Good designs of Expo Converters? Was: [sdiy] Multiplexed Expo Converter?

[Antti Huovilainen]

On Mon, 3 Aug 2009, cheater cheater wrote:

> So you can't really adjust the signal going from the EC into the 
> demultiplexer.

Obviously you do the oversampling (digital bandlimiting) in digital domain 
and then use analog lowpass filter for the post S&H filteering.

> I think the idea is to simply use a smoothing filter. What about a 4th
> order thingie at 0db=FS/2?

1. Overkill
2. Reaching 0dB is next to impossible (and pointless). -0.1dB is feasible.
3. Major issues with overshoot and ringing

You want to avoid overshoot and so you're stuck with critically damped 
filters which means more or less just cascaded RC stages (even if using 
one opamp to implement two of them).

> I bet this sort of thing has to exist in chip form already, I mean we're 
> not really discovering some new technology here.

Yes. It's called "Oversampling delta-sigma DAC". Point being that it's 
much cheaper _in commercial quantities_ to put stuff in digital domain and 
keep the analog side  simple. For DIY somewhat less so. Still, if you use 
linear CV control, you can do the exponential conversion in digital and 
can then filter the S&H outputs with say 2nd order lowpass filters or add 
more oversampling and use combined 1st order lowpass / V-I-converter.

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova