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ADS1248 Unipolar Supply with +/-2.5V input signal

Mayank Gupta
Prodigy 240 points
Other Parts Discussed in Thread: ADS1248, ADS1220, ADS124S08, ADS124S06

Hi TI team,

We use ADS1248 regularly for sensing 4-20mA signals in Unipolar mode 0-5V. However, off-late we learnt that we were using it wrongly by grounding the ADC negative terminal.The right way is to provide a Common Mode voltage (i.e. 2.5V)  to ADC negative input.

But at this point, I want to explore the possibilities of measuring bipolar signals (+/-2V) using unipolar supply of 0-5V? Now that ADC Negative input is provided a common mode voltage of 2.5V - I should be able to read signals from 0.5-2.5V as negative and 2.5V - 4.5V as positive. If yes, then can we use internal reference of 2.048V for this arrangement or we have to use external reference of 0-5V?

Regards

  • 0
    TI__Guru* 98025 points
    Mayank,


    The way you have described it is accurate. Ideally, you would want an input signal that has a common-mode voltage of +2.5V when the input supply is 0 to 5V. However, you certainly can make measurements with the negative input tied to +2.5V and have the positive input go from 0.5V to 4.5V. This would give you a measurement input range that would measure -2V to +2V.

    In this case, you could still use the internal reference voltage of 2.048V. The reference only needs to match the magnitude of the positive input full scale, and doesn't require the voltage to be larger or at different offset voltages within the 0 to 5V range. I would note that the ADS1248 has a limit on the reference such that the reference positive input is AVDD - 1V. This limits the reference to 4V when AVDD is 5V.

    I've mentioned this in previous posts, but if you need a similar device where the PGA can be bypassed (so that the negative input can be attached to ground), the ADS1220 is capable of making such measurements. Also the ADS124S08 and ADS124S06 will be available available soon, and are capable of this type of measurement as well.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 240 points
    Hi Joseph,

    Thanks for your reply.

    I had read your earler post about ADS1220, but here we are trying to interface various industrial sensors which include thermocouple that may require PGA. Additionally, we are using unused ADS1248 pins as GPIOs to save on isolation cost.

    Regards