ADS8684: ADC reading value keep changes on every scan

Rakesh Patel1

Part Number: ADS8684

Hello,

I am using ADS8684 with dspic33 microcontroller its integrated with channel 0, when I scan channel 0 the ADC values keep changes even if voltage remain same.

My code is as per below:

void adcInit()

{
  adcCmdreg(MAN_Ch_0);
}



void main()

{

uint16_t result = 0;
UINT8 MSB = 0;
UINT8 LSB = 0;

while(1)

{

adcCS_Low();
SPIPut(2,reg);
SPIPut(2,0);
MSB = SPIPut(2,0);
LSB = SPIPut(2,0);
adcCS_High();
result = ( MSB << 8) | LSB;

}

}

Any Help.

Thanks

Rakesh

over 5 years ago

0 Dale Li over 5 years ago

TI__Mastermind 49575 points

Hi Rakesh,

Can you please provide a few oscilloscope plots for CS, SCLK, SDO and SDI? This will allow us to verify the timing. Please place the probes right at the pins of the device.

Also, can you please measure the voltage on the REFIO and REFCAP pin?

If you have a schematic available, we could review as well.

Thanks&regards,

Dale

0 Rakesh Patel1 over 5 years ago in reply to Dale Li

Prodigy 50 points

Thank You Dale,

Schematic as per below attached.

REFIO PIN and REFCAP both have voltage: 4.48V

I don't have oscilloscope but have Logic analyzer can send data soon.

0 Rakesh Patel1 over 5 years ago in reply to Rakesh Patel1

Prodigy 50 points

Hello Dale,

I am able to write and read the command values now.. after turning the SPI mode to 0.

But still there is no consistency in repeat result of ADC.

My voltage between PIN 16 and 17 is 1.78Volt and ADC gives results as per below: sometime its give value 0 which is strange.

Even I change the PIn 16 and 17 voltage to 0.7Volt, it gives me same below result.

0x7FF1
0x7FF6
0x8F4C
0x8004
0x8002
0x8005
0x8037
0x8054
0x7FF0
0x7FF7
0x0000
0x90A2
0x8006
0x8002
0x8005
0x8037
0x8053
0x7FF0
0x7FF6
0x0000
0x8003
0x8001
0x8005
0x8037
0x8054
0x7FF1

My code is as per below

void main()
{
  adcInit();
  while(1)
   {
      adread();
   }
}


uint16_t  adcCmdreg(UINT8 reg)
{
    uint16_t  result = 0;
    UINT8 MSB = 0;
    UINT8 LSB = 0;
    
    adcCS_Low();
    SPIPut(2,reg);
    SPIPut(2,0);
    MSB = SPIPut(2,0);
    LSB = SPIPut(2,0);
    adcCS_High();
    
    result = ( MSB << 8) | LSB;
}
uint16_t adcReadReg(UINT8 reg)
{
    uint16_t  result = 0;
    UINT8 MSB = 0;
    UINT8 LSB = 0;
    
    adcCS_Low();
    SPIPut(2,((reg << 1) | 0x00));
    SPIPut(2,0);
    MSB = SPIPut(2,0);
    LSB = SPIPut(2,0);
    adcCS_High();
   
    result = MSB<<8;
    result |= LSB;
    
    return result;
}

void adcInit()
{
    adcCmdreg(MAN_Ch_0);
    __delay32(10000);
    adcCmdreg(AUTO_RST);
    __delay32(10000);
}

void adread(void)
{
    UINT16 adcData;
    __delay32(10000);
    adcData = adcReadReg(NO_OP);
    __delay32(10000);
}

0 Dale Li over 5 years ago in reply to Rakesh Patel1

TI__Mastermind 49575 points

Hi Rakesh,

The 4.48V on the REFIO and REFCAP is incorrect, the voltage on these pins should be 4.096V, please use a precision multimeter to double check.

It's really hard to see your schematic because it's not clear. However, a couple of errors have been found in your circuit:

1. Daisy pin should be connected to the ground instead of VCC_5V.

If an application uses a stand-alone device, the DAISY pin is connected to GND. see the Figure 70 and section 8.4.1.1.5 in the datasheet.

2. ADS8684 can only accept single-ended input signal and the AIN_xGND pin should be connected to the ground. You are connecting a differential signal to the ADC's input.

3. Pin 1(SDI) and pin 2(/RST/PD) should be connected to DVDD (VCC_3V3) if a pull-up is expected, not VCC_5V.

4. A solid ground plane is strongly recommended, and a single point connection between SGND and GND is not recommended.

Best regards,

Dale

0 Rakesh Patel1 over 5 years ago in reply to Dale Li

Prodigy 50 points

Thank You Dale,

I did change as per your suggestion.. I applied external voltage at AIN_0P and AIN_xGND.. when vary the voltage i did get the ADC count..

ADC has to be read as per below code. rest initialization will be as per previous code

void adread(void)
{
adcData = adcCmdreg(MAN_Ch_0);
__delay32(10000);
adcData = adcReadReg(NO_OP);
__delay32(10000);
}

Thanks for your support Dale.

Rakesh P

Data converters

Data converters forum

ADS8684: ADC reading value keep changes on every scan