Re: Anti-aliasing filtering for interleaved sampling approach
- From: Jerry Avins <jya@xxxxxxxx>
- Date: Tue, 12 Feb 2008 22:16:02 -0500
Alexz wrote:
Hello guys and girls, my first posting here.
On some other, hardware design-related forum there was a question raised
by someone whose goal is to built a sampling system to achieve 500 MHz
sampling rate by means of two 250 Msps ADCs while driving them with
samlping closk featured by 180 deg. phase shift. So that in time domain
would be sampling by two channels in interleaved manner. Then the entire
sampled sequence is to be composed from two memory buffers for further
processing.
My question is as follows: as we all know, sampling theory requires
anti-aliasing filtration (an LPF at half or lower of samlping rate) in
order to avoid distorting the sampled original spectrum by aliased
components.
If considering each cnannel (out of two) separately (as physycal sampling
is attained), it appears we need to have 125 MHz (half of 250 Msps rate)
LPF in analog domain prior to sampling to act as anti-aliasing filter.
However, that approach apparently defeats the whole purposes of the system
- to achieve effective rate of 500 MHz (i.e. effective bandwidth of 250
MHz).
On the other hand, if we assess the entire system in the whole, we know
that we intend to obtain 500 MHz samlping rate, so that the analog
anti-alias filtration should assume 250 MHz cut-off LPF rather then 125
MHz as if we would follow the previous approach. However, in such case we
may impact the spectral content of each separately sampled (physically)
channel.
So, what should be the recommended (or correct) aprpoach to anti-alias
filtration in that particular case ?
I don't understand your confusion. After you have collected your 500M samples in a second and arrayed them in time-serial order, it doesn't matter a whit (in theory) how they were obtained. For all that matters, there could be 500 samplers, each collecting one sample per second. For the sequence to be free of aliases, there can be no components as high as 250 MHz.
Practical considerations impose several constraints.
In order to ensure that components at and above 250 MHz are suitably diminished, the practical upper limit of the analog anti-alias filters' cutoff will be about 200 MHz.
The signals captured by the several samplers must not have any differential lag. That is easily achieved by using a single filter and making the leads from it to the samplers about the same length.
The clock phases must be accurately enough spaced so that the signal changes a small fraction of a LSB. Clock jitter is bad enough. Periodic clock jitter will produce aliases that may be troublesome.
Moreover, the samplers must be closely matched for response time, analog frequency response, offset, gain, and linearity. Obviously, fewer samplers minimize the difficulty. Two are used when there is no alternative. Back when ADCs were trimmed by hand, one system using four was built and made to work in the laboratory. Even matching two samplers is not a project to be undertaken lightly.
Jerry
--
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