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ADS124S08: Sampling Errors

Sergi Tommaso
Prodigy 10 points
Part Number: ADS124S08

Hello,

I have a problem with the ADS124S08 device.
I use the device to detect the temperature with a PT100.
I use the 4-wire configuration with low side Rref as shown in the figure ([NM] = not mounted).

 

Below I report the configuration

IDAC1 = 2mA, IDAC2 off

PGA disabled

Sinc filter selected

SPS = 2.5

AIN3 = positive input channel, AIN4 = negative input channel

Everything works fine. Sampling for 10 minutes I have a maximum variation of 0.002 ° C on the acquired samples.
But on a batch of 10 devices 2 have a variation of 0.15 ° C.

To solve the problem just change the ADS124S08 and the system returns to work correctly with a maximum variation of 0.002 ° C.

Why does this happen? is it an electrostatic discharge problem?
Changing the ADC is very expensive.
Can anyone help me out?

Thanks so much.

  • 0
    TI__Guru** 112795 points

    Hi Sergi,

    Welcome to the E2E forum! I can't say why you are seeing these particular issues, but I can point to some issues with your schematic and configuration. 

    If you are using the low-side reference as the ADC reference, then there is a violation of the input voltage.  The external reference voltage is specified as a minimum of 0.5V.  Your 4-wire RTD is using 2mA and 120 Ohm reference resistor which only allows for 0.24V.  So you will need to increase the reference resistor value.

    The external reference inputs can have the input buffer enabled or disabled.  If the reference buffer is enabled, then you will need to make sure that the voltage at the REFN1 is biased above AGND.  You could target 200mV bias.  For additional information see the following FAQ.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 10 points

    Thanks Bob,
    I will put your suggestions into practice and let you know.

    Stefano