FPGA Central

Hi !

I have the following question: If I use an AD7738 (Analog Devices) or
ADS1256 (TI) 24bit delta/sigma ADC with "fast channel switching" (I think
it's some mechanism to flush the digital filter so that you don't have to
throw away 6+ samples before you get a valid one), what effect on the anti-
aliasing filter requirements and transfer function does the channel
switching have ?

It works like this:
select Channel 1 -- wait for input stage to settle -- sample -- select
channel 2 - wait for input stage to settle -- sample ... and so on.

Which frequencies do I have to remove with the anti-aliasing filter now ?


Usually (without channel switching) an RC filter is often sufficient.
Only frequencies around multiples of 1/4 of the clock frequencies (ADS1256)
have to be removed. But is this really also the case when channel switching
is used ?

Please give some advice on literature, if possible.

Moritz v. Buttlar


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remove 123 for correct email address

Most of these "fast-switching" sigma-delta ADCs simply use an
decimation filter with a shorter impulse response, i.e. no longer than
the output word rate. The result is often a sinc^3 instead of a sinc^5
frequency response, which increases your noise bandwidth a little bit,
and makes the nulls less wide. Sometimes they use a composite filter
like sinc(f/f1)*sinc^3(f/f2), where the first factor is just there to
get a good 50/60 Hz null.

The resulting decimation filter "is what it is", and you don't have to
do anything special if you like what you see in the data sheet. As
always, your analog anti-alias filter needs to have good attenuation
at multiples of the modulator rate. In the case of multiple channels,
you should have multiple antialias filters. If you try to do muxing in
front of a single antialias filter, then your mux speed will be
limited by its step response.

David L. Rick
Hach Company

The "reply-to" address in my header has been disabled following the
last round of spam-bot attacks. Humans can now email me at
davidDOTrickAThachDOTcomREMOVE.



"Moritz v. Buttlar" <[email protected]> wrote in message news:<[email protected]>...
> Hi !
>
> I have the following question: If I use an AD7738 (Analog Devices) or
> ADS1256 (TI) 24bit delta/sigma ADC with "fast channel switching" (I think
> it's some mechanism to flush the digital filter so that you don't have to
> throw away 6+ samples before you get a valid one), what effect on the anti-
> aliasing filter requirements and transfer function does the channel
> switching have ?
>
> It works like this:
> select Channel 1 -- wait for input stage to settle -- sample -- select
> channel 2 - wait for input stage to settle -- sample ... and so on.
>
> Which frequencies do I have to remove with the anti-aliasing filter now ?
>
>
> Usually (without channel switching) an RC filter is often sufficient.
> Only frequencies around multiples of 1/4 of the clock frequencies (ADS1256)
> have to be removed. But is this really also the case when channel switching
> is used ?
>
> Please give some advice on literature, if possible.
>
> Moritz v. Buttlar