Electronic – AD5933 is not working with external clock

I am using AD5933 impedance measurement chip. There is an option to connect an external clock. I am using a function generator to give 250 kHz, 2-volts peak square wave signal to MCLK pin, but it's not working.

The communication (I2C) between controller and AD5933 is good.

(a) I have used the internal oscillator and I can measure impedance from 5 kHz to 100 kHz frequency range. I have measured different resistor and capacitor combinations using the internal oscillator. Now, I decided to measure impedance below the 5 kHz frequency range and according to the UG-364 application note, we have to use an external oscillator to apply a clock frequency of 250 kHz to use the frequency range below 5 kHz.

(b) I used a function generator to apply an external clock frequency shown in the attached picture, but the AD5933 is not responding. In the code, you can see the while loop, which is waiting for valid real and imaginary value by reading the flag set (2nd bit) in the status register of the AD5933. The 2nd bit of the status register does not go high, which means the AD5933 is not getting valid real and imaginary values.

(c) Schematic picture of my circuit is below.

(d) I am using 3.3 VDC and my output excitation range is 1 which means output excitation voltage is 1.98 Vp-p with DC offset 1.48 V.

Please share your experience of how can I connect the external clock to the AD5933 MCLK pin.

This is my code sequence:

1. Standby mode

2. Enable external oscillator

3. Initialize sweep

4. Start sweep

This is my code:

#include "Wire.h"

#define SLAVE_ADD 0x0D
#define ADD_PTR 0xB0

#define START_FREQ_REG1 0x82
#define START_FREQ_REG2 0x83
#define START_FREQ_REG3 0x84

#define FREG_INCR_REG1 0x85
#define FREG_INCR_REG2 0x86
#define FREG_INCR_REG3 0x87

#define NUM_INCR_REG1 0x88
#define NUM_INCR_REG2 0x89

#define CYCLE_REG1 0x8A 
#define CYCLE_REG2 0x8B 

#define REAL_REG1 0x94
#define REAL_REG2 0x95

#define IMG_REG1 0x96
#define IMG_REG2 0x97


#define CRL_REG 0x80
#define CRL_REG1 0x81
#define STATUS_REG 0x8F


void programReg();
void runSweep();
void writeData(int addr, int data);
byte getFrequency(float freq, int n);
int readData(int addr);

const float MCLK = 250*pow(10,3); // AD5933 Internal Clock Speed 16.776 MHz for formula
const float start_freq = 100; // Set start freq, < 100Khz (for formula)
const float incre_freq = 10; // Set freq increment (for formula)
const int incre_num = 10; // Set number of increments; < 511

byte value;
 int count=0;
char state; 
double gain;
const double pi = 3.141592654;


void setup() {

    Wire.begin();
    Serial.begin(115200);


  //nop - clear ctrl-reg
 writeData(CRL_REG,0x0);



    programReg();
}


void loop(){

         if(Serial.available()>0) {
     state = Serial.read();


      switch(state) {
        case 'A':  
          programReg();
          break;

        case 'B':
          runSweep();

          break;




      }
    }

}




void programReg(){



    // Start frequency of 1kHz
    writeData(START_FREQ_REG1, getFrequency(start_freq,1));
    writeData(START_FREQ_REG2, getFrequency(start_freq,2));
    writeData(START_FREQ_REG3, getFrequency(start_freq,3));


    // Increment by 1 kHz
    writeData(FREG_INCR_REG1, getFrequency(incre_freq,1)); 
    writeData(FREG_INCR_REG2, getFrequency(incre_freq,2)); 
    writeData(FREG_INCR_REG3, getFrequency(incre_freq,3));




    // Points in frequency sweep (150), max 511
    //writeData(NUM_INCR_REG1,0x00 );
    //writeData(NUM_INCR_REG2, 0x96);

 writeData(NUM_INCR_REG1, (incre_num & 0x001F00)>>0x08 );
 writeData(NUM_INCR_REG2, (incre_num & 0x0000FF));

  // Set settling cycles
  writeData(CYCLE_REG1, 0x00);
  writeData(CYCLE_REG2, 0x64);


}


void runSweep() {
    short re=0;
    short img=0;
    double freq=0;
  double mag=0;
  double  MAG=0;
  double phase=0;

  double impedance=0;

    int i=0;

  programReg();

    // 1. Standby '10110001'
    writeData(CRL_REG, 0xB0);




 // Enable external oscillator
 writeData(CRL_REG1, 0x8);




    // 2. Initialize sweep '00010001'
writeData(CRL_REG, 0x11);

     delay(20);

    // 3. Start sweep '00100000'
    writeData(CRL_REG, 0x21);   



while((readData(STATUS_REG) & 0x07) < 4 ) {  // Check that status reg != 4, sweep not complete

        delay(20); // delay between measurements

        int flag = readData(STATUS_REG)& 2;

        //Serial.println("");
        //Serial.println(readData(STATUS_REG));


        if (flag==2)

        {


            byte R1 = readData(REAL_REG1);
            byte R2 = readData(REAL_REG2);
            re = (R1 << 8) | R2;

            R1  = readData(IMG_REG1);
            R2  = readData(IMG_REG2);
            img = (R1 << 8) | R2;

            freq = start_freq + i*incre_freq;




     Serial.print(freq);
      Serial.print("\t");






        mag = sqrt(pow(double(re),2)+pow(double(img),2));


        if (count==0)
      {
       gain=1.0/(mag*34000);

       count=1;
      }


      if (count!=0)
      {
       impedance =1.0/(gain*mag);

       Serial.println(impedance);



        }


            //Increment frequency
            if((readData(STATUS_REG) & 0x07) < 4 ){
     // if((readData(STATUS_REG) & 0x04) == 0 ){
                writeData(CRL_REG,0x31);

       // writeData(CRL_REG,(readData(CRL_REG) | 0x01) | 0x30);
                i++;
      }
        }
             else {
      //Power down

   writeData(CRL_REG,0xA0);
  //writeData(CRL_REG,(readData(CRL_REG) & 0x07) | 0xA0);
             }
     p++;
        ///}
}
}










void writeData(int addr, int data) {

 Wire.beginTransmission(SLAVE_ADD);
 Wire.write(addr);
 Wire.write(data);
 Wire.endTransmission();
 delay(1);
}


int readData(int addr){
    int data;

    Wire.beginTransmission(SLAVE_ADD);
    Wire.write(ADD_PTR);
    Wire.write(addr);
    Wire.endTransmission();
  delay(1);

    Wire.requestFrom(SLAVE_ADD,1);

    if (Wire.available() >= 1){
        data = Wire.read();
    }
    else {
        data = -1;
    }

    delay(1);
    return data;    
}


byte getFrequency(float freq, int n){
    long val = long((freq/(MCLK/4)) * pow(2,27));
    byte code;

      switch (n) {
        case 1:
          code = (val & 0xFF0000) >> 0x10; 
          break;

        case 2:
          code = (val & 0x00FF00) >> 0x08;
          break;

        case 3:
          code = (val & 0x0000FF);
          break;

        default: 
          code = 0;
      }

    return code;  
}

double phase_sweep(double img, double re)
{
  double phase;
  if ((re>0)&(img>0))
  {
    phase = atan(double(img)/double(re));
    phase = (phase*180)/pi;  
  }
    else if((re>0)&(img<0))
    {
    phase = atan(double(img)/double(re));
    phase = (phase*180)/pi+360;  
    }
    else if ((re<0)&(img<0))
    {
      phase = -pi + atan(double(img)/double(re));
      phase = (phase*180)/pi;
    }
    else if ((re<0)&(img>0))
    {
      phase = pi + atan(double(img)/double(re));
      phase = (phase*180)/pi;
    }

    return phase;
  }

Please help me make my AD5933 run with an external oscillator.