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

[cheater cheater]

Isn't the S&H going to be 'analog' though, just discrete time?
So you can't really adjust the signal going from the EC into the demultiplexer.
I think the idea is to simply use a smoothing filter. What about a 4th
order thingie at 0db=FS/2?
I bet this sort of thing has to exist in chip form already, I mean
we're not really discovering some new technology here.

Cheers
D.

On Mon, Aug 3, 2009 at 1:28 PM, Antti Huovilainen<ajhuovil at cc.hut.fi> wrote:
> On Sun, 2 Aug 2009, Scott Nordlund wrote:
>
>>> The other important question is: where do you get good oversampling
>>> filters that would do a good job of reconstructing the S&H'd signal
>>
>> Um... oversampling analog filters?  Though the sample and hold output is
>> discrete time, it's not digital, so I'd be curious to see what that
>> entails..
>
> You limit the highest frequency present in the digital signal to N*2 times
> below the samplerate. This means digital filtering (except for sine which is
> trivial case).
>
>> You probably don't want a filter anyway unless you're explicitly going for
>> audio rate modulation or VCF/VCA signals.
>
> You do. Just set a simple RC filter time constant so that it gets within
> 10-20% of the final value in one sample. This will get rid of quite a bit of
> the steppiness.
>
> If you have dynamic range to burn (say, you use 16 bit dac), you can extend
> the DAC range about 30% above and below, set RC time constant to 1-2 samples
> and use elementary 1st order differential equations to adjust the digital
> part so that you get smooth interpolation of the output while keeping
> ability to have down to 1 sample attack times.
>
> Antti
>
> "No boom today. Boom tomorrow. There's always a boom tomorrow"
>  -- Lt. Cmdr. Ivanova
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