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ADS1248 internal reference

rakesh joshi
Prodigy 70 points
Other Parts Discussed in Thread: ADS1248, ADS1247

I am using -2.5 V to 2.5 v to power analog side of ADS1248. I need a clarification regarding VREFCOM pin .

Page number 7 in data sheet says it need to be connected to mid point if bipolar supply is used (which is 0v).

Page number 29 says VREFCOM should be connected to AVSS. I tried connecting it to AVSS which is giving me spurious data. 

FYI I am using ADS1248 as general purpose ADC. 

Thanks

Rakesh 

  • 0
    TI__Guru* 97805 points
    Rakesh,


    I'm aware of the confusion in the datasheet about this topic and we're working on getting a new version of the datasheet out to clear this up.

    Generally, you want to tie the VREFCOM to GND but tying it to AVSS may also work. You want to makes sure that VREFCOM is tied to a low impedance AC ground which is required for the stability of the reference.

    With some bipolar supplies, the supplies may not be able to easily sink and source current with a low impedance, that's why the GND is preferable and why tying VREFCOM to AVSS may not always work.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 70 points
    I connected VREFCOM to 0 V. I measured voltage between VREFOUT-VREFCOM, it is 1.58 V instead of 2.048V.

    And my data looks spurious in this case too.

    thanks
    Rakesh
  • 0 in reply to rakesh joshi
    TI__Guru* 97805 points
    Rakesh,


    Can you get an oscilloscope and verify that the reference is oscillating? If not, do you have it loaded down? Are you pulling more current than you expect?

    Also, do you have capacitance from VREFOUT to VREFCOM? You need between 1uF and 47uF of capacitance directly between the two nodes.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 70 points

    I have 1 uF capacitor between VREFOUT and VREFCOM . I will definitely verify signal using oscilloscope.

    FYI If I use 0 to 5V as AVSS and AVDD respectively. I get 2.048V  between VREFOUT and VREFCOM

    Thanks
    Rakesh

  • 0 in reply to Joseph Wu
    Prodigy 70 points
    I have 1 uF capacitor between VREFOUT and VREFCOM . I will definitely verify signal using oscilloscope.
    FYI If I use 0v and 5v for AVSS and AVDD respectively . I get 2.048v between VREFOUT and VREFCOM
  • 0 in reply to rakesh joshi
    TI__Guru* 97805 points
    Rakesh,


    What are you using as your supply? Are you using the TI ADS1248EVM? or do you have your own board and schematic?

    Do you have a schematic that you are able to share? Even a partial schematic may help. There are so many pins to hook up with the ADS1248, that it's easy to miss a connection. Do you have the board hooked up to anything else?

    Once you get the oscilloscope, check to see that the reference isn't oscillating. If it isn't check to see that the reference isn't loaded.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 50 points

    Hi Joseph,

    I have a similar problem with a ADS1247. We have 0V on VREFOUT. I checked the output with the oscilloscope and it is not oscillating (stable at 0V).

    Here is the schematic of the ADS1247:

    Regards, Felix

  • 0 in reply to Felix Weilenmann
    TI__Guru* 97805 points
    Felix,


    Are you getting 0V output from VREFOUT to VREFCOM? Do you get any output from the reference at all even if the output voltage is incorrect? In general, I don't see anything wrong with your schematic. Do you have supply bypass caps for VAA-2V6 also?

    Are you sure that you've programmed the device correctly to turn on the reference? I would make sure that the VREFCON bits were set to 01 so that the internal reference is always supposed to be on, so you don't have to worry about cases where START is low.

    Have you verified that the device is receiving your WREG commands by reading back all of your registers?


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 50 points
    Hi Joseph,

    Thanks for your quick reply,

    We found the problem: AVDD was not correct and therefore only the digital part of ADS1247 was working (I read back the configuration registers and it was correct).

    Thanks, Felix