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ADS114S08: ADS114S08

Moshe Levy
Prodigy 230 points

Part Number: ADS114S08

Hello

We are using the ADS114S08 in our design. our interface is SPI (CS not connected).

- we encounter issues with sampling.
   - Sometimes we get all zeros reading of our thermistor

   - Sometimes, the ADC provides sample reading (i.e: we see value), but, if we try to read again, the last value is stuck even if we change temperature. But, this happens only sometimes. one other times the ADC works fine for a while.

 - when value is stuck, we tried to reset the ADC - not always solved the stuck value issue. but power cycle helps.

Are you familiar with such phenomena? 

my circuit is attached for reference.

  • 0
    TI__Guru** 113765 points

    Hi Moshe,

    Usually when there are issues taking place that give abnormal operation some form of transient is entering the system.  As you are not using CS, you may want to enable the SPI timeout feature in case a transient is causing an issue with communication.

    I see there is external diodes for transient protection on the inputs.  However there is actually very little protection on the pins sourcing the IDAC current.  Sometimes a series diode can help with transients greater than the AVDD supply.  It is also possible to limit transient currents by using a series resistor on the IDAC pins.  Whether or not this is feasible will depend on the current and the voltage drop across the resistance.

    When you see that the ADC appears to be stuck, you could verify by reading the status register to see if the device has reset.  As you are also using the IDACs, you could verify the internal reference is working by measuring the voltage at REFOUT.

    I notice that the AVDD supply appears to have a ferrite in series with the supply.  I would suggest that you try replacing with either a 0 Ohm resistor or one that is below 10 Ohms (1 to 2 Ohms is enough to create a low pass filter).  The problem with using ferrites is the ADC will demand very short duration, large currents from the AVDD supply from time to time that can be choked by the ferrite.  Examples are initial power-up, turning on the internal reference, turning on the IDACs, conversion start, etc..

    I also see a filter resistor on REFN0.  This can be an issue when connected to AGND and I would recommend replacing with a 0 Ohm resistor.  As it is connected to AGND, the reference buffer must be turned off.  When the reference buffer is turned off, there will be a voltage drop created across the resistor that will add a gain error to the measurement.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 230 points

    Hi Bob,

    thank you for the detailed reply. very informative. will try and update if resolved