1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

Clamping output to gnd at very low threshold?

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by pavanbabut, Jun 13, 2018.

  1. pavanbabut

    pavanbabut Member

    Joined:
    Apr 27, 2012
    Messages:
    263
    Likes:
    0
    I have a digital potentiometer setup as voltage divider that doesn't quite go down to 0V and can only go as low as 30mV. Is there any logical way to clamp this to GND when it is at this level, for example, using a combination of analog switch / comparator or likewise? Would like it to be adjustable since these DACs have quite a bit of tolerance.

    thanks
     
    Last edited: Jun 13, 2018
  2. dknguyen

    dknguyen Well-Known Member

    Joined:
    May 6, 2006
    Messages:
    7,549
    Likes:
    162
    You mean like a giant sub-mohm MOSFET wired to pull down the output of the digital pot whose gate is driven by a comparator whose output fires whenever the input falls below ~30mV? If so, I kind of wonder how you would ever get out of that state since you have a feed forward loop where the MOSFET drives the output to zero and stays at zero if the output is driven to zero.
     
  3. JimB

    JimB Super Moderator Most Helpful Member

    Joined:
    Sep 11, 2004
    Messages:
    6,701
    Likes:
    651
    Location:
    Peterhead, Scotland
    ONLINE
    A rather broad question with not much supporting information.

    Which digital potentiometer? A link to its datasheet would be useful.

    How are you using the digital pot? A schematic would be useful.

    JimB
     
    • Like Like x 1
  4. dave miyares

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    2
    Likes:
    -10


     
  5. pavanbabut

    pavanbabut Member

    Joined:
    Apr 27, 2012
    Messages:
    263
    Likes:
    0
    I am not sure how this can be done or if it is even possible, but you never know.
     
  6. dknguyen

    dknguyen Well-Known Member

    Joined:
    May 6, 2006
    Messages:
    7,549
    Likes:
    162
    Well, it's a digital pot so your processor should know when it's trying to get 0V out of the pot. In which case, drive the MOSFET gate directly with the processor and use software to coordinate the MOSFET and the pot.
     
  7. pavanbabut

    pavanbabut Member

    Joined:
    Apr 27, 2012
    Messages:
    263
    Likes:
    0
    I am using AD5292 with a 5V reference connected to it A terminal while B is connected to GND. W is connected as reference voltage to a 10BIT DAC.

    Hope that helps
     
  8. dave miyares

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    2
    Likes:
    -10


     
  9. pavanbabut

    pavanbabut Member

    Joined:
    Apr 27, 2012
    Messages:
    263
    Likes:
    0
    Actually, thats a good idea. Now you got me thinking that I can use one of the GPIO pins on my SPI interface to drive a switch.

    Thanks a bunch
     
  10. dknguyen

    dknguyen Well-Known Member

    Joined:
    May 6, 2006
    Messages:
    7,549
    Likes:
    162
    What MOSFET are you planning on using? MOSFETs take a finite amount of time to turn on because the gate capacitance needs to be charged. Especially very low RDson MOSFETs (high gate capacitance/charge) being driven by the weak, low-current GPIO of a regular IC. Don't forget to use a logic level MOSFET.

    Of course, if you needed speed then you could drive the gate with a a dedicated low-side gate driver IC triggered from the GPIO. Which also releases you from the bounds of a logic level MOSFET since gate driver ICs have a separate supply voltage input used to drive the gate. But you probably don't need speed. Just putting it out there in case you did.
     
  11. pavanbabut

    pavanbabut Member

    Joined:
    Apr 27, 2012
    Messages:
    263
    Likes:
    0
    I am guessing like a simple analog switch like ADG1219 should work fine for my purpose.
     
  12. dknguyen

    dknguyen Well-Known Member

    Joined:
    May 6, 2006
    Messages:
    7,549
    Likes:
    162
  13. Pommie

    Pommie Well-Known Member Most Helpful Member

    Joined:
    Mar 18, 2005
    Messages:
    10,720
    Likes:
    422
    Location:
    Brisbane Australia
    Something doesn't add up. Your maximum error should be 10mV (5V/1024 * 2) - the 2 is the max error at position zero. Your normal error should be zero!!

    A schematic would help.

    Mike.
     
  14. JimB

    JimB Super Moderator Most Helpful Member

    Joined:
    Sep 11, 2004
    Messages:
    6,701
    Likes:
    651
    Location:
    Peterhead, Scotland
    ONLINE
    I just had a look at the datasheet to see if I could find the minimum resistance of the potentiometer, but I could not see it.

    Rather than trying to short circuit the output at zero output, why not insert an offset adjustment, maybe using an op-amp, to take the residual output to zero.

    JimB
     
  15. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

    Joined:
    Apr 17, 2007
    Messages:
    7,983
    Likes:
    1,079
    Location:
    Loveland, CO USA
    Which part?
    What are you passing through the pot? DC or AC and what frequency?
    Bread board? or PCB ? or point to point wiring? or ......
     
  16. pavanbabut

    pavanbabut Member

    Joined:
    Apr 27, 2012
    Messages:
    263
    Likes:
    0
    Its 20K version with a DC 5V reference voltage output from AD586 connected to terminal A.
     
  17. AnalogKid

    AnalogKid Well-Known Member

    Joined:
    Sep 22, 2011
    Messages:
    1,377
    Likes:
    191
    Location:
    Ohio - GMT-5
    ONLINE
    To fix 2000's tech, consider 1950's tech - a very small reed relay.

    ak
     
  18. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

    Joined:
    Apr 17, 2007
    Messages:
    7,983
    Likes:
    1,079
    Location:
    Loveland, CO USA
    Is the B terminal really connected to the ground reference of the 10 bit DAC?
    If you short out the W terminal to ground of the "pot" what is the reading on the DAC?
    ----edited----
    It is easy to have 30mV drop in ground in a bread board.
     
    Last edited: Jun 14, 2018
  19. pavanbabut

    pavanbabut Member

    Joined:
    Apr 27, 2012
    Messages:
    263
    Likes:
    0
    Just tested this and the DAC output is Zero when I short it. When I remove the short it goes back to 30mV. This setup is on a PCB. B terminal is connected to GND layer.
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice