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ADS1220 - Internal Temperature Sensor

Carlos Saravia Jurado
Prodigy 20 points
Other Parts Discussed in Thread: ADS1220

Dear Sirs,
I have a question about the internal temperature sensor of ADS1220. I am measuring data from a thermocouple. I want to read the data from thermocouple and then take the temperature from the internal sensor to make the cold junction compensation. Here comes my query, I have to wait until the next cycle or I can read the data from the temperature sensor in the same cycle. For example, I am using the sample rate at 20 SPS. So, do I have to read the thermocouple data and wait 50 ms and read the temperature from the internal sensor to make the CJC? or I can read them at the same cycle?.

Thanks,

Carlos Saravia Jurado

PS. Sorry for my english.

  • 0
    TI__Guru** 113765 points

    Hi Carlos,

    The TC is one measurement and the internal temperature sensor is another.  When you change from the TC measurement to the internal temperature sensor measurement you will restart the conversion, so the additional time will start after the the last SCLK falling edge of the internal mux change. So yes, these are two seperate measurements.

    One thing you can do to speed up the process is to change the register value(s) at the same time as you are reading the data (full-duplex communication).  For the internal temperature sensor you may find that you get just as good results by increasing the data rate to a higher speed.  for example, you can change the speed from 20sps to 1000sps when you change from the TC measurement to the internal temperature sensor measurement.  When the internal sensor has finished you can then change back to the TC and also reduce the speed back to 20sps.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 130 points
    I would like to add that while the information in Bob B reply is correct, my guess is that the internal temperature sensor reading is a single-shot, thus changing the SPS will not make a difference, especially if no averaging is performed. (The manual is unclear whether there is such internal averaging).
    There is normally no need to measure the reference per each TC reading (20SPS), as the ref does not change momentarily. A separate reading per one second seems more than sufficient.
    At 20 SPS (both TC and ref), a full reading cycle dose not take more than 300 micros (including internal set delays), there is sufficient time between TC readings to perform reference reading at any time or rate.
    My experience shows that the measured internal temperature changes very slowly (warming device) and is very stable within many seconds, thus I found no reason for fast and multiple readings and averaging.

    Samtal
  • 0 in reply to Samuel Tal
    TI__Guru** 113765 points

    Hi Samtal,

    The temperature sensor is converted in the same way as the analog inputs, so data rate does matter with respect to the amount of digital filtering.  However, as the temperature sensor has only 14-bit performance you are not likely to see much issue relative to noise with respect to higher data rates as compared to the lower data rates.

    Best regards,

    Bob B