Beware digital pots.

[nigelwright7557]

I designed a simple mixer with a mcp4231. It sounded ok on the breadboard.
I carefully followed the datasheet in designing the circuit.

It worked so I got a 6 channel pcb made. I noticed it when testing that even though the pot was set to zero a small amount of signal was getting through. I missed this on my breadboard.
So the pot still has a bit of resistance even at the end of its travel. (0-129)
So I tried turning the pot off which failed miserably because I had wired the pot with terminal b as input !

So I reversed the A+B terminals and got a new pcb made.
Turning the pot off at power up now worked but terminal A was floating to ground and so the input voltage caused the digital pot to latch up and put out a broken up signal !

So I now power up the digital pot with a value of 1 (not off) and this charges up the input capacitor to 2v5 and stops the break through.
The pot can then be switched off completely when the volume equals zero.

What a carry on to get something that I thought would be simple to work.
The datasheet says clearly that the A+B terminals can be used any way round which isnt true. Microchip seem to just say they are looking into it and it has now been a number of weeks.

[thaumaturge]

"Beware Digital Pots"

I always have. Never liked them. Far prefer a motorized if remote is desired.

Doc