• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 45 subscribers
  • Views 638 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS1248 calculation

Lelumpolelum
Prodigy 100 points
Other Parts Discussed in Thread: ADS1248

Hi,

Could you check my algorythm. In my application im measuring temperature using RTD.

First: measure Vref (voltage on Rref 820Ohm on my board) which is about 2460mV.

Vref=(2048*ADC_result)/0x7f ff ff

Second : Use measured voltage to calculate proper voltage value for RTD.

Vrtd=(Vref * ADC_result)/0x7f ff ff

If i dont measure Vref and write to Vref Variable 2460 (Rref  in my board is 820Ohm, 820*3mA =2460mV) it works fine,  but if i do like i show above i get error Voltage from ADC

for 0'=100Ohm i get 148mV ( proper value is 150mV). I get 150mV when i write Vref as a constant (2460).

 

  • 0
    TI__Guru** 113765 points

    Lelumpolelum,

    If you are using the constant current source to derive your reference voltage (ratiometric measurement) then the calculation is more simpler...which is just your second equation.  This makes somewhat of an assumption....you need to use a high grade reference resistor that is precision (1% or better.)  If you use a 0.5% resistor and you know the current you can calculate the reference voltage by Ohm's law and the resistor value will be 820 +/- 4.1. 

    If you are using the internal reference to make your measurement, then again you only need the second equation as your 820 ohm resistor in only used for biasing up the ADC input to within the common mode input range.  The 820 ohm resistor has no direct effect on the input value as compared to the reference.

    So, which reference are you using?

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 100 points

    If i understand you correctly. My english is quite poor. To measure RTD (3-wire) Voltage im using as a reference voltage which is on 820Ohm resistor (1%). But my application would work in different temperature for example it must work in high temperature 50' and in low temperature -10' ,reference resistor in different temperature would change his resistance. So im currently  struggling with this issue. To do RTD voltage measurement temperature independent, first i measure voltage on 820 resistor using internal reference which is 2.048 (capacitor 47U). I get error value, around 2430mV (2430 / 3mA=810 Ohm)should be at least 2435.4 mV  (2435.4 /3 = 811.8Ohm). Next i want save this measured voltage and use this variable as a reference voltage for RTD measurement. Maybe IDAC is not so accurate. I strongly believe that this current source from IDAC is exactly 1.5mA  :).

    Thanks for fast replay.

    Best regards

    Tomek

  • 0 in reply to Lelumpolelum
    TI__Guru** 113765 points

    Lelumpolelum,

    Ok, I now understand what you are trying to do.  Basically you cannot get an accurate reading for the 820 ohm resistor, as the measurement is outside the common mode input range of the ADS1248 (the device cannot measure to ground unless it is using bi-polar supplies.)

    So the actual measurement is a ratio of the two resistances where the ADC code result is equal to the RTD resistance/820 *7fffff.  So you need to know the value of one of the resistances by either measuring the reference resistor (820 ohm) or by calculating the resistance by using a calibrated temperature and using the ADC code value to calculate the value of the reference resistor.

    As far as the reference resistor itself, make sure that it is has a very low temperature coefficient.  You probably want to use something like a 0.5% or, better yet, 0.1%.  For example, look for a 820 ohm resistor that is 0.1% and 25ppm per degree C.

    Also remember that the RTD is only linear for a very small range so you will need to calculate for non-linearities as well.

    Best regards,

    Bob B