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ADS1158 Open Sensor Detection

Gary Morris
Prodigy 40 points
Other Parts Discussed in Thread: ADS1158

We are using the ADS1158 sensor bias facility in order to detect open circuit conditions at the analogue inputs.  The datasheet comments:

"Note that the interaction of the multiplexer resistance with the current source may lead to degradation in converter linearity. It is recommended to enable the current source only periodically to check for open inputs and discard the associated data."

Can you provide any empirical data to support this statement with regards both frequency of activation of this mode which would be considered more than 'periodically' and to what extent the linearity could be affected please?  Is this an issue with the 1.5uA current source selected as well as the 24uA?

Thanks,

Gary.

  • 0
    TI__Mastermind 44550 points

    Hi Gary,

    Welcome to the TI E2E Forums!

    I don't have any measurement data to show the effects of open-sensor detection on the ADC's linearity. However, I did do a quick simulation to illustrate the concept I think the datasheet is trying to convey...

    When applying 1V to the ADC's input and enabling the open-sensor detect current sources, a small error occurs in the ADC measurement, due to the voltage drop across the MUX impedance. However, this error becomes much more significant as the input impedance is increased, for example, if an RC filter is added to the input. I think the bottom line is that the open-sensor detect current sources introduce error into the measurement, so they should be turned off when trying to measure the input voltage with accuracy in mind.

    Best Regards,
    Chris