ADC and undersampling

Started by Pawel January 27, 2004
Hello,

As a result of a measurement I obtain a bandlimited signal centered at
144 kHz (12kHz Bandwidth). I was planning to use undersampling 
(Fs=192kHz) to fold this signal to 48kHz in the digital domain.
I am having a difficulty to find an ADC for this purpose. The only
ADCs I could find (I need at least 18 bit resolution) that support
Fs=192kHz had too small analog bandwidth, f.ex. cirrus CS5361.

Is there an ADC that could suit my application?

Thanks!

Pawel
Pawel wrote:
> Hello, > > As a result of a measurement I obtain a bandlimited signal centered at > 144 kHz (12kHz Bandwidth). I was planning to use undersampling > (Fs=192kHz) to fold this signal to 48kHz in the digital domain. > I am having a difficulty to find an ADC for this purpose. The only > ADCs I could find (I need at least 18 bit resolution) that support > Fs=192kHz had too small analog bandwidth, f.ex. cirrus CS5361. > > Is there an ADC that could suit my application?
Why not sample a much lower frequency? 48 will not work (the 0 frequency ends up in themiddle of your band of interest) but something a bit lower (6 KHz lower or so) should work. Take care that the quality of the sample-and-hold is more important with this input... the time of sampling is much more critical at high frequencies. Thomas
On Tue, 27 Jan 2004 21:51:54 +0100, Zak wrote:

> Pawel wrote: >> Hello, >> >> As a result of a measurement I obtain a bandlimited signal centered at >> 144 kHz (12kHz Bandwidth). I was planning to use undersampling >> (Fs=192kHz) to fold this signal to 48kHz in the digital domain. >> I am having a difficulty to find an ADC for this purpose. The only >> ADCs I could find (I need at least 18 bit resolution) that support >> Fs=192kHz had too small analog bandwidth, f.ex. cirrus CS5361. >> >> Is there an ADC that could suit my application? > > Why not sample a much lower frequency? 48 will not work (the 0 frequency > ends up in themiddle of your band of interest) but something a bit lower > (6 KHz lower or so) should work. > > Take care that the quality of the sample-and-hold is more important with > this input... the time of sampling is much more critical at high > frequencies. > > > Thomas
I think what he is saying is that the ADC frontend doesn't have enough bandwidth to do under-sampling. I know some high speed ADC's are designed for this, but I'm not too sure about these slower (192 kHz) ones. If he went to an even slower ADC, his problems would only get worse, presumably. Mac
ADS1625, ADS1626, ADS8383, AD7679 ??

> > As a result of a measurement I obtain a bandlimited signal centered at > 144 kHz (12kHz Bandwidth). I was planning to use undersampling > (Fs=192kHz) to fold this signal to 48kHz in the digital domain. > I am having a difficulty to find an ADC for this purpose. The only > ADCs I could find (I need at least 18 bit resolution) that support > Fs=192kHz had too small analog bandwidth, f.ex. cirrus CS5361. > > Is there an ADC that could suit my application? >
Where are you getting that? The data sheet clearly states that the 0.1dB passband at "quad speed" sampling is 0.24 x Fs which puts you at 46KHz, your 48KHz is 0.25 x Fs - negligible additional attenuation 0.14dB? And this is the digital filter- not the analog passband.
> Why not sample a much lower frequency? (6 KHz lower or so) should > work.
If you use very low sample rates, remember to obey the nyquist rule for the highest (baseband?) output frequency. Jim Adamthwaite.
Fred Bloggs <nospam@nospam.com> wrote in message
news:<4017AD09.9020501@nospam.com>...
> > > > As a result of a measurement I obtain a bandlimited signal centered at > > 144 kHz (12kHz Bandwidth). I was planning to use undersampling > > (Fs=192kHz) to fold this signal to 48kHz in the digital domain. > > I am having a difficulty to find an ADC for this purpose. The only > > ADCs I could find (I need at least 18 bit resolution) that support > > Fs=192kHz had too small analog bandwidth, f.ex. cirrus CS5361. > > > > Is there an ADC that could suit my application? > > > > Where are you getting that? The data sheet clearly states that the 0.1dB > passband at "quad speed" sampling is 0.24 x Fs which puts you at 46KHz, > your 48KHz is 0.25 x Fs - negligible additional attenuation 0.14dB? And > this is the digital filter- not the analog passband.
The INPUT is 144KHz

GPG wrote:
> Fred Bloggs <nospam@nospam.com> wrote in message
news:<4017AD09.9020501@nospam.com>...
> >>>As a result of a measurement I obtain a bandlimited signal centered at >>>144 kHz (12kHz Bandwidth). I was planning to use undersampling >>>(Fs=192kHz) to fold this signal to 48kHz in the digital domain. >>>I am having a difficulty to find an ADC for this purpose. The only >>>ADCs I could find (I need at least 18 bit resolution) that support >>>Fs=192kHz had too small analog bandwidth, f.ex. cirrus CS5361. >>> >>>Is there an ADC that could suit my application? >>> >> >>Where are you getting that? The data sheet clearly states that the 0.1dB >>passband at "quad speed" sampling is 0.24 x Fs which puts you at 46KHz, >>your 48KHz is 0.25 x Fs - negligible additional attenuation 0.14dB? And >>this is the digital filter- not the analog passband. > > > The INPUT is 144KHz
The analog input is aliased into 48KHz in the digital domain, it is the digital domain that is filtered, not the analog, and that passband spec refers to the digital domain .
On 27 Jan 2004 12:21:33 -0800, pawel.kluczynski@comhem.se (Pawel)
wrote:

>Hello, > >As a result of a measurement I obtain a bandlimited signal centered at >144 kHz (12kHz Bandwidth). I was planning to use undersampling >(Fs=192kHz) to fold this signal to 48kHz in the digital domain. >I am having a difficulty to find an ADC for this purpose. The only >ADCs I could find (I need at least 18 bit resolution) that support >Fs=192kHz had too small analog bandwidth, f.ex. cirrus CS5361. > >Is there an ADC that could suit my application? > >Thanks! > >Pawel
Hi, here are the frequency ranges (in kHz) within which you can have your Fs sample rate: Fs_ranges = 150.0000 -to- 276.0000 100.0000 -to- 138.0000 75.0000 -to- 92.0000 60.0000 -to- 69.0000 50.0000 -to- 55.2000 42.8571 -to- 46.0000 37.5000 -to- 39.4286 33.3333 -to- 34.5000 30.0000 -to- 30.6667 27.2727 -to- 27.6000 25.0000 -to- 25.0909 Zak is right, 48 kHz won't work. Good luck, [-Rick-]

Rick Lyons wrote:
> On 27 Jan 2004 12:21:33 -0800, pawel.kluczynski@comhem.se (Pawel) > wrote: > > >>Hello, >> >>As a result of a measurement I obtain a bandlimited signal centered at >>144 kHz (12kHz Bandwidth). I was planning to use undersampling >>(Fs=192kHz) to fold this signal to 48kHz in the digital domain. >>I am having a difficulty to find an ADC for this purpose. The only >>ADCs I could find (I need at least 18 bit resolution) that support >>Fs=192kHz had too small analog bandwidth, f.ex. cirrus CS5361. >> >>Is there an ADC that could suit my application? >> >>Thanks! >> >>Pawel > > > Hi, > here are the frequency ranges (in kHz) > within which you can have your Fs sample rate: > > Fs_ranges = > > 150.0000 -to- 276.0000 > 100.0000 -to- 138.0000 > 75.0000 -to- 92.0000 > 60.0000 -to- 69.0000 > 50.0000 -to- 55.2000 > 42.8571 -to- 46.0000 > 37.5000 -to- 39.4286 > 33.3333 -to- 34.5000 > 30.0000 -to- 30.6667 > 27.2727 -to- 27.6000 > 25.0000 -to- 25.0909 > > Zak is right, 48 kHz won't work. > > Good luck, > [-Rick-] >
Ummm- maybe you are confused by the term bandwidth. This means Fc+/-6KHz and -3dB, and at quad speed this is 0.03 x Fs. Looking at the passband ripple graph which a nominal 0.03dB peak variation- your low sample rates mean the signal is spread over a correspondingly larger percentage of the passband- you see the *full* ripple error variation which is of the same order of magnitude as using the 192KHz Fs. Luck is for dummies and programmers, analysis is for engineers.