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Reference limit for ADS7952

Andrey5924
Intellectual 350 points
Other Parts Discussed in Thread: ADS7952

Hello,

Please advice if I can use ADS7952 with external reference 3.00V if the IC is powered from 3.3V.

(From the data sheet it is unclear to me. It is written Vref max = 3.0V. for what rail voltage?

Also confusing for me note that typical Vref resistance 100kOhm. I plan to buffer my reference and buffer output will be ~ 35Ohm. Will it work.?

If it will not work, please advice substitution 12 channel device which will work at 3.3V power supply and 3.00V reference.

Thank you, appreciate your help here.

Andrey

  • 0
    TI__Guru**** 180981 points

    Hi Andrey,

    This is a really great question and I'd like to take this opportunity to ask you (and others by the way) to give me and my team a bit of feedback.  The ADS7952 and it's 11 sister parts are limited on the analog input voltage by the +VA rail (see page 3 of the data sheet).  This family also has a 2X REF option, so obviously if you have a 3.3VA rail, you can't use a 3.0V reference and run in the 2x mode.  You can't have a +2.7VA rail and run with a 3.0V reference either, so perhaps this is the clarity issue you mentioned above.  My question to you is: How can we make this clearer in the data sheet?  Would a table with +VA rail vs. reference limits help?

  • 0 in reply to Tom Hendrick
    Intellectual 350 points

    Hi Tom,

    I believe I know the answer to your question.

    And the answer to is in the way you answered to my question. Because you actually did not answer my question if you look at your answer grammatically. I do not mean to be rude but this is perfect example.

    Here was my question 1:

    >> Please advice if I can use ADS7952 with external reference 3.00V if the IC is powered from 3.3V.

    proper way to answer it would be to say:

    "yes you can use ADS7952 with external refeence 3.00v ...

    or 

    " no you can not use AD7952 ...". 

    Instead you go right into details and I appreciate you willingness to share the details because your details are extremely useful ... but you still did not answer my question. (Can you maybe answer it this time?). 

    Here was my question 2:

    "Also confusing for me note that typical Vref resistance 100kOhm. I plan to buffer my reference and buffer output will be ~ 35Ohm. Will it work.?"

    The grammatically correct way to answer this in English should be to say: "Yes it will." or "No it will not."

    And you are not answering.

    The same in Data Sheet - the descriptions and presentation of Data in your Data Sheet are not clear.

    I hope my comments do not make you upset. English is not my first language. I studied it in the age of 30 already. And it is written in English Grammar books, how to answer the questions properly. 

    I guess making your data sheets as clear and accessible as possible is in you interest: If it would be easy for engineers to understand advantages of your Part you will get more sales, right?

    BTW I still do not understand how 2x mode works. 

    I powered up your Part from 3.3V,applied  Reference Voltage 3.0V if I run in 2x Mode, will I be able to measure form 0 to 6V?

    Yes/No? 

    If 'yes' this is remarkable feature which you want people to know about. 

    Please answer me.

    Regards,

    Andrey




  • 0 in reply to Andrey5924
    TI__Guru**** 180981 points

    Touche Andrey!

    My apologies, I was working on some other issues this afternoon, data sheet clarification items  to be specific, and I got caught up in my own quest for answers without giving your actual questions the attention they deserve.

    Yes - you can use the ADS7952 with a 3V reference with +VA of 3.3V.  For the reference question, 100K is the typical reference input impedance.  If you run the device from 3.3V with a 3V reference, you cannot run the input to 6V as this would exceed the maximum input limitations defined on page 4 of the data sheet. 

    Thank you for the feedback!  Your comments are welcomed and I appreciate your candor.

  • 0 in reply to Tom Hendrick
    Intellectual 350 points

    Thank you Tom,

    1. I got the answer I needed and now I can commit to your IC. Hence we both won a little bit. 

    2. To answer your question how to specify Range of Reference Voltage:

    sometimes it is specified with reference to power rail (just like digital inputs)

    Something like that: V ref = V++ - 0.3v  (0.3v shy of supply rail.)

    If it follows such relation of course.

    3. One more question - if I do not plan to use GPIO's on your Part, what should I do with them? Let them be or tie them to Gnd or what?

    Andrey

  • 0 in reply to Andrey5924
    TI__Guru**** 180981 points

    Hi Andrey,

    Thank you again for the feedback!  The GPIOs on the ADS7952 default to 'inputs', so terminating them to GND would be fine.  They can also be used as alarm, power down and input range selection functions if you don't need them for general I/O purposes.  This is described in Table 10 on page 38 of the data sheet.