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ADS1235EVM: Register setup for two differential channels

Tony Boehm
Prodigy 40 points
Part Number: ADS1235EVM
Other Parts Discussed in Thread: ADS1235, ADS1261

I've been using the ADS1235EVM kit successfully with AC Excitation to a load cell with sense lines connected. I have also been able to run a separate instance of the application with a Thermistor connected without issue. The thermistor wired for a single ended measurement with the  VCC connected to the 5V through hole on the board, GND connected to GND on J4 and the Thermistor circuit voltage fed to AIN3/AIN1 pin of J2.  If possible I'd like to read both the load cell and the thermistor values from the same evaluation kit simultaneously. I am willing to use AC Excitation for the Thermistor and convert the output to a differential signal if necessary but I haven't seen a way to setup the evaluation kit with two differential inputs. Is it possible to run the ADS1235EVM with a differential and single ended measurement (it doesn't look like it from the data sheet). Alterntatively, can the ADS1235EVM be setup for two, simultaneous differential inputs?

The registor ssttings for the Load Cell configuration are:

[ADS1235 Register Settings]
ID C1
STATUS 04
MODE0 14
MODE1 61
MODE2 C0
MODE3 64
REF 0A
OFCAL0 00
OFCAL1 00
OFCAL2 00
FSCAL0 00
FSCAL1 00
FSCAL2 40
RESERVE0 FF
RESERVE1 00
RESERVE2 00
PGA 00
INPMUX 34

The register settings for the Thermistor are:

[ADS1235 Register Settings]
ID C1
STATUS 00
MODE0 14
MODE1 01
MODE2 00
MODE3 40
REF 05
OFCAL0 00
OFCAL1 00
OFCAL2 00
FSCAL0 00
FSCAL1 00
FSCAL2 40
RESERVE0 FF
RESERVE1 00
RESERVE2 00
PGA 00
INPMUX 4E

Thanks,

  • 0
    TI__Guru** 112715 points

    Hi Tony,

    Welcome to the E2E forum! The ADS1235 has a single ADC and a mux used to select the input.  Unfortunately there is no way to take simultaneous readings of 2 different input selections.

    Also, the ADS1235EVM was not designed to take conversion results with alternating input selections.  To evaluate in this way you would need to use an external processor with a program designed specifically for this purpose.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 40 points

    I suspected as much from the dev kit however, my final application will have a host processor that could alternate between the two channels if it is possible. It appears that the limitation may not be the muxed inputs but rather the limitation on the number/configuration of references. Is that correct?

  • 0 in reply to Tony Boehm
    TI__Guru** 112715 points

    Hi Tony,

    There are many ways that the measurements can be made and one of the reasons we have fixed the EVM operation to one measurement at a time.  It is quite doable to make the measurements as you have initially shown by writing the desired configuration and then reading the result from that conversion.  So you can use AC excitation for the load cell, and then periodically make the thermistor measurement as single-ended using a different reference.  If you feel you need more reference options or input channels you can look at the ADS1261.

    I did go back and look at your configuration which appears to be correct for the load cell measurement (although you most likely would want to apply some gain).  However, for the thermistor measurement it would appear that you are using an incorrect configuration.  The input mux settings that you are using for the thermistor is 0x4E which would use the AIN1/PGA N Input open input combination where the AIN- input is floating.  Also, as AIN 1 is in use for the load cell measurement, you could use the AIN4/AIN5 input combination (0x78) for the thermistor measurement. To make this measurement single-ended, connect AIN5 to AGND.

    Best regards,

    Bob B