• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 46 subscribers
  • Views 641 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

TIPD188: Is possible to use the same approach of ratiometric and ac excitation with ads1256?

Laurindo Neto
Prodigy 30 points
Part Number: TIPD188
Other Parts Discussed in Thread: ADS1256, , ADS1262

Dear all,

after taking a look at ADS1262 datasheet (used on TIPD188), Vref must be positive (pag 41). However, in order to mantain a ratiometric relation togheter with the proposed "ac" excitation, Vref must sometimes be negative (once Vref=VrefP-VrefN and the excitation switchs over time). As I suppose TIPD188 works fine, maybe this is a fault on datasheet. Hence, would it be possible to use the same approach with ads1256?

best regards,

  • 0
    TI__Mastermind 44550 points

    Hi Laurindo,

    Welcome to the TI E2E Forums!

    The ADS1262 has an internal reference multiplexer that is able to switch the positive and negative references inputs; therefore by (correctly) programming the device registers it is possible to always maintain a positive reference voltage.

    For most other delta-sigma ADCs, you would need to implement some kind of external switching to ensure that the reference voltage is always positive. 

    While you could do something similar with the ADS1256, do keep in mind that its reference voltage is limited to 2.6V, so it wouldn't be my first choice to use in such an application where a larger reference voltage is usually desirable.

    Best regards,
    Chris

  • 0 in reply to Christopher Hall
    Prodigy 30 points

    Hi Christopher,

    Thanks for your short reply.  By the way, nice article you did about ac excitation.

    Regarding the switch inside ads1262 for the reference pins, it was my bad I haven't seen before about it at ads1262 datasheet. But, I couldn't get the reason why at tipd188 design you change the reference polarity usign the switch and then, at ads1262 you change back this polarity to keep positive. It is not the same as maintaining always fixed and use the switch to change only load cell's excitation?

    Best regards

  • 0 in reply to Laurindo Neto
    TI__Mastermind 44550 points
    Hi Laurindo,

    Thank you!

    You certainly could configure the reference voltage to be fixed and not have to worry about switching the reference. However, in the TIPD188 design we wanted to measure back the reference voltage directly at the bridge to ensure that the design was truly ratiometric.

    The closer you can measure the excitation voltage to the bridge, the better the matching will be. It is for this same reason why some load cells have 6-wires instead of 4... the extra two wires are "sense" wires that allow you to measure back the excitation voltage directly at the bridge. When using a 4-wire load cell you'll have some voltage drop in the wires that connect the excitation source to the bridge.

    I hope that helps!
    Chris
  • 0 in reply to Christopher Hall
    Prodigy 30 points

    Thanks a lot Chris. I got your point. I will go for a design w/ ads1262 and I  will also give a try with current source excitation