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ADS1255: ADS1255 single-end AINCOM pin

Part Number: ADS1255

Hi team,

I want to confirm that ADS1255 while in a single-end design, the AINCOM need directly connect to VREF or AVDD?

And the cap between VREFP/VREFN need to be 47uf? if 10uf, any influence?

Thanks.

Best regards,

Mia Ma

A

  • Hi Mia,

    If you want to make a single-ended measurement with the ADS1255, you can select AIN0, AIN1, or AINCOM for either AINP or AINN (see the MUX register on page 31). In other words, you do not need to make single-ended measurements against AINCOM, you could also make a single-ended measurement between AIN0 and AIN1 by tying AIN1 to ground and applying a voltage to AIN0.

    You can modify the cap between VREFP and VREFN to whatever you want, but the larger cap value filters more noise at the expense of longer settling time. This usually isn't an issue if the reference is buffered, since the reference is basically outputting a constant DC voltage.

    -Bryan

  • Hi Bryan,

    Does it mean, while single-end application:

    If customer only use AIN0 for a positive input voltage, AIN1 to ground; then how about this sequence AINCOM link? Vref? VDD?

    And another case, if use AIN0 for input voltage, AIN1 floating, AINCOM should connect to GND?

    And a additional question, the diode of AINCOM /AIN0 pin in PAGE 14 Figure 6 is internal, no need to add diode external, right?

    Thanks.

    Best regards

    Mia Ma

  • Hi Mia,

    I am not sure I understand your first question. What does "how about this sequence AINCOM link? Vref? VDD" mean?

    You can connect any input pin (AIN0, AIN1, or AINCOM) to GND for a single-ended input. And then you can apply your voltage to any other analog input as shown in the MUX register on pg. 31. The ADS1255 has a very flexible input mux

    The ESD diodes are there to protect from mild overvoltage events, but many customers require additional diodes to protect against additional overvoltage or transient events. Also, current limiting resistors might be required to ensure that the current into the pins does not exceed that specified in the absolute maximum ratings. So it really depends on the customer's specific use-case.

    If you want to learn more about electrical overstress, please check out this FAQ: https://e2e.ti.com/support/data-converters/f/73/t/951268?tisearch=e2e-sitesearch&keymatch=faq%3Atrue

    -Bryan

  • Hi Bryan,

    In page 14, there is a graph as below in the datasheet.

    Saying:

    When using ADS1255/6 for single-ended measurements, it is important to note that common input AINCOM does not need to be tied to ground. For example, AINCOM can be tied to a midpoint reference such as +2.5V or even AVDD.

    So I am confusing the AINCOM connection?

    Thanks

    BR

    Mia

  • Hi Mia,

    You can see from Figure 6 that the AINCOM pin is basically like any other analog input. It could instead be called AIN8 instead of AINCOM. So you can take single-ended measurements against AINCOM, but you do not need to.

    The datasheet recommends taking single-ended measurements against AINCOM, but again, this is not required if for some reason it is not possible (e.g. a layout or routing issue).

    -Bryan

  • Hi Bryan,

    'The datasheet recommends taking single-ended measurements against AINCOM', so this case AINCOM should connect directly with GND?

    So which condition  ' AINCOM does not need to be tied to ground, can be tied to a midpoint reference such as +2.5V or even AVDD'  is  suggest? AINCOM not using for input or ground?

    BR

    Mia

  • Hi Mia,

    I think we are running in circles a bit here. Can you tell me what the customer wants to do, and I can tell you if it is acceptable?

    -Bryan

  • Hi Mia,

    I wanted to acknowledge that this post was closed out offline. The customer just wanted to be able to measure a single-ended input with the ADS1255, tying AINCOM to GND and applying a signal to AIN0, which is acceptable assuming the buffer is off.

    -Bryan