Help with a aplitude respons?
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From: Gery on 24 Aug 2006 03:24 Im looking at an aplitude response of an ADC fr?n Linear Technologies, LTC2440 (Figure 11 on page 19 in the data sheet). http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1155,C1001,C1152,P1771,D3135 The deep notches seems to come from a Sinc4 filter, but where are the other smaller notches comming from?
From: Gery on 24 Aug 2006 04:45 Now I have a working MATLAB file that generates the same amplitude response: ============================================================== OSR = 32768; fCLK = 9e6; Fs1 = fCLK/5; Fs2 = Fs1/OSR; Fs3 = Fs2/4; Fs4 = Fs3/2; f = linspace(0,240,10000); H1 = ( ( 1 - exp(1).^(-j*OSR*2*pi*f/Fs1) ) ./ ( OSR*(1 - exp(1).^(-j*2*pi*f/Fs1))) ).^4; H1_amp = 20*log10(abs(H1)); H1_phase = unwrap(angle(H1))*180/pi; H2 = 0.5*(1 + exp(-j*2*pi*f/Fs3)); H2_amp = 20*log10(abs(H2)); H2_phase = unwrap(angle(H2))*180/pi; plot(f,H1_amp+H2_amp), grid on axis([0 240 -140 0]) axis square hold on % Aliasing of f for i=1:length(f) f(i) = f(i) - Fs4*floor(f(i)/Fs4); if f(i) > (Fs4/2) f(i) = Fs4 - f(i); end end plot(f,H1_amp+H2_amp ,'r') legend('amplitude response','aliasing at output when reducing the rate') ============================================================== The ADC works at 1.8 MHz sampel rate and has an amplitude response of the generated graph. And then it sends out the samples at 6.87 Hz. And if im correct this causes aliasing so every multiple of 6.87 Hz folds back to DC. Why would they make an ADC like that? "Gery" <gery(a)ddd.com> wrote in message news:44ed54e2$0$75040$14726298(a)news.sunsite.dk... > Im looking at an aplitude response of an ADC fr?n Linear Technologies, > LTC2440 (Figure 11 on page 19 in the data sheet). > > http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1155,C1001,C1152,P1771,D3135 > > The deep notches seems to come from a Sinc4 filter, but where are the > other smaller notches comming from? > > >
From: Richard Owlett on 24 Aug 2006 06:42 Gery wrote: > Im looking at an aplitude response of an ADC fr?n Linear Technologies, > LTC2440 (Figure 11 on page 19 in the data sheet). > > http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1155,C1001,C1152,P1771,D3135 > > The deep notches seems to come from a Sinc4 filter, but where are the other > smaller notches comming from? > > > The plot is not "amplitude response" but "NORMAL MODE REJECTION".
From: Gery on 24 Aug 2006 06:48 "Richard Owlett" <rowlett(a)atlascomm.net> wrote in message news:12er0jq10d03a02(a)corp.supernews.com... > Gery wrote: > >> Im looking at an aplitude response of an ADC fr?n Linear Technologies, >> LTC2440 (Figure 11 on page 19 in the data sheet). >> >> http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1155,C1001,C1152,P1771,D3135 >> >> The deep notches seems to come from a Sinc4 filter, but where are the >> other smaller notches comming from? >> >> >> > > The plot is not "amplitude response" but "NORMAL MODE REJECTION". Ok (different name for the same thing i think). But it is the digital filters response.
From: Jerry Avins on 24 Aug 2006 10:24
Gery wrote: > "Richard Owlett" <rowlett(a)atlascomm.net> wrote in message ... >> The plot is not "amplitude response" but "NORMAL MODE REJECTION". > > Ok (different name for the same thing i think). But it is the digital > filters response. Not at all the same. Look up terms you don't understand. http://zone.ni.com/devzone/nidzgloss.nsf/webmain/0507703AE80CA60A8625687B006123BF?OpenDocument Jerry -- Engineering is the art of making what you want from things you can get. ??????????????????????????????????????????????????????????????????????? |