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ADS1220: Questions about ADS1220 Input range.

Duckhyun Kim1
Prodigy 20 points
Part Number: ADS1220
Other Parts Discussed in Thread: MSP430F5528

I am using ADS1220 to test the performance(INL, DNL) of my 10bit Dac chip.

The drift and offset of ADS1220 will become as a variable in measuring performance, but we will not take this variable seriously because the resolution of the ADS1220 is much higher than my DAC chip(is it right?).

I have succeeded in reading the voltage by using the ADS1220, but I had one big problem.

My DAC output range is 1-3V(2V difference), ADS1220 input range is also maximum 2.5V difference voltage range. DAC, ADC are controlled by only one MCU(MSP430F5528), they share only one ground.

In this situation, ADS1220 only can read the 0-2.5V range, so that cannot read my DAC output (1-3V range) even if I use any other voltage reference mode (default of not default mode).

In this situation, can I read the 1-3V voltage range by using the ADS1220?

Thanks for your cooperation.

  • 0
    TI__Guru** 113765 points

    Hi Duckhyun,

    Welcome to the E2E forum!  The ADS1220 has an internal reference of 2.048V, but an external reference can be applied to better match your input range.  There is an internal register setting that allows for the AVDD supply to be used as a reference.  You can also apply another voltage to the reference inputs.

    As your DAC output range is 1V to 3V, you could also use the internal reference by making a pseudo-differential measurement. To make this type of measurement you could set your DAC output to AIN0 and set AIN1 to 2V.  For the conversion result you would need to add 2V to the output result.

    As an example, 1V is applied to AIN0 and 2V would be applied to AIN1.  The output result will be equivalent to -1V, and by adding 2V to the result your final result is 1V.  If 3V is applied to AIN0, the output code result would relate to 1V.  If you add the 2V to the 1V your result is 3V.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 20 points

    Hello Bob Benjamin,

    I am really appreciated to your best quality answer.

    It is very helpful information to me.

    I have one of very simple question about your answer. 

    Is there any reason to set the AIN1 to 2V?

    I think it is better to set the AIN1 to 1V because negative voltage is difficult to handle in the MCU.

    Thank you very much for your kind answer. 

    Best regards,

  • 0 in reply to Duckhyun Kim1
    TI__Guru** 113765 points

    Hi Duckhyun,

    I used 2V for AIN1 to cover the complete full-scale range of the internal reference (2.048V).  However this assumes that your AVDD supply is at least 4V.  If AVDD is 3.3V then you could use the analog supply as the reference and measure the DAC output directly.

    If the analog supply is 2.5V, then you will have an issue with 3V output of the DAC being outside of the input range of the ADS1220.  If the analog supply is large enough to accept a 3V input, then you can certainly measure the 2V output range of the DAC by setting AIN1 to 1V and adding 1V to your conversion results.  How you decide to make your measurement is up to you.

    Best regards,

    Bob B