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ADS1262: Open sensor causes overrange reading on Thermocouple channel 1 and increases value on thermocouple 2 channel 2

Alex Moran
Prodigy 30 points
Part Number: ADS1262

Tool/software:

I have a multi channel system running on a stm32 mcu based pcb. For the most part it takes and displays thermocouple readings well but I've noticed when I run the functions in my code to read both channels it tends to have a greater percentage of error on one channel if the other channel doesn't have a sensor plugged in. For example

Both sensors plugged in:

Channel 1 = 0.04mv             Channel 2 = 0.04mv

Channel 1 Plugged in with Channel 2 unplugged

Channel 1 = 0.08mv            Channel 2 = 312.5mv

Channel 2 Plugged in with Channel 1 unplugged

Channel 1 = 312.5mv           Channel 2 = 0.08mv

The gain is set to 8 so the 312.5 is just the high limit of the voltage range it reads But why would one sensor reading overrange because its open have an effect on the other sensor when I'm changing the mux to a different channel between readings and is there a way around this? This only happens when I call the functions to read both of the sensors when one is unplugged. Its like getting an over voltage reading causes some of the reading to bleed into the other channel. Its very confusing because I wasn't aware an ADC could have that problem

  • 0
    TI__Mastermind 45915 points

    Hi Alex,

    From your test, the large value was only seen on the channel that was not plugged in. Can you clarify 'have an effect on the other sensor '? Can you share your schematic?

    BR,

    Dale

  • 0 in reply to Dale Li
    Prodigy 30 points

    Right, the large value only happens on a channel that is not plugged in which is normal im assuming but the "effect" im seeing is that when one sensor isnt plugged in and reads open it also increases the reading value other channel that is plugged in by around 0.04mv from what im seeing. It almost seems like the overrange on the open sensor bleeds into the reading on the other channel. Here's the schematic for reference.

  • 0 in reply to Alex Moran
    TI__Mastermind 45915 points

    Hi Alex,

    I understood your concern, you were talking about 0.08mV vs. 0.04mV on the channel that plugged in a thermocouple. I guess you were testing the THERM1+/THERM1- channel (AIN0/AIN1) and the THERM2+/THERM2- channel (AIN8/AIN9) as shown in the schematic, they are adjacent channels on the terminals J400, but they are not physically adjacent channels on ADS1262.

    Did you check other channels (not those two channels) on the ADC when you were doing the test?

    Can you short the input of the channel that is not connected to a thermocouple and check the output of another channel that has a thermocouple?

    BR,

    Dale