How to return the conversion result from a ADS8317, need C code assistance

Hello Laurent,

The full-scale input range of the ADC is set by the reference voltage you are using. In particular, -Vref <= Vin <= +Vref. The corresponding output range is -32767 <= output code <= +32767. For your reference, table 1 and figure 44 of the datasheet provide clear illustrations of how the ADC input voltage range maps to its output code range.

With regard to your particular case, VIN = IN+ - IN- = 2.41V. If you are using a 2.5V reference voltage, notice that your input is closer to the positive end of the input range (+2.5V) than the negative end (-2.5V). So you should expect an output that is closer to the positive end of the output range as well, i.e, something close to +32767 (0x7FFF).

In order to calculate the exact output code, the "LSB size" of the ADC needs to be computed as follows:

LSBsize = full-scale input range/total number of output codes = (2.5V - (-2.5V))/2^16 = 76uV/code

Therefore, the output code for a 2.41V input should ideally be int(2.41V/76uV) = +31588 (0x7B64). From your plot, it appears that the ADC output is 0x671C (+26396) (considering MISO over 16 rising SCK edges starting with the 7th rising edge that occurs after the falling edge of /CS). The output error is over 5000 codes which is much larger than the max combined offset error, gain error, INL and DNL of the part (which is not more than 50 LSB). Please make sure you are operating the ADC according to datasheet recommendations and that your input and reference sources are precise and remain stable even during conversion.

Best Regards,
Harsha

Hello,

I use the following schematic and SPI code below :

The voltage reference used is +3.00V

I consider the following calculation : V+ = Vpress = ( ADC / 32768 ) x Vref

I think i could have made a mistake in the code source because i can't get a correct returned ADC value.

When sensor outputs 2.416V, i should expect to get ADC value =(32768*2.416) / 3.00 = 26389

The SPI code returns an ADC result about 401 ADC steps less than expected.

Would you please confirm the following way to return ADC value is correct according to my schematic ?

Many thanks for your help and support.

unsigned int ADS8317_read(void)
{
        unsigned char SPICON1Save;
	unsigned char SPISTATSave;

	ADS8317_CS_IO = 0;        				// Enable CS
	ADS8317_SPI_IF = 0;       				// Clear SPI Flag

	/*********************************/
	ADS8317_SSPBUF = 0x00;					// Send dummy value (0x00) in order to get the FIRST 8 bit byte
	while(!ADS8317_SPI_IF);ADS8317_SPI_IF = 0;              // Wait until data is shifted out
	byte1 = ADS8317_SSPBUF;
	/*********************************/

	/*********************************/
	ADS8317_SSPBUF = 0x00;					// Send dummy value (0x00) in order to get the FIRST 8 bit byte
	while(!ADS8317_SPI_IF);ADS8317_SPI_IF = 0;              // Wait until data is shifted out
	byte2 = ADS8317_SSPBUF;
	/*********************************/

        /*********************************/
	ADS8317_SSPBUF = 0x00;					// Send dummy value (0x00) in order to get the FIRST 8 bit byte
	while(!ADS8317_SPI_IF);ADS8317_SPI_IF = 0;              // Wait until data is shifted out
	byte3 = ADS8317_SSPBUF;
	/*********************************/

	ADS8317_CS_IO = 1;        				// Disable CS
	ADS8317_SPIEN = 0;					// Disable the interface


        ADC_result=(byte1&0x03)*65536UL;           // masked for two LSBs only
        ADC_result|=byte2*256UL;
        ADC_result|=byte3;
        ADC_result>>=2;                                         // Final shift drops unwanted two LSbs of third byte

	return ADC_result;
}