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:
- 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?
- 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?
- 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!