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?
Help with a aplitude respons?
Started by ●August 24, 2006
Reply by ●August 24, 20062006-08-24
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@ddd.com> wrote in message
news:44ed54e2$0$75040$14726298@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?
>
>
>
Reply by ●August 24, 20062006-08-24
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".
Reply by ●August 24, 20062006-08-24
"Richard Owlett" <rowlett@atlascomm.net> wrote in message news:12er0jq10d03a02@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.
Reply by ●August 24, 20062006-08-24
Gery wrote:> "Richard Owlett" <rowlett@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. �����������������������������������������������������������������������
Reply by ●August 24, 20062006-08-24
Gery wrote:> "Richard Owlett" <rowlett@atlascomm.net> wrote in message > news:12er0jq10d03a02@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. > > >Whoops - while reading " NORMAL MODE REJECTION" was 'thinking' " *COMMON* MODE REJECTION"
Reply by ●August 24, 20062006-08-24
Jerry Avins wrote:> > Gery wrote: > > "Richard Owlett" <rowlett@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?OpenDocumentNope wrong again. That does appear to be the amplitude response. They are just advertising what this particular amplitude response is good for. -jim ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =----
Reply by ●August 24, 20062006-08-24
jim wrote:> > Jerry Avins wrote: >> Gery wrote: >>> "Richard Owlett" <rowlett@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 > > Nope wrong again. That does appear to be the amplitude response. They > are just advertising what this particular amplitude response is good > for.Frequency response of a device is related to its output as a function of its excitation frequency. Its normal mode rejection has nothing to do with the device's excitation, but rather to the effect of induced noise on its connecting circuitry. Those are different in my view. Noise at power-line frequency and its harmonics is apparently canceled by integrating the signal for a whole number of periods of the line. Such integration affects noise and signal equally. Underlying that is the native frequency response of the device itself. It can be a serious mistake to confound the two. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●August 24, 20062006-08-24
Jerry Avins wrote:> Frequency response of a device is related to its output as a function of > its excitation frequency. Its normal mode rejection has nothing to do > with the device's excitation, but rather to the effect of induced noise > on its connecting circuitry. Those are different in my view.The graph shown is frequency response of the filtering of the delta sigma ADC. The graph shows amplitude vs. frequency - You can dance all around that fact if you wish. -jim ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =----
Reply by ●August 25, 20062006-08-25
One more question about aliasing at the ADC output. When using the over sampling ration (OSR) 32768 the ADC has an output rate of 6.867 Hz. What I want to know is when i input a 13.5 Hz signal, does it alias back to about 0.234 Hz? And if so, why do they make an ADC like that? I have used an Texas Instruments ADS1251 ADC before, and it has a Sinc5 filter that has all its notches at multiples of the output rate frequency (like it should be). "jim" <"sjedgingN0sp"@m@mwt.net> wrote in message news:1156438467_7839@sp6iad.superfeed.net...> > > Jerry Avins wrote: > >> Frequency response of a device is related to its output as a function of >> its excitation frequency. Its normal mode rejection has nothing to do >> with the device's excitation, but rather to the effect of induced noise >> on its connecting circuitry. Those are different in my view. > > The graph shown is frequency response of the filtering of the delta > sigma ADC. The graph shows amplitude vs. frequency - You can dance all > around that fact if you wish. > > -jim > > ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet > News==---- > http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ > Newsgroups > ----= East and West-Coast Server Farms - Total Privacy via Encryption > =----
