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ad5235 A to B resistance measurement not stable with Ohm meter

Category: Hardware
Product Number: ad5235B-250
Software Version: none

Hello,

I'm am testing an ad5235B-250 digital pot.  I can write and read to the spi registers, and am powered by a clean 3.3V signal and the output pot is unloaded. 

When I take an Ohm meter and measure across A to B on either port, the resistance value changes.

I expect it to read the same value (~250K) every time as it's fully unloaded. 

For example, I program the port zero wiper to 0 using B10100000.  I can read this back and confirm the write worked. 

From a0 to b0 I get 187K (should be 250K).  a to wiper is 183K, b to wiper is 100.  So where did the 4K go?  

Then I program the port zero wiper to 512 using B10100000.  I can read this back and confirm the write worked. 

From a0 to b0 I get 208Kohm (should be 250K).  a to wiper is 105K, b to wiper is 108K.  So this is 203K, it should be 208K (should be 250K).

Why is the a to b reading changing with programming, and why does a to wiper to b add up to something different than a to b?

s there something I'm doing wrong to make this thing stable?

Is there some reason a benchtop meter would be messing things up?  Does it need a DC bias across the pot or something?

Thanks,

Dave.

Thread Notes

Parents Reply
  • Hi Dave,

    The leakage current from the device can affect the ohmmeter readings, adding unwanted errors. You can put an op-amp as a buffer and measure the voltage at the buffer output. Another method is to use a source-meter where a precision current source/sink is fed through one of the terminals with the other terminal held at GND or some other level and voltage is measured across the same terminals.

    Best Regards,

    Yokki

Children
  • Thanks for the reply.  To investigate further, I put a 3.08V supply in series with a 264200 Ohm resistor in series with the A port and grounded the B port.  The vout at A was 1.17V which means the series resistance from A to B was approx 160K.  So I think there is an issue with the AD5235 that I have, I would expect somewhere close to 250K.  

    Any tips to further debug this?  Same setup as I describe above.  I would think not matter what the programming, it should always be 250K across A to B.  

  • Hi,

    What is connected to the wiper terminal? have you tried shorting B to wiper then measure it through the wiper?

    Best Regards,

    Yokki