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
Reply by robert bristow-johnson●April 25, 20122012-04-25
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."
Reply by steveu●April 25, 20122012-04-25
>On 4/24/12 9:40 AM, steveu wrote:
>>> On 24.04.2012 8:38, HardySpicer wrote:
>>>> Do you still require an anti-aliasing filter with sigma-delta A/D's?
I
>>>> am guessing yes but very low order.
>>>> A/D I am testing seems to have an excellent roll-off without any
>>>> filter at all.
>>>>
>>>>
>>>> Hardy
>>>
>>> A simple RC filter would suffice in most cases. You may find some
>> . 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. Without those you may have problems when you try irradiating
the
>> board with a few hundred MHz. The main filter just can't cope with that
>> because a) the capacitor across the inputs is probably of the kind of
value
>> that sucks at RF, and b) the differential quality of the chip's inputs
>> falls apart at sufficiently high frequencies.
>
>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. :-)
>the one thing that worries me about a diff input driven by two separate
>op-amp buffers is a mismatch of gain between the two buffers. if
>they're mismatched, you won't have much CMRR. maybe if the op-amps were
>true envelope followers (rather than inverting amps as depicted) there
>would be little of that.
Steve
Reply by Allan Herriman●April 25, 20122012-04-25
On Tue, 24 Apr 2012 11:04:04 -0400, robert bristow-johnson wrote:
> On 4/24/12 9:40 AM, steveu wrote:
>>> On 24.04.2012 8:38, HardySpicer wrote:
>>>> Do you still require an anti-aliasing filter with sigma-delta A/D's?
>>>> I am guessing yes but very low order.
>>>> A/D I am testing seems to have an excellent roll-off without any
>>>> filter at all.
>>>>
>>>>
>>>> Hardy
>>>
>>> A simple RC filter would suffice in most cases. You may find some
>>> useful information in this article:
>>> http://www.analog.com/library/analogDialogue/Anniversary/15.html
>>>
>>> --
>>>
>>> Alexander
>>
>> 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. Without those you may have problems when you
>> try irradiating the board with a few hundred MHz. The main filter just
>> can't cope with that because a) the capacitor across the inputs is
>> probably of the kind of value that sucks at RF, and b) the differential
>> quality of the chip's inputs falls apart at sufficiently high
>> frequencies.
>
> 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?
Yes, for differential mode signals (i.e. the signals you want).
For common mode signals, the time constant = RC
> the one thing that worries me about a diff input driven by two separate
> op-amp buffers is a mismatch of gain between the two buffers. if
> they're mismatched, you won't have much CMRR. maybe if the op-amps were
> true envelope followers (rather than inverting amps as depicted) there
> would be little of that.
1% resistors are very cheap (< 1 cent), whereas capacitors don't usually
have a tolerance of better than 10% or so. 1% is possible, but
expensive. Mismatch between the two caps to ground will result in common
mode to differential conversion that is frequency dependent.
This may not matter if you are only interested in a restricted band, e.g.
< 20kHz, but watch out for your switching power supply (50-500 kHz)
ending up in-band after sampling.
Regards,
Allan
Reply by HardySpicer●April 25, 20122012-04-25
On Apr 25, 10:10�am, Tim Wescott <t...@seemywebsite.com> wrote:
> On Tue, 24 Apr 2012 14:37:17 -0700, HardySpicer wrote:
> > On Apr 25, 1:48�am, Tim Wescott <t...@seemywebsite.please> wrote:
> >> On Mon, 23 Apr 2012 21:38:36 -0700, HardySpicer wrote:
> >> > Do you still require an anti-aliasing filter with sigma-delta A/D's?
> >> > I am guessing yes but very low order.
> >> > A/D I am testing seems to have an excellent roll-off without any
> >> > filter at all.
>
> >> You need an anti-alias filter, but it only needs to hold the aliases
> >> down at the sigma-delta sampling rate (which is fast), and have a
> >> rolloff that is acceptable at the speed at which you sample out of the
> >> ADC (which is slow).
>
> >> So generally an RC is enough, although of course you should still think
> >> about it.
>
> >> --
> >> Tim Wescott
> >> Control system and signal processing consultingwww.wescottdesign.com
>
> > Can you not therefore rely on the "natural" roll-off due to amplifiers
> > etc if you are dealing with "low" frequencies. eg my frequencies are
> > only up to 20kHz
> > and my clock is 40MHz.
>
> Very possibly. �Make sure you don't have any late-stage high frequency
> noise being introduced, and if not -- go to town.
>
> --
> My liberal friends think I'm a conservative kook.
> My conservative friends think I'm a liberal kook.
> Why am I not happy that they have found common ground?
>
> Tim Wescott, Communications, Control, Circuits & Softwarehttp://www.wescottdesign.com
so convenient these Sigma-Delta A/Ds. Best things since sliced FPGAs!
Hardy
Reply by Tim Wescott●April 24, 20122012-04-24
On Tue, 24 Apr 2012 14:37:17 -0700, HardySpicer wrote:
> On Apr 25, 1:48 am, Tim Wescott <t...@seemywebsite.please> wrote:
>> On Mon, 23 Apr 2012 21:38:36 -0700, HardySpicer wrote:
>> > Do you still require an anti-aliasing filter with sigma-delta A/D's?
>> > I am guessing yes but very low order.
>> > A/D I am testing seems to have an excellent roll-off without any
>> > filter at all.
>>
>> You need an anti-alias filter, but it only needs to hold the aliases
>> down at the sigma-delta sampling rate (which is fast), and have a
>> rolloff that is acceptable at the speed at which you sample out of the
>> ADC (which is slow).
>>
>> So generally an RC is enough, although of course you should still think
>> about it.
>>
>> --
>> Tim Wescott
>> Control system and signal processing consultingwww.wescottdesign.com
>
> Can you not therefore rely on the "natural" roll-off due to amplifiers
> etc if you are dealing with "low" frequencies. eg my frequencies are
> only up to 20kHz
> and my clock is 40MHz.
Very possibly. Make sure you don't have any late-stage high frequency
noise being introduced, and if not -- go to town.
--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?
Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
Reply by HardySpicer●April 24, 20122012-04-24
On Apr 25, 1:48�am, Tim Wescott <t...@seemywebsite.please> wrote:
> On Mon, 23 Apr 2012 21:38:36 -0700, HardySpicer wrote:
> > Do you still require an anti-aliasing filter with sigma-delta A/D's? I
> > am guessing yes but very low order.
> > A/D I am testing seems to have an excellent roll-off without any filter
> > at all.
>
> You need an anti-alias filter, but it only needs to hold the aliases down
> at the sigma-delta sampling rate (which is fast), and have a rolloff that
> is acceptable at the speed at which you sample out of the ADC (which is
> slow).
>
> So generally an RC is enough, although of course you should still think
> about it.
>
> --
> Tim Wescott
> Control system and signal processing consultingwww.wescottdesign.com
Can you not therefore rely on the "natural" roll-off due to amplifiers
etc if you are dealing with "low" frequencies. eg my frequencies are
only up to 20kHz
and my clock is 40MHz.
Hardy
Reply by robert bristow-johnson●April 24, 20122012-04-24
On 4/24/12 9:40 AM, steveu wrote:
>> On 24.04.2012 8:38, HardySpicer wrote:
>>> Do you still require an anti-aliasing filter with sigma-delta A/D's? I
>>> am guessing yes but very low order.
>>> A/D I am testing seems to have an excellent roll-off without any
>>> filter at all.
>>>
>>>
>>> Hardy
>>
>> A simple RC filter would suffice in most cases. You may find some useful
>> information in this article:
>> http://www.analog.com/library/analogDialogue/Anniversary/15.html
>>
>> --
>>
>> Alexander
>
> 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. Without those you may have problems when you try irradiating the
> board with a few hundred MHz. The main filter just can't cope with that
> because a) the capacitor across the inputs is probably of the kind of value
> that sucks at RF, and b) the differential quality of the chip's inputs
> falls apart at sufficiently high frequencies.
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?
the one thing that worries me about a diff input driven by two separate
op-amp buffers is a mismatch of gain between the two buffers. if
they're mismatched, you won't have much CMRR. maybe if the op-amps were
true envelope followers (rather than inverting amps as depicted) there
would be little of that.
--
r b-j rbj@audioimagination.com
"Imagination is more important than knowledge."
Reply by Tim Wescott●April 24, 20122012-04-24
On Mon, 23 Apr 2012 21:38:36 -0700, HardySpicer wrote:
> Do you still require an anti-aliasing filter with sigma-delta A/D's? I
> am guessing yes but very low order.
> A/D I am testing seems to have an excellent roll-off without any filter
> at all.
You need an anti-alias filter, but it only needs to hold the aliases down
at the sigma-delta sampling rate (which is fast), and have a rolloff that
is acceptable at the speed at which you sample out of the ADC (which is
slow).
So generally an RC is enough, although of course you should still think
about it.
--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Reply by steveu●April 24, 20122012-04-24
>On 24.04.2012 8:38, HardySpicer wrote:
>> Do you still require an anti-aliasing filter with sigma-delta A/D's? I
>> am guessing yes but very low order.
>> A/D I am testing seems to have an excellent roll-off without any
>> filter at all.
>>
>>
>> Hardy
>
>A simple RC filter would suffice in most cases. You may find some useful
>information in this article:
>http://www.analog.com/library/analogDialogue/Anniversary/15.html
>
>--
>
>Alexander
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. Without those you may have problems when you try irradiating the
board with a few hundred MHz. The main filter just can't cope with that
because a) the capacitor across the inputs is probably of the kind of value
that sucks at RF, and b) the differential quality of the chip's inputs
falls apart at sufficiently high frequencies.
Steve