steve wrote:> Jerry Avins wrote: > >>steve wrote: >> >> ... >> >> >>>Analog Devices quote from web site >>>"For oversampling to work, samples need to be taken at a rate no > > faster > >>>than 10 times the analog bandwidth" >> >>Could you say where that is on the website? The context is crucial. >>However high the oversampling ratio, the original signal can be >>reconstructed. What does "working" consist of? >> >>Jerry >> >>P.S. I garbled part of my previous message. It should have been (in >>part) "The signal is contaminated by any noise produced by the > > sampling > >>chain within the signal's band, and any noise outside the band can be >>filtered out regardless of the sample rate." >>-- >>Engineering is the art of making what you want from things you can > > get. > > ����������������������������������������������������������������������� > > see part 8 of > > http://www.analog.com/Analog_Root/static/techSupport/designTools/interactiveTools/accelerometer/adxl202.html > > Another thing I didn't understand was why the bandwidth is reduced by > oversampling, I thought the whole purpose of oversampling was to > maintained the signal bandwidth at reduced noise at the expense of > faster a/d conversions. I certainly can see if you oversampled but kept > the sampling rate constant you would reduce the bandwidth, but I > wouldn't call that oversampling (and from the online calculator, the > sample rate does increase with increase samples, yet the bandwidth > still drops).I don't see where it says that higher sample rates reduce the bandwidth. As for oversampling by more than ten not working, I will lay a small bet that that's an artifact of the way the program works and doesn't refer to the real world. I think the author just figured that nobody would want more than 10, just as IBM figured that nobody would want more than 640 K of RAM in a PC. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Maximum useable oversampling rate10x analog signal bandwidth?
Started by ●January 22, 2005
Reply by ●January 23, 20052005-01-23
Reply by ●January 23, 20052005-01-23
"I don't see where it says that higher sample rates reduce the bandwidth." Well the title for one, "Option: Reduce noise by oversampling (at expense of bandwidth)" And the first sentence too, also just type in a oversample rate and you will see it recompute the lower bandwidth and display it in "bandwidth after oversampling" you must of read the wrong web page to miss it
Reply by ●January 23, 20052005-01-23
steve wrote:> Jerry Avins wrote: > >>steve wrote: >> >> ... >> >> >>>Analog Devices quote from web site >>>"For oversampling to work, samples need to be taken at a rate no > > faster > >>>than 10 times the analog bandwidth" >> >>Could you say where that is on the website? The context is crucial. >>However high the oversampling ratio, the original signal can be >>reconstructed. What does "working" consist of? >> >>Jerry >> >>P.S. I garbled part of my previous message. It should have been (in >>part) "The signal is contaminated by any noise produced by the > > sampling > >>chain within the signal's band, and any noise outside the band can be >>filtered out regardless of the sample rate." >>-- >>Engineering is the art of making what you want from things you can > > get. > > ����������������������������������������������������������������������� > > see part 8 of > > http://www.analog.com/Analog_Root/static/techSupport/designTools/interactiveTools/accelerometer/adxl202.html > > Another thing I didn't understand was why the bandwidth is reduced by > oversampling, I thought the whole purpose of oversampling was to > maintained the signal bandwidth at reduced noise at the expense of > faster a/d conversions. I certainly can see if you oversampled but kept > the sampling rate constant you would reduce the bandwidth, but I > wouldn't call that oversampling (and from the online calculator, the > sample rate does increase with increase samples, yet the bandwidth > still drops). >It's for that specific part -- the part reads out an analog reading as a duty cycle of a fixed-frequency square wave, so your actual sampling rate is fixed. When they say "oversampling" they should be saying "digital filtering". I believe that the factor of 10 is for the physical sampling rate vs. the analog bandwidth, not the internal lowpass filtering. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●January 23, 20052005-01-23
steve wrote:> "I don't see where it says that higher sample rates reduce the > bandwidth." > > Well the title for one, > > "Option: Reduce noise by oversampling (at expense of bandwidth)" > > And the first sentence too, also just type in a oversample rate and you > will see it recompute the lower bandwidth and display it in "bandwidth > after oversampling" > > you must of read the wrong web page to miss itI remembered the title after I hit "send". Here's my take: the bandwidth is reduced by averaging back to what it would have been before over- sampling allowed the bandwidth to increase. If that's it, it's a crock. I'll bet again that the auther is tied to a codec with a built-in anti-alias filter that tracks the sample rate. If the anti-alias cutoff stayed fixed, averageing wouldn't reduce external noise. The filter would effectively have done the averaging by holding down the bandwidth. I get the impression that it's not an article I'd want to learn from. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●January 23, 20052005-01-23
Tim Wescott wrote:> steve wrote: > > > Jerry Avins wrote: > > > >>steve wrote: > >> > >> ... > >> > >> > >>>Analog Devices quote from web site > >>>"For oversampling to work, samples need to be taken at a rate no > > > > faster > > > >>>than 10 times the analog bandwidth" > >> > >>Could you say where that is on the website? The context is crucial. > >>However high the oversampling ratio, the original signal can be > >>reconstructed. What does "working" consist of? > >> > >>Jerry > >> > >>P.S. I garbled part of my previous message. It should have been (in > >>part) "The signal is contaminated by any noise produced by the > > > > sampling > > > >>chain within the signal's band, and any noise outside the band canbe> >>filtered out regardless of the sample rate." > >>-- > >>Engineering is the art of making what you want from things you can > > > > get. > > > >=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF= =AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF= =AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF> > > > see part 8 of > > > >http://www.analog.com/Analog_Root/static/techSupport/designTools/interactiv= eTools/accelerometer/adxl202.html> > > > Another thing I didn't understand was why the bandwidth is reducedby> > oversampling, I thought the whole purpose of oversampling was to > > maintained the signal bandwidth at reduced noise at the expense of > > faster a/d conversions. I certainly can see if you oversampled butkept> > the sampling rate constant you would reduce the bandwidth, but I > > wouldn't call that oversampling (and from the online calculator,the> > sample rate does increase with increase samples, yet the bandwidth > > still drops). > > > It's for that specific part -- the part reads out an analog readingas a> duty cycle of a fixed-frequency square wave, so your actual sampling > rate is fixed. When they say "oversampling" they should be saying > "digital filtering". I believe that the factor of 10 is for the > physical sampling rate vs. the analog bandwidth, not the internal > lowpass filtering. > > -- > > Tim Wescott > Wescott Design Services > http://www.wescottdesign.comOk, I think I see now, I read the datasheet, I don't completely understand what they are talking about but it sure looks like it a limitation of the part (PWM output is variable but limited .5 to 10ms) thanks
Reply by ●January 24, 20052005-01-24
Flies in the face of DeltaSigma oversampling by 256X or 512X etc. My understanding is that for a second order DS converter every doubling of oversampling buys about 9 dB in SNR. Dave "steve" <bungalow_steve@yahoo.com> wrote in message news:1106449193.210985.208270@z14g2000cwz.googlegroups.com...> Analog Devices comments on their web site that for oversampling to work > the sample rate cannot be over 10 times the analog signal bandwidth. So > this defines the upper bounds on how much you can reduce the noise > floor of an analog signal. So I am thinking what magical about 10x, but > I suppose they are implying that if you had a 100hz bandwidth limited > signal and oversampled it 1024 times at 1Ghz rate all the samples would > be the same value anyway since if the signal is truly bandlimited at > 100Hz it couldn't change quick enough to be different between samples. > So whats the question? Well I guess I actually answered it as I am > writing this post, funny how writing the question down does that! > > Thinking about it some more I suppose oversampling rates greater then > 10x the signal bandwidth could help to reduce internal a/d noise, clock > noise, switching power supply noise and any other high frequency noise > that sneaks in after the external analog filter or though an external > a/d voltage reference. >
