# Q: Unexpected behavior from digital Rheostat

I have no idea whats warranting this behavior from the MCP4162 device

Starting from 500mV(pp) - 2.8V(pp) a weird "wave" like distortion happens in between the two voltages. (Towards the negative cycle).

This only happens at low frequencies such as 20Hz and lower, however at higher frequencies this no longer occurs even with the voltage range described above.

Input 1 = Input (Yellow)

Input 2 = Output (Green)

The behavior:

Diagram

simulate this circuit – Schematic created using CircuitLab

• Never used them, but watch out for ": Non-linearity is affected by wiper resistance (RW), which changes significantly over voltage and temperature." and considering the long time constant from C2 , watch out for excessive xxx uA current related distortion during startup. – Sunnyskyguy EE75 Dec 13 '18 at 4:18
• I see I see, but does it make sense to only be distorted between those voltages? I was thinking of scrapping this and trying out this guy ww1.microchip.com/downloads/en/DeviceDoc/11195c.pdf – Pllsz Dec 13 '18 at 4:28
• These digipots use FETS. As you apply LARGE voltages, the FETs experience major channel-gate voltage changes, and the on-resistance will change a lot, which changes your resistance and changes your gain. Is this effect discussed in the datasheet? – analogsystemsrf Dec 13 '18 at 4:30
• Thats interesting, I thought they would use latches/relays. I have no idea. I am not experience what so ever in these type of applications. I tried looking for anything but I couldnt find anything related to what to avoid, etc. Why is that when I apply a large voltage is good, and a small voltage, but in between its not good? Does that explain its behavior ? – Pllsz Dec 13 '18 at 4:32
• Neither end of the digital pot should ever exceed the supply voltage of the digital pot. You don't show the supply voltage for the digital pot. Measure the voltage at both side of the pot when you see the distortion (one side on each channel), using DC coupling on both channels. – Spehro Pefhany Dec 13 '18 at 4:45