potentiometer
I'm trying to replace an analog pot with a digital one for the volume control in a guitar amp. I have a digital pot – MCP4141 put instead of the analog one. However, checking on the oscilloscope seems like "zero level" is in the middle of the signal, so everything below this level gets cut off. How could I "move up" whole wave above the ground level? Does have to be a resistor put between "B" and ground? Or something else?
I finally found the problem! While initializing the ports on the PIC the CS and INC lines on the digital pot were momentarily being cleared long enough to initiate a decrement command but not long enough for a store command. The correct value was being stored and the device would decrement that correct stored value each power cycle giving the illusion that a new value had been stored. By setting the CS and INC lines high before port initialization (didn't know I could do that even) I avoided the unwanted decrement.
BSF CS ; SET CS
BSF INC ; SET INC
;
MOVLW 0X1F ;
MOVWF TRISC ;
MOVLW 0x80
MOVWF TRISB ;
CLRF TRISA ;
CLRF ANSEL ; CONFIGURE AS DIGITAL IO
CLRF ANSELH ;
CLRF PORTA ; INITIALIZE PORT A
CLRF PORTB ; INITIALIZE PORT B
CLRF PORTC ; INITIALIZE PORT C
Well you can modify the linear pot to get an almost logarithmic response by adding a resistor from the CCW side to the wiper. Because of its simplicity this might be worth a try. Here's what the idea looks like: -
For the pedal circuit you need this might be ideal. It's not perfect of course but it reduces the risk of buying linear pots because these can be made "almost logarithmic by adding one resistor.
One thing to remember is that if the design calls for a 1M pot and you have a 1M resistor across wiper to CCW, the end to end pot value will change as the wiper advance towards CW. This would probably mean that you should use a 2M pot and 2M resistor.
Some experimentation is required and if you could post a circuit I'm sure better advise could be given. Pictures gleened from here
EDIT - picture of circuit attached with my best guess at log/lin: -
Best Answer
None of the 'pot' pins can be allowed to go (much) beyond the power supply voltages.
If you have a digital pot with 0/5V supply you need to keep the voltages within that range. Since you are dealing with audio signals, they are AC, and you could bias the input at 2.5V with a couple of resistors and your coupling capacitor, so that the analog input (and output) sit at +2.5V (thump on start-up). Or use +/-2.5V supplies. And make sure you never get too much input amplitude or you could destroy the chip.
It's often not that easy to replace a pot with a digital pot in an existing circuit- you end up redesigning.
There are some digital pots (eg. AD7376) that will work with relatively high and bipolar supply voltages, while retaining digital inputs referenced to ground but they tend to be relatively expensive. Depending on the function, you might also do much better with a log taper digital pot in an audio application.