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Design >> Mixed-Signal Design >> sigma delta digital output?
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Message started by chungmnig on Apr 20^th, 2006, 12:25am

Title: sigma delta digital output?
Post by chungmnig on Apr 20^th, 2006, 12:25am

I has a question about the digital output of comparator.
Like attached file , the comparator has two output Vo+ and Vo-
Then i want to get the PCM signal , is only get Vo+ to do next step(FFT analysis)?
or get the differential output (Vo+) - (Vo-) to do next step?

I had already checked that , there are have some difference.
Which are the correct output , single ended or differential ?
or just see what application i used?
thanks~~!!

Title: Re: sigma delta digital output?
Post by shpongle on Apr 20^th, 2006, 5:18am

differential !

Title: Re: sigma delta digital output?
Post by chungmnig on Apr 20^th, 2006, 6:46am

Thanks for your reply~~!
You say differential , but why ?
Could you tell me the reason.
thanks!

Title: Re: sigma delta digital output?
Post by jbdavid on Apr 21^st, 2006, 12:45am

because thats how it was drawn?
differential signaling is ususally indcated when outputs are NOT rail-rail outputs (lower swing means Higher speed possible)
so the "real" signal is 1 if V(out+,out-) > 0 and 0 if V(out+,out-) <0

best way to tell is to look at the stage driving the comparator output ..
if its a diff pair driving a constant current or resistive load, you can't get rail-rail operation..
if its a Low Speed comparator (not very likely in signal path) then you can use complementary (something like an CMOS inverter)
outputs.. and the inverted version is just for convenience (probably less power than adding another inverter later?? but
for a circuit drawn this way, its NOT what I would expect if I push down into the comparator.

Title: Re: sigma delta digital output?
Post by vivkr on Apr 21^st, 2006, 2:19am

Hi David,

The comparator output(s) from a delta-sigma ADC need to be processed by a digital
filter that follows it. Additionally, these signals are also used to control the switching
of the DAC.

Keeping these two points in mind along with the fact that the signal sent to the DAC must
be the same as the one sent to the filter, would it not be strange if the comparator outputs
were not rail-rail digital signals ?

Regards
Vivek