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AD7792 with Thermocouple Question.

Question asked by piwdebar on Jun 27, 2013
Latest reply on Jul 2, 2013 by piwdebar

This my schematic of ADC Part 

ADC Channel 1  -  Thermocouple Type K

ADC Channel 2  -  PTC Thermistor 1k@25C (Cold Junction Compensation) with Exitation Current 1mA

ADC Channel 3  -  External Reference Voltage ( R 2k 0.1% ) with Exitation Current 1mA

ad7792_sch.JPG

and Value of Register in AD7792 in the initial , i have to read data of all register in my IC AD7792

Status Register  =  0x80

Mode Register  =  0x000A

Configuration Register  =  0x0710

Data Register  =  0x0000

ID Register  =  0x4A

IO Register  =  0x00

Offset Register  =  0x8000

Full Scale Register  =  0x5xxx  ; i can't remember xxx

 

Then i set IO Register = 0x03  to use Exitation Current at 1mA  for Vref at 2V  and  Voltage at PTC Thermistor 1k@25C ( 1V at 25C)

and set ADC Channel 1 Configuration Register         

VBIAS1 = 0, VBIAS0 = 1        // use vbias ch 1

BO = 1                                  // use burnout current

U/B = 1                                 // unipolar data

BOOST = 1                         

G2 = 1, G1 = 1, G0 = 0   //  gain = 64

REFSEL = 0                    // external ref

BUF = 1

CH2 = 0, CH1 = 0, CH0 = 0          // Select AIN1+ , AIN1-

 

and set ADC Channel 2 Configuration Register         

VBIAS1 = 0, VBIAS0 = 0        // vbias disable

BO = 0                                  // burnout current disable

U/B = 1                                 // unipolar data

BOOST = 0                        

G2 = 0, G1 = 0, G0 = 0   //  gain = 1

REFSEL = 0                    // external ref

BUF = 0

CH2 = 0, CH1 = 0, CH0 = 1          // Select AIN2+ , AIN2-

 

I have to read data in Singel Conversion Mode

 

From Thermocouple Ch1 data (Room Temperature about 31 - 32 C) =   796  (integer)

if short circuit Thermocouple  data = 764

if open circuit Thermocouple  data = 65535 or  0xFFFF

 

From PTC Thermistor Ch2 data (Room Temperature about 31 - 32 )  =  33652   ( This data i think it's right)

 

My question is thermocouple data it's right ?  becase when change value 796 to voltage  =  379.562 uV  in the ITS-90 table equal about 9.52 C

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