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ADS1248 - RTD Application

Baha Burhanov
Prodigy 10 points
Other Parts Discussed in Thread: ADS1248

Hello,

I want to use two wire RTD measurement application with ADS1248, 

http://www.ti.com/europe/downloads/2-%203-%204-Wire%20RTD%20Measurement.pdf

I use following configuration :

ADS1248_MUX0_REG_VALUE (0xC7)
#define ADS1248_VBIAS_REG_VALUE (0x00) 
#define ADS1248_MUX1_REG_VALUE (0x30) 
#define ADS1248_SYS0_REG_VALUE (0x0F)
#define ADS1248_IDAC0_REG_VALUE (0x0C) 
#define ADS1248_IDAC1_REG_VALUE (0x0F)

with scope i see that voltage on 1k resistor is correct, but data that received in adc is wrong.

How i calculate the voltage ? and what is my internal reference (2.048V) ? what additional processed should i do for reading adc ?

Thank you very match .  

  • 0
    TI__Guru** 113765 points

    Hi Baha,

    Your register settings show that you have the burnout current source on, and you are using the internal reference voltage.  First of all turn off the burnout current when taking the measurement ( change to MUX0=0x07).  This additional current will alter the value of voltage dropped across the RTD.

    Secondly, if you have the Rbias resistor connected as shown in the presentation, you should select the reference so that your measurement becomes ratiometric.  In this way noise and drift will cancel.  The reference voltage will be the IDAC current times the value of the resistor.  This resistor should be of high quality and a precision resistor.  This would produce a 1V reference if using a 1k ohm resistor with 1mA of current (MUX1=x20).  if you continue to use the internal reference, then yes the reference voltage is 2.048V.  The measured voltage of the RTD will be the IDAC current times the RTD resistance for the current temperature.  One count (LSB) is VRef/PGA/(2^23 - 1) or about 244nV for a 2.048V reference (119nV for 1V reference.)  So to work it backwards you just solve Ohm's Law for resistance:

    RTD resistance = # of codes * (244nV)/1mA

    What was the actual code you are getting?

    Best regards,

    Bob B