digital potentiometer

Discussion in 'General Electronics Chat' started by pazpaz, Sep 19, 2014.

  1. pazpaz

    Thread Starter New Member

    Nov 14, 2013
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    Hello all,
    In my circuit I am using digital potentiometer MCP4641.
    can you tell me how output voltage is related to wiper code
    Rab = 5 KOHM
    wiper code is sent is 135
    MCP4641 is 8 bit poentiometer
    End A is connected to 5V supply
    End B is connected to ground.
    In this case what voltage and resistance I should expect at the wiper end?
    regards
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. #12

    Expert

    Nov 30, 2010
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    Here's the datasheet for the pot in question:
     
  4. pazpaz

    Thread Starter New Member

    Nov 14, 2013
    19
    0
    Hello,
    I have gone through the datasheet. but I am still confused. if I ask you that I am setting wiper code = 0x50 then what will be the Rw resistance? and what voltage it will be developed across Vwb if 'A' terminal is connected to 5V and 'B' terminal is connected to GND. 'W' terminal is output voltage generated by digital pot.

    regards
     
  5. #12

    Expert

    Nov 30, 2010
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    Learning as I go...an 8 bit control has 255 steps. If each one is an equal part of 5k ohms, then each step must be 19.6 ohms or 1/255 times the Vcc. Then there is the Rw (resistance of the analog switch in series with the wiper) which is rather difficult to interpret from the datasheet. For 5.00 volts, 135/255 x 5 = 2.647 volts.

    Am I making sense or just randomly stabbing at the wrong answer?
     
  6. pazpaz

    Thread Starter New Member

    Nov 14, 2013
    19
    0
    Hi,
    when I am transmitting wiper code = 135 (decimal) then at the output of the wiper pin voltage is 0.023V wrt GND
    regards
     
  7. #12

    Expert

    Nov 30, 2010
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    Well, that seems wrong if your only load is the meter you are measuring with.
    Does 135 mean you commanded the pot to go to step 135 out of 255 steps?
    or is that what you're asking us?
     
  8. pazpaz

    Thread Starter New Member

    Nov 14, 2013
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    yes I commanded the pot to got to step 135 out of 255 steps i.e wiper position is at 135 step.
     
  9. #12

    Expert

    Nov 30, 2010
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    You should expect 2.647 volts. That is not what you're getting, so we need to examine your circuit and your method of giving instructions to the pot. Please try to present the information by uploading it.
     
  10. pazpaz

    Thread Starter New Member

    Nov 14, 2013
    19
    0
    I am using wiper command address = 0x90 and value to be sent is 135 (0x87). This is the only thing I am doing for pot and circuits is
     
  11. #12

    Expert

    Nov 30, 2010
    16,278
    6,789
    I've hit my limit for tonight. Somebody else take over, please.
     
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