The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Mixed-Signal Design >> comparator and pads in a sigma delta modulator
https://designers-guide.org/forum/YaBB.pl?num=1184966134

Message started by filipe on Jul 20^th, 2007, 2:15pm

Title: comparator and pads in a sigma delta modulator
Post by filipe on Jul 20^th, 2007, 2:15pm

To supply a Sigma Delta Modulator, I have two separated sources: an analog and a digital voltage supply. where should I supply the compartor??? With the VDD from digital or analog Supply??? Why?? Have you some references for this???

And the VDD of the PADs, where should I connect it??? VDDa or VDDd??? Why???

Thanks

Title: Re: comparator and pads in a sigma delta modulator
Post by Berti on Jul 23^rd, 2007, 10:17pm

You need the seperate your circuit blocks into agressors and victims. Since the comparator
decision might be influence by spikes on the supply I usually connect the comparator to
the analog supply (and provide local decoupling to avoid switching noise propagate through
the circuit).
The pad-ring is usually splited in an analog and a digital section (or even more sections
for larger chips). However, if you don't want to split the pad-ring you can still supply the
pad-ring with a seperate supply and provide analog and digital supply directly to the core.

Hope this helps, regards.

Title: Re: comparator and pads in a sigma delta modulator
Post by filipe on Jul 26^th, 2007, 11:59am

Yes, it has been very helpful.
In the case of the local decoupling to the comparator. Should I put a capacitor in parallel with the comparator source??? Which order???
Thanks

Title: Re: comparator and pads in a sigma delta modulator
Post by Berti on Jul 26^th, 2007, 10:17pm

For good decoupling you should put a resistor between the sensitive supply and
your comparator supply (the resistors decouples the two circuits). However, often
the speed of the comparator is critical and one cannot tolerate any voltage drop
over a resistor.
Therefore I recommenend to add only capacitance between vdd and gnd. Make
sure that the capacitance is close to the source (in order to keep the switching
noise localy). The capacitance value will depend on your comparator.

Regards