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

[Antti Huovilainen]

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"
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