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ADS112C04: Measuring 3 thermocouples with 4 ADC inputs

Marvin Gasis Arcangel
Expert 8990 points
Part Number: ADS112C04

Hi Experts,

Our customer is requesting that we check their ADS112C04 schematic, kindly refer to the details below:

The customer have a space constrained application where they are measuring 3 K-thermocouples and would like to use only one ADS112C04 ADC.

For this design, all thermocouple junctions are electrically isolated. The internal temperature sensor is used to measure the cold junction temperature and then the respective inputs are selected and each thermocouple is sampled using the internal reference and 90sps conversion rate. The overall sample rate will range between 1 and 10 Hz.

Two 1M resistors are used to create the mid-supply reference connected to pin AIN3. The negative leads of each thermocouple are connected to AIN3 through a 1k resistor; the positive leads are connected through another 1k resistor to the remaining AIN0, -1, & -2 input channels. 0.1uF ceramic capacitors are connected between AIN3 and the 3 other ADC input channels. The BCS are not currently used for open-circuit detection.

DVDD and AVDD are at 2.8V. DVDD is bypassed with only a 1uF ceramic capacitor and AVDD is filtered with a 1k/100MHz ferrite and 1uF/10nF ceramic capacitors.

Can you please advise if there will be any potential issues with this configuration? Interference between two thermocouple channels?

Kindly refer to the schematic below:

Thanks for the help.

Regards,

Marvin

  • 0
    TI__Guru** 112895 points

    Hi Marvin,

    Generally the pullup/pulldown (R11/R12) would be placed outside of the input filter.  Also, the ferrite on the analog supply could cause startup issues by choking high current demands.  If filtering on the supply is necessary, we suggest using a low Ohm (in the range of 1 to 10 Ohms) RC filter instead.

    There is the small potential for some crosstalk as it relates to noise as each of the thermocouples are AC coupled with the capacitor.  Where this might be a problem is if long unshielded thermocouple wires are used that can pick up EMI/RFI.  This may or may not be a significant issue in in principle this circuit should work.

    Best regards,

    Bob B