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ADS1261: Is bipolar input measurement possible or not?

Joonas Hollo
Prodigy 80 points

Part Number: ADS1261
Other Parts Discussed in Thread: ADS124S08, ADS125H02

I have tried to understand the ADS1261B performance in bipolar input measurement. Our support told that using external 5V reference it should be possible to achieve almost ±5V input range.

How that ±5V input is possible if AVss can be max -3V?

Anyhow is it still possible to have roughly ±2,5V input if we use AVDD to AVSS as 5V and then tie DGND +2,5V?

Below the message that we have received from TI:
"Hi Joonas,  I had a quick look at the data sheet and wanted to ask you a few questions.  You could change the input range to almost +-5V if you use the Analog Vcc reference running of 5V rather than 2.5V reference  ( and use an external  Voltage reference to drive Vcc )  but the PGA limits what you can actually achieve,  but this can be extended by bypassing the PGA. "

  • 0
    TI__Guru 58981 points

    Hi Joonas,

    I think we first need to clarify what you mean by bipolar input measurement: are you looking for an ADC that support negative voltages with respect to ground (absolute bipolar)? Or are you looking for an ADC that can support positive and negative input voltages with respect to one analog input (differential bipolar)? The ADS1261 (and many other delta-sigma ADCs) support differential bipolar, while a bipolar power supply is required for absolute bipolar.

    To help explain the difference, I had created this FAQ a while back: https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/1048636/faq-can-my-adc-measure-negative-voltages?tisearch=e2e-sitesearch&keymatch=faq%3Atrue

    Please review this information and let me know if it answers your questions.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 80 points

    Hi Bryan,

    Thanks for the quick reply. FAQ that you posted gave good clarification for the issue. We need to have ADC with true negative voltages measured. So it´s absolute bipolar and some signals are single ended. To conclusion, we need to seek for another model of ADC. ADS124S08 seemed to be rather good option while there is sensor supply (IDAC) and integrated PGA. Availability is of course the biggest challenge for many components like is with this as well. I have tried to browse ti pages for the best ADC match but still struggling with all these options. Probably the ADS124S08 will help us to seek other similar products from the portfolio. Is there any ADC with ±5V input while ADS124S08 is ±2,5V?

  • 0 in reply to Joonas Hollo
    TI__Guru 58981 points

    Hi Joonas,

    The ADS125H02 could be another option for you. The ADS125H02 can support a ±15V input directly using a ±18V supply. The ADS125H02 is also very similar to the ADS1261 and ADS124S08 in terms of feature set (IDACs, PGA, etc.), performance, and size.

    Please review the device datasheet and let me know if you think this device can work for your application.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 80 points

    Hi Bryan,

    Forgot to mention about channel requirements. 6-ch would be the optimal but also the least needed.

  • 0 in reply to Joonas Hollo
    TI__Guru 58981 points

    Hi Joonas,

    Would the ADS125H02 be acceptable to you if it had more channels? Are there any other challenges you see designing with this specific ADC? So far I am aware that you need a 24-bit, 6-ch (or more) ADC that has a PGA, IDACs, and can support at least ±5V absolute input voltage.

    Additionally, would you be willing to use an external mux to increase the number of channels? The ADS125H02 has integrated GPIOs that could be used to control the mux so you do not need to bring additional control lines from the MCU.

    Let me know if this is an option for you and I can suggest a few mux options.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 80 points

    Hi Brian,

    We have gone through our requirements and decided to go for ADS124S08 for first POC. It seems that sampling rate should be just enough for our application. Really much appreciated your support, Thanks!

    - Joonas

  • 0 in reply to Joonas Hollo
    TI__Guru 58981 points

    Sounds good, Joonas.

    Since it seems like this questions has been resolved, please start a new thread for any additional questions and we will support you there

    -Bryan