Simultaneous ADC Sampling

Hi,
 
I am wishing to perform Simultaneous Sampling an two ADC Channels (AN0 and AN1) on a dsPIC33FJ128MC804. I have been through the datasheet and family reference guide to derive my ADC and DMA initialisation code. For test purposes I have attached AN0 to an incoming sine wave ~250KHz and AN1 to +3.3V (VREF+). I am implementing scatter/gather mode, but I am hoping to see BufferA contain AN0 data and BufferB the results for AN1 OR BufferA[0] = AN0 and BufferA[1] = AN1. However, I am seeing what looks like 3 samples of AN0 and one AN1, and so one across both buffers.
 
My Code is below:

  
 // Define Message Buffer Length for ECAN1/ECAN2
 #define  MAX_CHNUM     2  // Highest Analog input number in Channel Scan
 #define  SAMP_BUFF_SIZE    25  // Size of the input buffer per analog input
 #define  NUM_CHS2SCAN   2  // Number of channels enabled for channel scan
 // Number of locations for ADC buffer = 14 (AN0 to AN13) x 8 = 112 words
 // Align the buffer to 128 words or 256 bytes. This is needed for peripheral indirect mode
 int  BufferA[MAX_CHNUM][SAMP_BUFF_SIZE] __attribute__((space(dma),aligned(256)));
 int  BufferB[MAX_CHNUM][SAMP_BUFF_SIZE] __attribute__((space(dma),aligned(256)));
 void ProcessADCSamples(int * AdcBuffer);
 /*=============================================================================
 ADC Initialisation for Channel Scan 
 =============================================================================*/
 void initAdc1(void)
 {
  AD1CON1bits.FORM   = 3;  // Data Output Format: Signed Fraction (Q15 format)
  AD1CON1bits.SSRC   = 2;  // Sample Clock Source: GP Timer starts conversion
  AD1CON1bits.ASAM   = 1;  // ADC Sample Control: Sampling begins immediately after conversion
  AD1CON1bits.AD12B  = 0;  // 10-bit ADC operation
  AD1CON1bits.SIMSAM  = 1; // Sample Channels selected by CHPS simultaneously
  AD1CON2bits.ALTS    = 1;
  AD1CON2bits.BUFM    = 0;
  AD1CON2bits.CHPS    = 1; // Converts CH0 + CH1 Simultaneously
  AD1CON1bits.ADDMABM = 0;  // DMA buffers are built in scatter/gather mode
  AD1CON2bits.SMPI    = 1; //(NUM_CHS2SCAN-1); // 4 ADC Channel is scanned
  AD1CON4bits.DMABL   = 5; // Each buffer contains 32 words 
  AD1CON2bits.CSCNA = 0;  // NO Scan Input Selections as Simultaneous Sampling
  AD1CON3bits.ADRC = 0;  // ADC Clock is derived from Systems Clock
  AD1CON3bits.ADCS = 63;  // ADC Conversion Clock Tad=Tcy*(ADCS+1)= (1/40M)*64 = 1.6us (625Khz)
         // ADC Conversion Time for 10-bit Tc=12*Tab = 19.2us
  AD1CHS0bits.CH0SB = 1;  //AN1 => CH0 Input
  AD1CHS0bits.CH0NB = 0;  //VREF- for CH0- Input
  AD1CHS0bits.CH0SA = 0;  //AN0 => CH0 Input
  AD1CHS0bits.CH0NA = 0;  //VREF- for CH0- Input
  AD1CHS123bits.CH123NB = 0; //CH1,CH2,CH3 VREF-
  AD1CHS123bits.CH123SB = 0; //Positive Input Selection
  AD1CHS123bits.CH123NA = 0; //CH1,CH2,CH3 VREF-
  AD1CHS123bits.CH123SA = 0; //Positive Input Selection
  //AD1CSSH/AD1CSSL: A/D Input Scan Selection Register
  // AD1CSSH = 0x0000;   
  //AD1CSSLbits.CSS0=1;   // Enable AN0 for channel scan
  //AD1CSSLbits.CSS1=1;   // Enable AN1 for channel scan
  
   //AD1PCFGH/AD1PCFGL: Port Configuration Register
  AD1PCFGL=0xFFFF;
  //AD1PCFGH=0xFFFF;
  AD1PCFGLbits.PCFG0 = 0;  // AN0 as Analog Input
  AD1PCFGLbits.PCFG1 = 0;  // AN1 as Analog Input 
  
  IFS0bits.AD1IF   = 0;  // Clear the A/D interrupt flag bit
  IEC0bits.AD1IE   = 0;  // Do Not Enable A/D interrupt 
  AD1CON1bits.ADON = 1;  // Turn on the A/D converter
     tglPinInit();
 }
 /*=============================================================================  
 Timer 3 is setup to time-out every 125 microseconds (8Khz Rate). As a result, the module 
 will stop sampling and trigger a conversion on every Timer3 time-out, i.e., Ts=125us. 
 =============================================================================*/
 void initTmr3() 
 {
  TMR3 = 0x0000;
  PR3  = 40;    // Trigger ADC1 every 2usec
  IFS0bits.T3IF = 0;  // Clear Timer 3 interrupt
  IEC0bits.T3IE = 0;  // Disable Timer 3 interrupt
  T3CONbits.TON = 1;  //Start Timer 3
 }
 
 // DMA0 configuration
 // Direction: Read from peripheral address 0-x300 (ADC1BUF0) and write to DMA RAM 
 // AMODE: Peripheral Indirect Addressing Mode
 // MODE: Continuous, Ping-Pong Mode
 // IRQ: ADC Interrupt
 void initDma0(void)
 {
  DMA0CONbits.AMODE = 0;   // Configure DMA for Peripheral indirect mode
  DMA0CONbits.MODE  = 2;   // Configure DMA for Continuous Ping-Pong mode
  DMA0PAD=(int)&ADC1BUF0;
  DMA0CNT = (SAMP_BUFF_SIZE*NUM_CHS2SCAN)-1;     
  DMA0REQ = 13;     // Select ADC1 as DMA Request source
  DMA0STA = __builtin_dmaoffset(BufferA);  
  DMA0STB = __builtin_dmaoffset(BufferB);
  IFS0bits.DMA0IF = 0;   //Clear the DMA interrupt flag bit
     IEC0bits.DMA0IE = 1;   //Set the DMA interrupt enable bit
  DMA0CONbits.CHEN=1;    // Enable DMA
 }
 /*=============================================================================
 _DMA0Interrupt(): ISR name is chosen from the device linker script.
 =============================================================================*/
 unsigned int DmaBuffer = 0;
 void __attribute__((interrupt, no_auto_psv)) _DMA0Interrupt(void)
 {
  if(DmaBuffer == 0)
  {
   ProcessADCSamples(&BufferA[0][0]);
   ProcessADCSamples(&BufferA[1][0]);
 //  ProcessADCSamples(&BufferA[10][0]);
 //  ProcessADCSamples(&BufferA[13][0]);
  }
  else
  {
   ProcessADCSamples(&BufferB[0][0]);
   ProcessADCSamples(&BufferB[1][0]);
 //  ProcessADCSamples(&BufferB[10][0]);
 //  ProcessADCSamples(&BufferB[13][0]);
  }
  DmaBuffer ^= 1;
  tglPin();     // Toggle RA6 
  IFS0bits.DMA0IF = 0;  // Clear the DMA0 Interrupt Flag
 }
  
 

 
Attached is a screenshot of my DMA buffers in the watch window where you can see the pattern. Ultimately, both AN0 and AN1 will be sine waves and I wish to calculate their phase relationship.
 
Any thoughts are welcomed.
 
Regards,
Tom

Hi,
 
I am designing an impedance measuring device. I think my problem lies in the following section of channel selection
 

 AD1CHS0bits.CH0SB = 1;  //AN1 => CH0 Input
 AD1CHS0bits.CH0NB = 0;  //VREF- for CH0- Input
 AD1CHS0bits.CH0SA = 0;  //AN0 => CH0 Input
 AD1CHS0bits.CH0NA = 0;  //VREF- for CH0- Input
 AD1CHS123bits.CH123NB = 0; //CH1,CH2,CH3 VREF-
 AD1CHS123bits.CH123SB = 0; //Positive Input Selection
 AD1CHS123bits.CH123NA = 0; //CH1,CH2,CH3 VREF-
 AD1CHS123bits.CH123SA = 0; //Positive Input Selection
 
 

 
Regards,
Tom

I would still like any phase determing suggestions, between the two signals.

Hi Tom,
Have a look here: http://www.mysick.com/saqqara/get.aspx?id=IM0015190 page 37.
You would need to have a good reference for Sin and Cos offset levels (either digital or analogue.)

Also, this might be helpful: http://www.freescale.com/files/product/doc/AN1942.pdf

Hi,
 
I would be interested to see what you have tried and which particular areas you are struggling with. More over, you should read the data sheet and the family reference guide.
 
Regards,
Tom

Nice chip! $30 though? And it's only 180 degrees range.
There are lots of chips for phase measurement purpose but they are mostly designed for motor resolver designs. They are not cheap either.

Hi,
 
I think that you might want to invetigate the inclusion of the following:
 

  AD1CON2bits.CHPS    = 1;  // Converts CH0 + CH1 Simultaneously
  AD1CON2bits.CSCNA  = 0;  // NO Scan Input Selections as Simultaneous Sampling
  AD1CON3bits.ADRC    = 0;  // ADC Clock is derived from Systems Clock
  
  AD1CHS0bits.CH0SB  = 0;  //AN0 => CH0 Input MUX B
  AD1CHS0bits.CH0NB  = 0;  //VREF- for CH0- Input MUX B
  AD1CHS0bits.CH0SA  = 1;  //AN1 => CH0 Input MUX A
  AD1CHS0bits.CH0NA  = 0;  //VREF- for CH0- Input MUX A
  
  AD1CHS123bits.CH123NB = 0; //CH1,CH2,CH3 VREF-
  AD1CHS123bits.CH123SB = 0; //Positive Input Selection
  AD1CHS123bits.CH123NA = 0; //CH1,CH2,CH3 VREF-
  AD1CHS123bits.CH123SA = 0; //Positive Input Selection
 

 
Obvisously you will need to change these parameters to suit your application, but they are important to the complete initialisation of the ADC.
 
Let me know how you get on!
 
Regards,
Tom