On 4/25/12 12:54 PM, steveu wrote:>> On 4/24/12 9:40 AM, steveu wrote:...>>> That's a good page, who's critically important part is >>> > http://www.analog.com/library/analogDialogue/Anniversary/Graphics/figure59lg.gif >>> . You want a capacitor between the inputs of a size that gives appropriate >>> rolloff in combination with the resistors. This filter has almost no >>> ability to inject differential noise, so its very tolerant of board layout. >>> Then you want two very small capacitors from the 2 inputs to ground, to >>> bypass RF....>> >> i'm curious what the RC time constant comes out to be. if the two R's >> are the same and the three C's are the same, is the corner frequency >> equal to >> >> 1/(2*pi*(2R)*(3/2 C)) ? >> >> the two R's are in series and the C across the diff input is in parallel >> to the two other C's in series. is that how it works out? > > Why would you make the three Cs the same? To get lousy results? I think you > should stick with what I wrote. :-)in order to do that, i have to read it carefully. i just went directly to the picture and the caps were unmarked, so i assumed they were the same. if the two C's in series are small, then we can forget about them. -- r b-j rbj@audioimagination.com "Imagination is more important than knowledge."
Anti-Aliasing filters with sigma-delta A/Ds
Started by ●April 24, 2012
Reply by ●April 25, 20122012-04-25
Reply by ●April 26, 20122012-04-26
On Apr 25, 1:11�pm, robert bristow-johnson <r...@audioimagination.com> wrote:> On 4/25/12 12:54 PM, steveu wrote: > > >> On 4/24/12 9:40 AM, steveu wrote: > ... > >>> That's a good page, who's critically important part is > > >http://www.analog.com/library/analogDialogue/Anniversary/Graphics/fig... > >>> . You want a capacitor between the inputs of a size that gives appropriate > >>> rolloff in combination with the resistors. This filter has almost no > >>> ability to inject differential noise, so its very tolerant of board layout. > >>> Then you want two very small capacitors from the 2 inputs to ground, to > >>> bypass RF. > > ... > > > > >> i'm curious what the RC time constant comes out to be. �if the two R's > >> are the same and the three C's are the same, is the corner frequency > >> equal to > > >> � � 1/(2*pi*(2R)*(3/2 C)) �? > > >> the two R's are in series and the C across the diff input is in parallel > >> to the two other C's in series. �is that how it works out? > > > Why would you make the three Cs the same? To get lousy results? I think you > > should stick with what I wrote. :-) > > in order to do that, i have to read it carefully. �i just went directly > to the picture and the caps were unmarked, so i assumed they were the > same. �if the two C's in series are small, then we can forget about them. > > -- > > r b-j � � � � � � � � �r...@audioimagination.com > > "Imagination is more important than knowledge."This might be more information than you need, but ...... There are 2 design methods for sigma delta converters, switched cap and continuous time. The switched cap variety often directly sample the input pins onto internal caps that might be fairly large ( up to 10 pf). In this case the input sampling will directly alias signals near the over sampling clock rate with little attenuation. You can usually figure out the over sampling clock rate from the data sheet. The other type of converter (continuous time modulator) presents a resistive input to the user and has an inherent sinc(x) roll off near the over sampling clock rate. So it is easier to drive and requires less anti-aliasing than switched cap designs. Bob
