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

Started by hrh1818 July 27, 2009
hrh1818 wrote:

> On Jul 29, 2:09&#2013266080;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. &#2013266080;Warning the document is >> > 31 pages long, only click on the link if you have a broadband internet >> > connection. &#2013266080;http://focus.ti.com/lit/an/slaa409a/slaa409a.pdf. &#2013266080;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. &#2013266080; Furthermore on page 6 the >> > document says the sampling frequency is 4096 samples per second which >> > gives a Nyquist frequency of 2048 cps. &#2013266080; 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&#2013266172;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&#2013266172;sse -- Regards F. Seuhs Mailto: friedrich.seuhs@hasos.com