• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 45 subscribers
  • Views 1755 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS1298 Reference & Input Range

paulo farina
Prodigy 140 points

Dear Kevin,

I am a bit glad i am not the only one who gets confused when reading the REFN REFP part of the datasheet. Since we have already decided to use the 1298 in a new project it would save me time if I understand it before mounting a 1st board version. We are thinking on using a +/- 2.5V suply, couse we need to have the patient at the 0V of the system. The questions are:

1) I must link VREFN to AVSS right?

2) I believed until now (considering the answer for question 1 is yes), I could link 0V to VREFP, but in fact it must be positive is that it?

3) If I use a +/-2.5V supply, gain = 1 and a xV VREFP wich voltages are the full scale to the converter?

Thanks for your attention,

Paulo Farina

  • 0
    TI__Mastermind 46290 points

    Hi Paulo,

    1) I must link VREFN to AVSS right?

    This is not necessarily true for external references, but in most cases I see this being the best way to realize a reference that will enable you to achieve the best performance. For the internal reference VREFN should be tied to AVSS.

    2) I believed until now (considering the answer for question 1 is yes), I could link 0V to VREFP, but in fact it must be positive is that it?

    Again, this really depends on what we're talking about - internal vs. external references. If this is using the internal reference you should replicate a similar circuit to what is shown in figure 35 of the datasheet and place a decoupling capacitor between VREFP and VREFN. If you're using an external reference I could see you maybe tying VREFP to 0V and VREFN to -2.5V to achieve a 2.5V reference.

    3) If I use a +/-2.5V supply, gain = 1 and a xV VREFP wich voltages are the full scale to the converter?

    The full-scale voltage the converter is capable of quantizing is also dependent on the negative reference input. If you were to write an equation for everything it would look something like this....

    (AINP - AINN) <= +/-(VREFP-VREFN)/GAIN

    So assuming you have VREFN grounded and use unity gain the simplified equation is...

    (AINP - AINN) <= +/-VREFP

    Keep in mind that this is differential inputs. While you can resolve down to -VREFP based on the differential voltage each analog input is confined to the range of AVSS-0.3V to AVDD+0.3V.

  • 0 in reply to Kevin Duke
    Prodigy 140 points

    Kevin,

    Thanks for the patience and clarity of responses. So now I am starting to believe the obvious works: I can connect VREFN to AVSS and VREFP to AVDD despite the fact that AVDD-AVSS > 4V. Is it true?

  • 0 in reply to paulo farina
    TI__Mastermind 46290 points

    Paulo,

    You can, but usually we strongly encourage customers to use the reference voltages prescribed in the datasheet because these are the circumstances that the device was characterized under and the circumstances that we can guarantee performance metrics on. The thread you had jumped into before must have been an incoherent day for me as I miss-answered the question - it's since been removed for it's inaccuracies.

    Usually increasing the reference voltage wont suffer you any tremendous consequences - the issue is usually in going the other way as noise contributions become more problematic when the reference voltage gets smaller. This decreases the LSB size and allows smaller noise signals to be quantized.