• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 79 subscribers
  • Views 690 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

RTD interfacing with MSP430F6721

ajimenez
Prodigy 120 points
Other Parts Discussed in Thread: MSP430F6721

I have seen in the forums that almost everybody uses an external IC like ADS124X or LMP90XXX to interface to the RTDs (using a constant current source passing through the RTD). I would like to use the SD24_B(24 bits ADC) of the MSP430F6721 microcontroller to read the voltage in the RTD making a voltage divider with the RTD and a 0,1% precision resistor. I have tried it in an evaluation board and it works well but I am afraid I won't arrive to an enough precision. I wait for tsuggestions before making the PCB. I need a very good precision because is for a heat meter.

Thanks

  • 0
    Guru 227245 points
    You don't provide enough information to give an advice.
    You know your RTD, you know the current, you know the temperature range, so you know the expected voltage range and the linearity (or lack of) and therefore where in the temperature range you will get which resolution and whether this will be sufficient.
    We don't.
    You may provide this information, you may as well do the calculations yourself (which is faster than waiting for one of us doing it for you - it should be not too difficult for you. Simple electronics and therefore something you should know if you started an electronics project)

    BTW, if you have a constant current passing through the RTD, then the resulting voltage is Rrtd*I, no mater how large (or what precision) any series resistor in a "voltage divider" might have. voltage dividers are for fixed voltage. On fixed current, you have independent voltages. Also, you can't use a voltage divider to split the voltage over the RTD, as then you'll have a partial current directed around the RTD (which makes the idea of a constant current fruitless) and leaves you with a very complex calculation.

    Not to mention that temperature is an average of the molecular movement energy. So below a certain point, precision is futile. It depends on the mechanical features of the sensor and how it is connected to the media you want to measure. If the voltage range is properly chosen, then the SD24 should be more than sufficient. But only you can know.
  • 0
    Prodigy 120 points
    I am going to use two JUMO 902425/10-815-1004-1-5-26-114-04-1500/340 heat meter RTD temperature probes. I will use a 3,6V battery to power 2 voltage dividers formed with a 0,1% 6,8K resistor in 3,6V and a probe in GND, the voltage reference will be another voltage divider powered by the same battery and formed with a 0,1% 6,8K resistor in GND and a 0,1% 10K resistor in 3,6V. The temperature range will be 0-100ºC
  • 0 in reply to ajimenez
    Intellectual 565 points

    Hi,

    The best reference I have seen for how to interface and linearize RTD's is Analog Devices AN-709. They show how to generate co-coefficients, for piece-wise interpolation, and calculate the error budget. TI's IQmath library makes the math easily do-able.

    Regards,

    Corky

**Attention** This is a public forum