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ADS1248 / burnout detection and internal MUX on resistance

Ryuji
Guru 20090 points

Hello,

Can we use burnout detection at the same time as measurement the thermocouple ?
If yes, could you please let me know the on resistance of internal MUX?
I would like to calculate the drop voltage by burnout current source via the internal MUX.

Best Regards,
Ryuji Asaka 

  • 0
    TI__Guru* 92715 points

    Ryuji-san


    I wouldn't use the burnout detection at the same time as a thermocouple measurement. Thermocouple measurement is usually a small voltage and requires significant PGA gain. I would guess (as you are guessing), that the burnout current source will give an error reacting with the mux resistance. Instead of using the burnout sources at the same time as the thermocouple measurement, I would make a burnout measurement periodically.

    I will ask, but I don't have any information on the resistance for the internal mux. Like other transmission gates, I would assume that the resistance depends on the input voltage.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Guru 20090 points
    Hello Joseph san,

    Thank you for your reply. I understood.
    If you get the resistance for internal mux, please let me know.

    Best Regards,
    Ryuji Asaka
  • 0 in reply to Ryuji
    TI__Guru* 92715 points
    Ryuji-san,


    I was able to look at this on one of the EVMs and tested the error caused by the BCS reacting with the input mux. I would guess that the mux resistance is about 400 Ohms.

    I set up the input bias to be about AVDD/2 and short the inputs. Then I check the input reading from the ADC with the BCS off, then at 0.5uA, 2uA, and 10uA. It is linear depending on the BCS value, but at 10uA, the total error is about 8mV, so for each channel, this becomes 4mV of error with a 10uA BCS or about 400 Ohms.

    There are two other things to consider. Remember that these currents are not calibrated or trimmed and they could be off by a significant percentage. Also, this resistance probably depends a little on the input voltage and temperature. Just understand that this measurement is approximate with a significant error.

    Since the BCS is a single control (not set up for each individual input channel), it is placed between the input mux and the ADC. Note that the excitation current sources are routed outside the mux and ADC and do not react with this resistance.


    Joseph Wu