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Original question:

ADS1000: ADC is railing when the board temperature rise

ADS1000: Strange values at -40ºC temperature

Vitor Silva2
Prodigy 60 points
Part Number: ADS1000
Other Parts Discussed in Thread: LM35, LMT87

Hi,

We are using an ADC ADS1000 with a LM4132EMF-3.3/NOPB to generate power supply and reference to the ADS1000. We also have an analog multiplexer TMUX1208PW to select one from the 7 analog signals to read. We are making measurements every 10ms from a diferent analog input.

Everything seems to be working fine until the board is powered up (cold start) at least at -40ºC. At this temperature, the NTC, LM35 and LMT87 values given by the ADS1000 are 2047 (corresponding to 3.3V). The LM35 voltage should be 950mV and we are reading 3.3V and the anemometer should be 1 to -1mV and we are reading 65mV. When we rise the temperature with the board working, at a temperature of near -18ºC on the PCB, the values pass from these wrong readings to the correct ones. It is interesting that the Anemometer readings passes from 65mV to -1mV from one moment to another.

I'm attaching the schematics of the MUX, ADC and 2 NTCs. All the analog signals circuitry is very similar. The reference circuit was already removed and the ADS1000 was supplied directly to the 3.3V rail. The behaviour was the same. Do you find any reason for this behaviour at -40ºC temperature?

Thanks!

Best regards,

Vítor

  • 0
    TI__Mastermind 22950 points

    Hi Vitor,

    It is a little difficult to make out some component values/names. Is it possible to upload a new image with a better resolution?

    Regarding your main concern, I would first like to state that you are operating at the very edge of the absolute minimum temperature that the device can operate at. While the device should still operate just fine, I am curious in what kind of controlled environment the device is being operated in? Are you taking measurements inside an oven? How accurate is the ambient temperature? Since the abs minimum temperature is -40C, I am worried that slight temperature variations are happening and the temperature is lower than -40C. 

    Additionally, have you been able to debug around the board? You say that the temperature sensor outputs should be specific values but have you measured right at the input pin of the ADC to rule out any other external circuit being negatively affected?

    Regards,

    Aaron Estrada