AD5206 inconsistent value settings

I breadboarded a simple circuit to check the functionality of using the AD5206 in rheostat mode to simulate RTDs for a battery management system.  I find that the resistor setting is incorrect around 1/10 of the time when the same instruction is given.  

  • I cannot see any issues with the timing on a scope (attached).  I am running this pretty slowly.  The clock rate is only 33 Hz.
  • I cannot detect any extra pulses or noise on the CLK or CS signals.  
  • Vdd = 5V and Vss=0V.  
  • Am I clocking the circuit too slowly?  
  • The outputs are open circuit right now.  I am checking the resistance using a Fluke DMM.  
  • Also, I did a test tying Terminal B to Ground and Terminal A to Vdd to form a voltage divider but the results were the same.  
  • I see this effect across multiple channels and using two different devices.
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  • 0
    •  Analog Employees 
    on Oct 23, 2017 7:37 AM

    Hi All,

    I believe few things are clearly missing here, and consequently results are not as expected.

    a digipot  is not more than resistors connected by switched, you can check some details here,

    http://www.analog.com/en/analog-dialogue/articles/digipot-specifications-and-architecture.html 

    The main problem specially when measuring with a DMM is the current sink/source by the internal switches, this generates a error in teh instrument as the DMM makes the calculations based on the current injected... but if this current is different, the calculation are as well.

    In addition, there are two things that makes things worst in your system, you are writing the device continuously, which means that the device is all time updating, and generating even worst specs in terms of current + your board is "floating"... is your GND connections good enough?

    As an intermediate solution, could you connect one of the digipot terminals to VDD, and using the DMM in current mode, connect the DMM between the second digipot terminal and GND.

    Is the value more constant?

    What are the value displayed in the DMM?

    Regards,

    Miguel

Reply
  • 0
    •  Analog Employees 
    on Oct 23, 2017 7:37 AM

    Hi All,

    I believe few things are clearly missing here, and consequently results are not as expected.

    a digipot  is not more than resistors connected by switched, you can check some details here,

    http://www.analog.com/en/analog-dialogue/articles/digipot-specifications-and-architecture.html 

    The main problem specially when measuring with a DMM is the current sink/source by the internal switches, this generates a error in teh instrument as the DMM makes the calculations based on the current injected... but if this current is different, the calculation are as well.

    In addition, there are two things that makes things worst in your system, you are writing the device continuously, which means that the device is all time updating, and generating even worst specs in terms of current + your board is "floating"... is your GND connections good enough?

    As an intermediate solution, could you connect one of the digipot terminals to VDD, and using the DMM in current mode, connect the DMM between the second digipot terminal and GND.

    Is the value more constant?

    What are the value displayed in the DMM?

    Regards,

    Miguel

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