[sdiy] Micro as a Linear to Exponential converter?

[Colin f]

> Can I just confirm one thing...
> 
> >I've used a PIC driving an LTC1597 DAC, followed by a 4051 
> then LT1112
> >op-amps to generate 8 linear CV outputs at 0.5ms refresh rate, with
> >RC-shaped portamento done in software, with good results.
> 
> So you'd drive the 4051 with  e.g. a Digital I/O pin from the 
> PIC right? I'm guessing that the frequency/speed that would 
> give you would not cause any noticeable fluctuations in the 
> CV by the time it came out of the op-amps on the outputs of your Mux?

You may need to use a level shifter between a PIC and a 4051 if you're
powering the 4051 from >5v, but I was using a ~4 volt DAC reference, so was
able to drive it direct.
The PIC drives the DAC and the mux.
The 4051 is turned off, the DAC is written with the value for the next
channel to be refreshed, there's a slight delay for the op-amp in the DAC
i/v convertor to arrive at the right place, then the 4051 output is enabled.
The output of the mux is a poly cap in front of an LT1112 voltage follower.
I would have put a single pole smoothing filter on the output of the op-amp,
but in the application in question, there is already filtering on the CV
input to the VCO, so I didn't bother.
All 8 channels are refreshed in less than 0.5ms.

The important thing is to keep out any offset voltage.
In a linear VCO, that becomes a scaling error, which you can't fix just by
using the tune knob.
The LT1112 has a maximum offset of 60uV, about 2% of the error on the best
TL072.
You can see the board in question at the bottom of this page:
http://www.sequentix.com/gx1/

Cheers,
Colin f