Faking a center tap on 3 terminal pot?

JohnRoberts said:

BTW the OP is talking about using a Digital pot,,, using two there are more options available.

Click to expand...

I wonder if it makes sense to use a dual digital pot for this purpose. The trick would be to get the two pots to update at the same instant, although I think that the user would never notice that one pot had been updated before the other. Even with 100 kHz I2C, which is slow, the update is still fast enough.

Andy Peters said:

JohnRoberts said:

BTW the OP is talking about using a Digital pot,,, using two there are more options available.

Click to expand...

I wonder if it makes sense to use a dual digital pot for this purpose. The trick would be to get the two pots to update at the same instant, although I think that the user would never notice that one pot had been updated before the other. Even with 100 kHz I2C, which is slow, the update is still fast enough.

Click to expand...

I have done a bunch of work with DPOTs and this is easy enough to deal with. The dual DPOTs only let you update one side or the other at a time, but this design would use multiple dual DPOTs so you could use two A sides of two different DPOTS to make one control.  Push the desired data update to both, then latch them both at the same time...

easy peasy...  8)

JR

  Switches are cheaper than dpots, why not use one pot and switch it from one side to the other? if you want to keep the noise gain constant at the pamps you could switch a resistor when the pot is switched off. (if there's a reason, if not you benefit from 'half?' the total noise gain all the time)

  I mean, is quite simple, one end of the pot to ground, the wiper where it belongs, the other end to the boost or the cut sides selected with one SPDT (break before make) and the optional resistor using a DPDT, many ways to wire it depending on PCB routing convenience. I think there are mechanical pots used this way in some EQs, selecting the amount on the pot and a switch to alternate between boost and cut.

JS

The bottom of this page has a pointer to center tapped 10K:

http://groupdiy.com/index.php?topic=33256.0

BI Technologies
P271-DS21BR10K

Available at Mouser, Digikey and Newark:

http://www.eciaauthorized.com/search/P271-DS21BR10K

Andy Peters said:

I wonder if it makes sense to use a dual digital pot for this purpose. The trick would be to get the two pots to update at the same instant, although I think that the user would never notice that one pot had been updated before the other. Even with 100 kHz I2C, which is slow, the update is still fast enough.

Click to expand...

Thats my (very limited) understanding so far - Im looking at SPI, and spent a lot of time looking for a distribution hub or similar, then thought to check  the amount of bits/transfer speed and realised, its so fast, it won't matter -  In theory...

Gustav

squarewave said:

The bottom of this page has a pointer to center tapped 10K:

http://groupdiy.com/index.php?topic=33256.0

BI Technologies
P271-DS21BR10K

Available at Mouser, Digikey and Newark:

http://www.eciaauthorized.com/search/P271-DS21BR10K

Click to expand...

Those look like mechanical pots, OP is asking about digital pots.

JR

Gustav said:

Andy Peters said:

I wonder if it makes sense to use a dual digital pot for this purpose. The trick would be to get the two pots to update at the same instant, although I think that the user would never notice that one pot had been updated before the other. Even with 100 kHz I2C, which is slow, the update is still fast enough.

Click to expand...

Thats my (very limited) understanding so far - Im looking at SPI, and spent a lot of time looking for a distribution hub or similar, then though to checked the amount of bits/transfer speed. In theory...

Gustav

Click to expand...

I designed an automatic mixer with 6 mono mic input channels and two stereo line inputs. The company I was designing for/with abandoned the digitally controlled analog automatic mixer project when the market embraced low cost digital consoles.  :'(

While they were dragging their feet, I built my own prototype with both VCA and DPOTs available to compare results. I fired up the DPOTs first and they worked so well I didn't even bother to finish populating the VCAs. To bring this on topic I wanted to control both Left and Right level for the stereo channels simultaneously so used the trick of using both A sides of a dual DPOT for one channel, and both B sides for the other channel so updates could be simultaneous.

I was controlling 10 DPOTs total so I rolled my own SPI routine in a microprocessor so I could perform updates quickly enough for audio gain control. This was years ago so details are not fresh in my mind, but something like 1 common spi clock to drive data to all the DPOTS,  I used two or three unique data sends, and three or so data latch lines to multiplex data to everything with as few as 3 pushes. In an automatic mixer, the loudest channels get priority for updates. 

In practice I could push data updates to the DPOTs fast enough to not hear artifacts while gain sharing between multiple channels in real time. I originally build in a zero crossing synchronization capability to reduce audibility of gain changes but didn’t seem to need that (but the product never reached the market so I probably would have refined it further).

For only a handful of DPOTs without update speed constraints of real time gain control you might be able to handle all with a stock I2S distribution scheme.

As has already been suggested, a single DPOT with a transfer gate switch to select between boost /cut could keep it simpler.

JR

PS: To use 2x DPOTs to mimic a single center tapped pot, send separate feed resistors to each DPOT wiper.