Why use an anti aliasing filter with an energy meter.?

hrh1818 wrote:

> On Jul 29, 2:09&#4294967295;am, Friedrich Seuhs <friedrich.se...@hasos.com> wrote:
>> hrh1818 wrote:
>> > The following document says on page 5 an anti aliasing filter is used
>> > on the A/D input channels of an energy meter. &#4294967295;Warning the document is
>> > 31 pages long, only click on the link if you have a broadband internet
>> > connection. &#4294967295;http://focus.ti.com/lit/an/slaa409a/slaa409a.pdf. &#4294967295;My
>> > understanding is you only need an anti aliasing filter if you want to
>> > reconstruct the signal or find the frequency content of a signal.
>> > Neither criteria apply to an energy meter. &#4294967295; Furthermore on page 6 the
>> > document says the sampling frequency is 4096 samples per second which
>> > gives a Nyquist frequency of 2048 cps. &#4294967295; But the anti aliasing filter
>> > is a single pole filter with the *+3db point at approximately 5000
>> > hz.
>>
>> > Hence I ask did the designers make a minor mistake or am I missing
>> > something?
>>
>> Defenitly yes!
>>
>> You should have a look how a sigma delta ADC is working!
>>
>> The OSR (output sample rate) of 4096 samples per second is not the sample
>> rate of the modulator. Simply spoken you have a 1 bit ADC with a
>> samplerate of a multiple (which is called the oversampling ratio) which
>> is typically *32, *64, *128, *256 ... So your analog sampling rate is
>> much higher. I have not looked in detail on the descripion of the MSP
>> (slau056h.pdf), but the maximum modulator frequency (= analog sampling
>> rate) is about 1 MHz. For this frequency you need the analog
>> anti-aliasing filter!. Converting the 1bit-datastreem (1Mbit/sec) into a
>> slower 16-bit datastreem is done via a digital low pass filter. This
>> filter should removes the frequencys above the digital OSR / 2 (4096 Hz /
>> 2). The digital filter of a sigma delta ADC is what defines most of the
>> performance of a sigma delta ADC. The passband is not flat and the
>> attanuation in the stopband may be not big enough. The passband ripple is
>> typically between +-0.1 .. 0.005 db and this counts if you want to make
>> precise measurments.
>>
>> Hope this helps to understand the principle.
>>
>>
>>
>> > Howard
>>
>> --
>> Freundliche Gr&#4294967295;sse -- Regards
>> Friedrich
> 
> Friedrich, your response is very helpful in helping me understand why
> an an anti aliasing filter is used in this application.  However, your
> use of the term OSR confuses me.   The information I found uses the
> term OSR to mean over sampling ratio not output sampling rate. For
> example check the graph on page 54 of
> http://focus.ti.com/lit/ds/slas626a/slas626a.pdf. Warning this document is
> 82 pages long. Notice in the graph the best
> SINAD is at a high OSR.  Whereas if OSR meant output sampling rate
> then the best SINAD would be at a low output sample rate.
> 
> Howard

This is a problem of the 3-letter shortcuts. 
I defined it in my response above as "output sample rate".
At closer looks to the application notes and user's guide (sla056h.pdf) in
the TI-documents it is defines as "over sampling ratio".
The graph on page 54 of slas626a.pdf is very confusing, making the
impression a wide continous range of values are possible. But only the
discrete values 32, 64, 128, 256, 512, 1024 are possible (see slau056h.pdf
page 29-24).
Sorry for my misinterpretion of OSR in my first response.
-- 
Freundliche Gr&#4294967295;sse -- Regards
F. Seuhs
Mailto: friedrich.seuhs@hasos.com