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variable resistor - digipot, DAC, or other?

Question asked by MKB-UNC on Aug 3, 2015
Latest reply on Aug 7, 2015 by KRZ

 

I have an AC+DC signal where the DC component is typical 2V and the AC portion is only 1 mV.  For now, I am after only the AC component.  This AC signal typically has a rise time of 50 ns and decays exponentially anywhere from microseconds to seconds.  I route the AC+DC signal through a simple voltage subtraction circuit which is manually controlled by a mechanical potentiometer that eliminates the DC component of the AC+DC signal.  The leftover AC signal is then amplified 400-fold.  This works except that the DC subtraction is not stable during the measurement of the AC signal.  The entire AC waveform sporadically jumps between 3 different positions – middle of the oscilloscope screen, centered in the lower half of the o’scope screen, and centered in the upper half of the o’scope screen.  I have tracked this to the physical instability of the mechanical potentiometer which apparently does not hold its resistor value absolutely fixed over time. 

 

Because of this, I believe I could benefit from a digital potentiometer (e.g. Analog Devices AD5270) to replace the mechanical one.  Perhaps there is another solution that I have not considered (e.g. DAC, resistor network/ladder, etc.) which would be a better fit.  Other questions follow:

 

  1. One of my concerns with the digital potentiometer is that there won’t be enough resistor values to accurately subtract the DC voltage.  If there are 2 channels, could I easily wire those 2 channels internally in series/parallel to achieve different resistance values? Should I be considering a DAC?
  2. For a digital potentiometer (e.g. Analog Devices AD5270), what does it mean to be 50 times programmable?  For my application, which would likely need routine, small adjustments, would I only get 50 uses out of this hardware?  Why does a digital potentiometer need to be programmed?  Aren’t the resistor values hardware-determined upon construction?
  3. For my application, would a poor resistor tolerance for a digital potentiometer (e.g. Analog Devices AD5270 although its tolerance is rated as 1%) translate to DC subtraction instability?  If so, does this mean I need to consider other electronics solutions?

  

Any thoughts and guidance will be greatly appreciated!

 

Kyle

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