Interfacing CVs and microcontrollers.

[reve]

So I'd like to interface my modular CV signals to a microcontroller based LFO. The microcontroller ADC is expecting 0-5v positive. My envelopes are going 0-10 positive and my analog LFOs are +/- 5v. I'd like to handle the input as efficiently as possible, so I was hoping to pick some of your collective brains.

It seems like a voltage divider followed by a diode clamp would get the incoming voltages where I need them to be. But I also want manual control – so I would also want a pot wired to 5v and ground. And I might want multiple CV inputs.

Aaaannnnd that's where I run into trouble. As I already have four opamps on the outputs of my simple digital project, I was hoping to get out of constructing like, a DC mixer. Is there a way around that? Could I just use a couple zeners and just straight clip the voltage or something to keep inside the 0-5 box?

Doing it right is more important than doing it easy here, but... I’d like both right and easy if all y'all have any suggestions.

[clee]

You're going to have to build a DC mixer if you want to mix CV inputs. Or bring each CV input to a PIC pin and share the A/D converter (the harder of the 2 I would think).

As far as protecting the PIC from overvoltage, I'm having the same issue with a Quantizer I'm building, I'd be interested in some ideas others have. Just using a zener and a resister will give you a very non-linear response near 5 volts.

Here are 2 bits of info:

This is nice, but the voltage break is changed by the input impeadance
http://books.google.com/books?id=1NcSBP2OA-QC&pg=PT278&lpg=PT278&dq=precision+clipper+op+amp&source=bl&ots=oLn5X37G35&sig=IFR5vOZfbEpnXTR4WEpw9TSXo2A&hl=en&ei=UH8RTLaeJZCUMvPC7cgF&sa=X&oi=book_result&ct=result&resnum=6&ved=0CDAQ6AEwBQ#v=onepage&q=precision%20clipper%20op%20amp&f=false

and this tread seems to be saying don't worry about it

http://www.electro-tech-online.com/micro-controllers/22046-i-o-pin-over-voltage-protection.html

but I'm not so sure...

[PickNick]

hi!

here is some info about modified Edruid LFO/sync in, FM in...:

http://www.acoupel.com/LFO/Schema/Schema%20LFO%20edruid.htm

and the original schema with optional external CVins:

http://www.electricdruid.net/forums.html?page=projects.lfo9

[reve]

Thanks for the responses, guys. Extra thanks to Wavemultiplier... I see now the problems with the nonlinearity in the zener solution -- it winds up working as a voltage divider. Boo. Hiss.

That said, it looks like both AVRs and PICs have internal protection diodes in place as long as you use a current limiting resistor in series. The bad is that it winds up biasing the other ADC inputs when the protection diodes kick in. :/

So here's my crazy idea: (for my scenario) we've gotta build a DC mixer, ya? So I've got an inverting opamp followed by a voltage follower. What if I used a rail to rail op amp powered by 0v/+5v as the voltage follower? A regular opamp would work for protecting the micro from overvoltage, but... it'd mess up the scaling, which needs to be real close to 0-5v.

Would that work?

[PickNick]

for -5/+5 cvin maybe?

http://www.acoupel.com/sequencer/schemas/ACS%20Sequencer%20schema.pdf

for multiple 1v/oct cvins///0-8v:

http://www.electricdruid.net/datasheets/VCLFO9DDatasheet.pdf

sorry if i misunderstood something

[clee]

Here is what I came up with the other day. I have not actually hooked this up to a PIC yet, the person doing the programing has it, but it SHOULD work. Resister values may change.

The circuit is putting out about -1 volt when going negative right now. I need to try a schottkey diode for D1 and see if it gets it to -.3v

R6 sets the CV from 0 to 5v R7 sets CV IN to no gain at the center, full gain with the wiper at CV IN and Inverted full gain with the wiper at GROUND

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[Rykhaard]

[clee]

So, don't build the circuit I posted. The 2 pots interact with each other. The clipper does work though.

Craig

[Rykhaard]

[Junk Rhythm]

[Rykhaard]