Osr = freq_params.ADCSinc2Osr;
  clks_cal.ADCSinc3Osr = freq_params.ADCSinc3Osr;
  clks_cal.ADCAvgNum = 0;
  clks_cal.RatioSys2AdcClk = AppIMPCfg.SysClkFreq/AppIMPCfg.AdcClkFreq;
  AD5940_ClksCalculate(&clks_cal, &WaitClks);		
	
	
	  SRAMAddr = AppIMPCfg.MeasureSeqInfo.SeqRamAddr + AppIMPCfg.SeqWaitAddr[0];
	   
           SeqCmdBuff[0] =SEQ_WAIT(WaitClks/2);
           SeqCmdBuff[1] =SEQ_WAIT(WaitClks/2);
      
		AD5940_SEQCmdWrite(SRAMAddr, SeqCmdBuff, 2);
		
		SRAMAddr = AppIMPCfg.MeasureSeqInfo.SeqRamAddr + AppIMPCfg.SeqWaitAddr[1];
		  
           SeqCmdBuff[0] =SEQ_WAIT(WaitClks/2);
           SeqCmdBuff[1] =SEQ_WAIT(WaitClks/2);

		AD5940_SEQCmdWrite(SRAMAddr, SeqCmdBuff, 2);

		
//	/* Maximum number of clocks is 0x3FFFFFFF. More are needed if the frequency is low */
//	if(WaitClks > 0x3FFFFFFF)
//	{
//		WaitClks /=2;
//		SRAMAddr = AppIMPCfg.MeasureSeqInfo.SeqRamAddr;
//		SeqCmdBuff[0] = SEQ_WAIT(WaitClks);
//		AD5940_SEQCmdWrite(SRAMAddr+11, SeqCmdBuff, 1);
//		AD5940_SEQCmdWrite(SRAMAddr+12, SeqCmdBuff, 1);
//		AD5940_SEQCmdWrite(SRAMAddr+18, SeqCmdBuff, 1);
//		AD5940_SEQCmdWrite(SRAMAddr+19, SeqCmdBuff, 1);
//	}
//	else
//	{
//		SRAMAddr = AppIMPCfg.MeasureSeqInfo.SeqRamAddr;
//		SeqCmdBuff[0] = SEQ_WAIT(WaitClks);
//		AD5940_SEQCmdWrite(SRAMAddr+11, SeqCmdBuff, 1);
//		AD5940_SEQCmdWrite(SRAMAddr+18, SeqCmdBuff, 1);
//	}
 
  return AD5940ERR_OK;
}


/* This function provide application initialize. It can also enable Wupt that will automatically trigger sequence. Or it can configure  */
int32_t AppIMPInit(uint32_t *pBuffer, uint32_t BufferSize)
{
  AD5940Err error = AD5940ERR_OK;  
  SEQCfg_Type seq_cfg;
  FIFOCfg_Type fifo_cfg;

  if(AD5940_WakeUp(10) > 10)  /* Wakeup AFE by read register, read 10 times at most */
    return AD5940ERR_WAKEUP;  /* Wakeup Failed */

  /* Configure sequencer and stop it */
  seq_cfg.SeqMemSize = SEQMEMSIZE_2KB;  /* 2kB SRAM is used for sequencer, others for data FIFO */
  seq_cfg.SeqBreakEn = bFALSE;
  seq_cfg.SeqIgnoreEn = bTRUE;
  seq_cfg.SeqCntCRCClr = bTRUE;
  seq_cfg.SeqEnable = bFALSE;
  seq_cfg.SeqWrTimer = 0;
  AD5940_SEQCfg(&seq_cfg);
  
  
  /* Reconfigure FIFO */
  AD5940_FIFOCtrlS(FIFOSRC_DFT, bFALSE);									/* Disable FIFO firstly */
  fifo_cfg.FIFOEn = bTRUE;
  fifo_cfg.FIFOMode = FIFOMODE_FIFO;
  fifo_cfg.FIFOSize = FIFOSIZE_4KB;                       /* 4kB for FIFO, The reset 2kB for sequencer */
  fifo_cfg.FIFOSrc = FIFOSRC_DFT;
  fifo_cfg.FIFOThresh = AppIMPCfg.FifoThresh;              /* DFT result. One pair for RCAL, another for Rz. One DFT result have real part and imaginary part */
  AD5940_FIFOCfg(&fifo_cfg);
  AD5940_INTCClrFlag(AFEINTSRC_ALLINT);

  /* Start sequence generator */
  /* Initialize sequencer generator */
  if((AppIMPCfg.IMPInited == bFALSE)||\
       (AppIMPCfg.bParaChanged == bTRUE))
  {
    if(pBuffer == 0)  return AD5940ERR_PARA;
    if(BufferSize == 0) return AD5940ERR_PARA;   
    AD5940_SEQGenInit(pBuffer, BufferSize);

    /* Generate initialize sequence */
    error = AppIMPSeqCfgGen(); /* Application initialization sequence using either MCU or sequencer */
    if(error != AD5940ERR_OK) return error;

    /* Generate measurement sequence */
    error = AppIMPSeqMeasureGen();
    if(error != AD5940ERR_OK) return error;

    AppIMPCfg.bParaChanged = bFALSE; /* Clear this flag as we already implemented the new configuration */
  }

  /* Initialization sequencer  */
  AppIMPCfg.InitSeqInfo.WriteSRAM = bFALSE;
  AD5940_SEQInfoCfg(&AppIMPCfg.InitSeqInfo);
  seq_cfg.SeqEnable = bTRUE;
  AD5940_SEQCfg(&seq_cfg);  /* Enable sequencer */
  AD5940_SEQMmrTrig(AppIMPCfg.InitSeqInfo.SeqId);
  while(AD5940_INTCTestFlag(AFEINTC_1, AFEINTSRC_ENDSEQ) == bFALSE);
  AD5940_INTCClrFlag(AFEINTSRC_ALLINT);
  /* Measurement sequence  */
  AppIMPCfg.MeasureSeqInfo.WriteSRAM = bFALSE;
  AD5940_SEQInfoCfg(&AppIMPCfg.MeasureSeqInfo);
  
  AppIMPCheckFreq(AppIMPCfg.FreqofData);

  seq_cfg.SeqEnable = bTRUE;
  AD5940_SEQCfg(&seq_cfg);  /* Enable sequencer, and wait for trigger */
  AD5940_ClrMCUIntFlag();   /* Clear interrupt flag generated before */

  //AD5940_AFEPwrBW(AppIMPCfg.PwrMod, AFEBW_250KHZ);

  AppIMPCfg.IMPInited = bTRUE;  /* IMP application has been initialized. */
  return AD5940ERR_OK;
}

/* Modify registers when AFE wakeup */
int32_t AppIMPRegModify(int32_t * const pData, uint32_t *pDataCount)
{
  if(AppIMPCfg.NumOfData > 0)
  {
    AppIMPCfg.FifoDataCount += *pDataCount/4;
    if(AppIMPCfg.FifoDataCount >= AppIMPCfg.NumOfData)
    {
      AD5940_WUPTCtrl(bFALSE);
      return AD5940ERR_OK;
    }
  }
  if(AppIMPCfg.StopRequired == bTRUE)
  {
    AD5940_WUPTCtrl(bFALSE);
    return AD5940ERR_OK;
  }
  if(AppIMPCfg.SweepCfg.SweepEn) /* Need to set new frequency and set power mode */
  {
    AD5940_WGFreqCtrlS(AppIMPCfg.SweepNextFreq, AppIMPCfg.SysClkFreq);
		AppIMPCheckFreq(AppIMPCfg.SweepNextFreq);
  }
  return AD5940ERR_OK;
}

/* Depending on the data type, do appropriate data pre-process before return back to controller */
int32_t AppIMPDataProcess(int32_t * const pData, uint32_t *pDataCount)
{
  uint32_t DataCount = *pDataCount;
  uint32_t ImpResCount = DataCount/4;

  fImpPol_Type * const pOut = (fImpPol_Type*)pData;
  iImpCar_Type * pSrcData = (iImpCar_Type*)pData;

  *pDataCount = 0;

  DataCount = (DataCount/4)*4;/* We expect RCAL data together with Rz data. One DFT result has two data in FIFO, real part and imaginary part.  */

  /* Convert DFT result to int32_t type */
  for(uint32_t i=0; i<DataCount; i++)
  {
    pData[i] &= 0x3ffff; /* @todo option to check ECC */
    if(pData[i]&(1L<<17)) /* Bit17 is sign bit */
    {
      pData[i] |= 0xfffc0000; /* Data is 18bit in two's complement, bit17 is the sign bit */
    }
  }
  for(uint32_t i=0; i<ImpResCount; i++)
  {
    iImpCar_Type *pDftRcal, *pDftRz;

    pDftRcal = pSrcData++;
    pDftRz = pSrcData++;
    float RzMag,RzPhase;
    float RcalMag, RcalPhase;
    
    RcalMag = sqrt((float)pDftRcal->Real*pDftRcal->Real+(float)pDftRcal->Image*pDftRcal->Image);
    RcalPhase = atan2(-pDftRcal->Image,pDftRcal->Real);
    RzMag = sqrt((float)pDftRz->Real*pDftRz->Real+(float)pDftRz->Image*pDftRz->Image);
    RzPhase = atan2(-pDftRz->Image,pDftRz->Real);

    RzMag = RcalMag/RzMag*AppIMPCfg.RcalVal;
    RzPhase = RcalPhase - RzPhase;
    //printf("V:%d,%d,I:%d,%d ",pDftRcal->Real,pDftRcal->Image, pDftRz->Real, pDftRz->Image);
    
    pOut[i].Magnitude = RzMag;
    pOut[i].Phase = RzPhase;
  }
  *pDataCount = ImpResCount; 
  AppIMPCfg.FreqofData = AppIMPCfg.SweepCurrFreq;
  /* Calculate next frequency point */
  if(AppIMPCfg.SweepCfg.SweepEn == bTRUE)
  {
    AppIMPCfg.FreqofData = AppIMPCfg.SweepCurrFreq;
    AppIMPCfg.SweepCurrFreq = AppIMPCfg.SweepNextFreq;
    AD5940_SweepNext(&AppIMPCfg.SweepCfg, &AppIMPCfg.SweepNextFreq);
  }

  return 0;
}

/**

*/
int32_t AppIMPISR(void *pBuff, uint32_t *pCount)
{
  uint32_t BuffCount;
  uint32_t FifoCnt;
  BuffCount = *pCount;
  
  *pCount = 0;
  
  if(AD5940_WakeUp(10) > 10)  /* Wakeup AFE by read register, read 10 times at most */
    return AD5940ERR_WAKEUP;  /* Wakeup Failed */
  AD5940_SleepKeyCtrlS(SLPKEY_LOCK);  /* Prohibit AFE to enter sleep mode. */

  if(AD5940_INTCTestFlag(AFEINTC_0, AFEINTSRC_DATAFIFOTHRESH) == bTRUE)
  {
    /* Now there should be 4 data in FIFO */
    FifoCnt = (AD5940_FIFOGetCnt()/4)*4;
    
    if(FifoCnt > BuffCount)
    {
      ///@todo buffer is limited.
    }
    AD5940_FIFORd((uint32_t *)pBuff, FifoCnt);
    AD5940_INTCClrFlag(AFEINTSRC_DATAFIFOTHRESH);
    AppIMPRegModify(pBuff, &FifoCnt);   /* If there is need to do AFE re-configure, do it here when AFE is in active state */
    //AD5940_EnterSleepS(); /* Manually put AFE back to hibernate mode. This operation only takes effect when register value is ACTIVE previously */
    AD5940_SleepKeyCtrlS(SLPKEY_UNLOCK);  /* Allow AFE to enter sleep mode. */
    /* Process data */ 
    AppIMPDataProcess((int32_t*)pBuff,&FifoCnt); 
    *pCount = FifoCnt;
    return 0;
  }
  
  return 0;
}

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AD5941 optimising for low frequency Impedance measurements

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