Using Digital Potentiometers

[grapefruit]

Does anyone have experience in using digital potentiometers?
I'm looking at using the microchip MCP42050. The data sheet says the voltages ranges for the pot terminals are from GND to Vdd. I'm quite happy to run that section of the circuit off 5V and use rail-rail op amps and a 2.5V half rail. I'll need the 5V for the micro anyway... But before I order some from overseas I just wanted to check if, in reality, they will actually handle ac signals all the way between GND and Vdd, and if there's any other thing to be aware of. I'll be using the pots to contol the frequency of state variable filters.

Regards,
Stew.

[MetalGuy]

I'm looking for a similar project from a long time without success. I even bought a book on microcontrollers only to confirm what I knew in advance - this is not for me. I mean I can learn it but for what? To spend month to build couple of projects? No thanks! I would prefer to pay somebody to do it for me.
Is there anybody interested /against reasonable amount of money/ in designing a project incliding 10-15 digital pots controlled via rotary encoders so we can get something similar to a programmable floor effects unit but using our analog effects instead. I'll be more than happyin being able to build up to 9-10 patches indicated by an LCD. Ready to solder PCBs and fully programmed PICs /or whatever/ can be offered separately or as kits.
Any takers?

[niftydog]

Digital pots will handle AC signals pretty much rail to rail. I have done this and it works quite nicely.

Something for both of you to be aware of, however, is that the maximum allowable wiper current is usually very low - around 1mA for most. While this is ok for AC signals (because they average zero) it's no good for DC usually - unless the DC is "buffered" by a high input impedance op amp or similar. This fact complicates the process of using them to "replace" conventional pots in stompboxes - as if it weren't already complicated enough!

Another potential pitfall is zipper noise (google it for more info.) I don't know what you're planning, grapefruit, but if you're hoping for a smooth, glitch free variable attenuator you're likely going to be dissapointed. You can get zero crossing detecting digipots, but this only reduces the noise, it doesn't eliminate it.

MetalGuy, I and many others love this idea, but it's just a little too much effort for the common joe at the moment - I'm yet to see anyone here implement this idea. I am intrigued however about the programmable BDDI and how they acheive this - but I ain't about to buy one just to find out!

[TELEFUNKON]

if there's any other thing to be aware of

zipper noise & limited resolution (# of steps)

[MetalGuy]

Looking at Marshall's JMP-1 schematic it doesn't seem very difficult to implement this in practice - obviously it's already done. Look at all those other digital FX units and amps.
About the effort that would go into it maybe you're right - it's not a one day project, but I'm sure that some of the people visiting this forum  are quite skilfull as far as these matters are concerned. Maybe other reasons not to see such projects are involved as well. I don't know.
Other than that I can live with with this zipper noise and I don't think 256 steps of attenuation  is low resolution. After all nothing is perfect, isn't it?

[Paul Perry (Frostwave)]

I was just looking at the specs for an old Analog Devices digital pot with memory, AD5232. I notice the 'cycle' life is 700,000 cycles, which sounds  lot....... until I thought, what if I used it as a tremolo element (yeah, maybe I LIKE zipper noise..) well, driving the pot with a LFO rate, looks like it would only last a week! but, I could well be completely wrong in my understanding of the operation of these things. I'd love to hear from anyone who knows how they behave under continual operation.

[grapefruit]

Thanks for the replies.
Zipper noise and resolution may not be a major problem in my application. I might get a few and see how it goes.

Stew.

[Brett Sinclair]

Heya,

I tried this using the Analog Devices AD5204 series and an 18F PIC running MIOS (www.ucapps.de)
The ucapps Midibox platform stuff is awesome as it's a modular MIDI-oriented project that can be used for synths, sequencers or just plain midi-controllers.
You can order PCB's from Europe or US...

You can download the simple MIOS application i wrote here http://compiler.kaustic.net/mios_udf/

I tried it with a big muff circuit and apart from the "zipper noise" (and considering the gain of the muff) it was actually pretty quiet (not a lot of annoying clock noise or other mixed digital/analog issues)...

cheers!

[Paul Perry (Frostwave)]

Brett, you are obviously up to speed on this stuff, is there any problem with the  "700,000 cycle" life? Does 'cycle' refer to the continual resetting you would experience, driving the unit to make a trem for example?

[grapefruit]

I haven't read the AD5232 datasheet but I'd be guessing that the cycle life is only a problem with the devices that have memory. You'd probably want to remember the setting in the micro eeprom anyway, so it wouldn't be an issue. There's plenty of digital pots that don't have memory. The reason I like the microchip ones is that they come in DIP and SOIC packages. A lot of the Maxim ones only come in TSSOP package, which is a little tricky to solder. And they're under $2 for a dual unit.

After all this (H11F3 optos, PWM, and digital pots) I'm thinking maybe OTA's are the best way to make multiple filters track each other. The noise is what put me off in the first place but there doesn't seem to be a decent way to do it otherwise.

[Brett Sinclair]

Hi Paul, i don't know about the cycle-life thing. The AD5204 i used have no memory, on powerup they reset to mid wiper position.
I haven't tried doing some LFO stuff.. yet.... 

I have used the Microchip ones as well, but i prefer the AD5204 ones, they can be daisychained easily and analog is on one side of the ic and digital on the other.

[MetalGuy]

Brett: Excellent site and projects. Only one problem  - MIDI is a black hole for me. Tried couple of times to grasp the concept without any luck. Maybe because I don't need it at all.
My needs are simple - a tube or hybrid preamp - EQ- noise gate - chorus - flanger - delay - reverb. This makes around 15  pots to control and settings to memorize and 10-20 switches/relays to control. A 0 to 9 LCD showing the number of  patches. Or I can put it this way - controlling digitally /via the  pots/  our analog effects. No MIDI or whatever. Is any of those projects capable of doing this and which one? Is it a big trouble to modify any of those to make it perform the functions I mentioned? Thanks!

[Sambo]

http://tomscarff.tripod.com/

Try this for midi designs. My fx switcher (16 loop effects switcher controlled by midi) is based around it in part. nice and easy to understand and is a nice starter

[MetalGuy]

Thanks for the link to that nice site but as I already mentioned I'm looking for a MIDI-less project..

[Brett Sinclair]

Brett: Excellent site and projects. Only one problem  - MIDI is a black hole for me. Tried couple of times to grasp the concept without any luck. Maybe because I don't need it at all.
My needs are simple - a tube or hybrid preamp - EQ- noise gate - chorus - flanger - delay - reverb. This makes around 15  pots to control and settings to memorize and 10-20 switches/relays to control. A 0 to 9 LCD showing the number of  patches. Or I can put it this way - controlling digitally /via the  pots/  our analog effects. No MIDI or whatever. Is any of those projects capable of doing this and which one? Is it a big trouble to modify any of those to make it perform the functions I mentioned? Thanks!

Everything and more is possible using the modules found on the ucapps.de website.
The beauty of the platform Thorsten (the guy behind it all) is that his "application framework" lets you handle any event you wish having when designing a digitally or midi controlled synth, effect or whatever... It allows for handling buttons, pots, encoders, driving leds, LCD displays, numeric led displays, graphic LCD displays, CV outputs, Commodore 64 SID synth chips, anything with a SPI interface etc.etc.etc.

What you don't need, you don't use... You can hook up only a few modules, or link multiple PIC cores and several LCD screens and a desk full of motorised faders.
Writing a MIOS application can be so simple and complex as you like. It can take a lot of time if you are new to programming assembler though.

You will need MIDI to connect the PIC board with your PC, because this offers a handy way of doing firmware updates and makes development very easy. No need to reburn the PIC everytime you change a line of code...

If you choose to check it out, be warned... it can be as addictive as building stompboxes...