Hello all, I have IR preamp with a gain of 64dB, the output noise is about 50mV peak to peak for a signal of 4V peak peak (max) and the bandwidth is 350kHz. I have a AD7626 (16 bits 10MSPS ADC) which is largly suffisient for my aplpication. I would want to know what would be the best method to filter the noise (which is white noise and 1/f noise which come from the IR detector) in my band of interest ...? is it possible ? Does the oversampling could help me to reduce it ? or does it only improves the quantification noise of the ADC ? Does it exist another (digital) way to help me to reduce the noise ? Does the averaging could help me ? I hope that you could help me. Regards
Oversampling technique need help...
Started by ●September 29, 2011
Reply by ●September 29, 20112011-09-29
On Sep 29, 3:47�am, "glias" <glias37@n_o_s_p_a_m.hotmail.com> wrote:> Hello all, > I have IR preamp with a gain of 64dB, the output noise is about 50mV peak > to peak for a signal of 4V peak peak (max) and the bandwidth is 350kHz. > I have a AD7626 (16 bits 10MSPS ADC) which is largly suffisient for my > aplpication. > I would want to know what would be the best method to filter the noise > (which is white noise and 1/f noise which come from the IR detector) in my > band of interest ...? is it possible ? Does the oversampling could help me > to reduce it ? or does it only improves the quantification noise of the ADC > ? > Does it exist another (digital) way to help me to reduce the noise ? Does > the averaging could help me ? > > I hope that you could help me. > RegardsThe way to approach this problem is to solve for system NF of the system. Once you get that number you can solve for system dynamic range that can be used to calculate the noise floor. No = (KTBGF) F = ((s/n)i)/((s/n)o) The dynamic range of the A/D can be calculated using: 6db*N + 10*log(Fs) � 10*log(Bwa) � 1.25db Where: KT is boltzman constant, B is analog bandwidth, G is amp gain, F is noise figure, N is A/D bits, Fs is sample rate and Bwa is the final analysis bandwidth. So calculate the NF of the amp, NF of the A/D and everything falls out. Regards, Hope this helps
Reply by ●September 29, 20112011-09-29
On Thu, 29 Sep 2011 05:47:17 -0500, glias wrote:> Hello all, > I have IR preamp with a gain of 64dB, the output noise is about 50mV > peak to peak for a signal of 4V peak peak (max) and the bandwidth is > 350kHz. I have a AD7626 (16 bits 10MSPS ADC) which is largly suffisient > for my aplpication. > I would want to know what would be the best method to filter the noise > (which is white noise and 1/f noise which come from the IR detector) in > my band of interest ...? is it possible ? Does the oversampling could > help me to reduce it ? or does it only improves the quantification noise > of the ADC ? > Does it exist another (digital) way to help me to reduce the noise ? > Does the averaging could help me ? > > I hope that you could help me. > RegardsYou have stated "the bandwidth", but presumably that's either the bandwidth of the noise or the bandwidth of the signal. What's the bandwidth of the signal? What's the bandwidth of the noise? You also don't clarify what you mean by "oversampling". There is no hard and fast line between "under sampling" and "over sampling" (an adequate sampling rate, in fact, can be severely "undersampled" from a frequency domain perspective, in fact). So -- what do you mean by oversampling? You state that your chosen ADC is largely sufficient for your application, but you don't seem to have everything pinned down yet, so how can you know? Noise reduction is accomplished by identifying the differences between the noise that you wish to discard and the signal that you wish to keep, then designing a filter that will pass signal more readily than it will pass noise. You have told us little about your noise, and nothing about your signal, so it is difficult for us to suggest anything. If you ignore quantization noise at the ADC, and if it is safe to assume that the bandwidth of the noise is higher than the bandwidth of the signal, then the sampling rate that is barely adequate is one where the effect of aliasing does not fold any significant amount of noise energy into your signal's spectrum. Then the digital method that you'd most likely want to use is simple low-pass filter (or bandpass, if the signal spectrum does not extend down to zero). The ratio of your raw signal to raw noise is 80:1. This is certainly far more noise than the raw quantization of the ADC (which should be on the order of hundreds of microvolts). Unless your noise spectrum is peculiarly quiet in the frequencies occupied by your signal spectrum, this indicates that a 16-bit ADC is overkill, and that -- _only_ from a noise standpoint -- you might be able to get away with a 12-bit or even an 8-bit ADC (note that the ADC's nonlinearity would still bite you, so you may well need the whole 16 bits, even if the bottom six are useless to you). This may help. It may not. Sampling isn't magic: http://www.wescottdesign.com/articles/Sampling/sampling.pdf -- www.wescottdesign.com
Reply by ●September 29, 20112011-09-29
On 9/29/2011 9:31 AM, me0223@yahoo.com wrote:> On Sep 29, 3:47 am, "glias"<glias37@n_o_s_p_a_m.hotmail.com> wrote: >> Hello all, >> I have IR preamp with a gain of 64dB, the output noise is about 50mV peak >> to peak for a signal of 4V peak peak (max) and the bandwidth is 350kHz. >> I have a AD7626 (16 bits 10MSPS ADC) which is largly suffisient for my >> aplpication. >> I would want to know what would be the best method to filter the noise >> (which is white noise and 1/f noise which come from the IR detector) in my >> band of interest ...? is it possible ? Does the oversampling could help me >> to reduce it ? or does it only improves the quantification noise of the ADC >> ? >> Does it exist another (digital) way to help me to reduce the noise ? Does >> the averaging could help me ? >> >> I hope that you could help me. >> Regards > > > The way to approach this problem is to solve for system NF of the > system. > > Once you get that number you can solve for system dynamic range that > can be used to calculate the noise floor. > > No = (KTBGF) > > F = ((s/n)i)/((s/n)o) > > The dynamic range of the A/D can be calculated using: > > 6db*N + 10*log(Fs) � 10*log(Bwa) � 1.25db > > Where: KT is boltzman constant, B is analog bandwidth, G is amp gain, > F is noise figure, N is A/D bits, Fs is sample rate and Bwa is the > final analysis bandwidth. > > So calculate the NF of the amp, NF of the A/D and everything falls > out. > > Regards, > Hope this helpsHardly! He asked how to filter. (Not that the question has an answer with the information given.) Jerry -- Engineering is the art of making what you want from things you can get.
Reply by ●September 29, 20112011-09-29
On Sep 29, 8:03�am, Jerry Avins <j...@ieee.org> wrote:> On 9/29/2011 9:31 AM, me0...@yahoo.com wrote: > > > > > > > On Sep 29, 3:47 am, "glias"<glias37@n_o_s_p_a_m.hotmail.com> �wrote: > >> Hello all, > >> I have IR preamp with a gain of 64dB, the output noise is about 50mV peak > >> to peak for a signal of 4V peak peak (max) and the bandwidth is 350kHz. > >> I have a AD7626 (16 bits 10MSPS ADC) which is largly suffisient for my > >> aplpication. > >> I would want to know what would be the best method to filter the noise > >> (which is white noise and 1/f noise which come from the IR detector) in my > >> band of interest ...? is it possible ? Does the oversampling could help me > >> to reduce it ? or does it only improves the quantification noise of the ADC > >> ? > >> Does it exist another (digital) way to help me to reduce the noise ? Does > >> the averaging could help me ? > > >> I hope that you could help me. > >> Regards > > > The way to approach this problem is to solve for system NF of the > > system. > > > Once you get that number you can solve for system dynamic range that > > can be used to calculate the noise floor. > > > No = (KTBGF) > > > F = ((s/n)i)/((s/n)o) > > > The dynamic range of the A/D can be calculated using: > > > 6db*N + 10*log(Fs) 10*log(Bwa) 1.25db > > > Where: KT is boltzman constant, B is analog bandwidth, G is amp gain, > > F is noise figure, N is A/D bits, Fs is sample rate and Bwa is the > > final analysis bandwidth. > > > So calculate the NF of the amp, NF of the A/D and everything falls > > out. > > > Regards, > > Hope this helps > > Hardly! He asked how to filter. (Not that the question has an answer > with the information given.) > > Jerry > -- > Engineering is the art of making what you want from things you can get.- Hide quoted text - > > - Show quoted text -I have a hard time telling if this is a homework problem or not. I was attempting to stimulate some thought re dynamic range and how he might use the concept of noise to solve his problem. It looks to me like he�s taking a measurement and reading the fuzz on a scope trace. But I don�t know. An interested student would ask where those equations come from and how he could use them to solve his problem. The concept of how to design a gain lineup, whether that be a op-amp and A/D or a downconvert with a A/D behind and solve for noise is all the same. Simple concepts such as where to place that narrowband filter and why elude many. Never did I say that he supplied enough information � but he does have access to the solution. He just needs to know what to solve for.
Reply by ●September 29, 20112011-09-29
>> in my band of interestThat's the first question: What is your band of interest. 350k appears to be the upper limit, but what about the low end? It depends on the application. A lowpass filter allows you to throw away samples afterwards without losing information. This is effectively the same as averaging. For example, if your noise were white and you'd keep 500k out of a 5M bandwidth, you'd remove 90 % of the noise => 10 dB improvement. If - you know the signal spectrum - and you know the noise spectrum - and you want to detect, whether or not the signal is received, the optimal solution is a textbook problem. Have a look here: http://en.wikipedia.org/wiki/Matched_filter at "giving us our normalized filter". Build a matched filter, compare the output against a threshold (see here, why: http://en.wikipedia.org/wiki/Neyman%E2%80%93Pearson_lemma) and you're probably done.
Reply by ●September 29, 20112011-09-29
On Thu, 29 Sep 2011 08:29:57 -0700, me0223 wrote:> On Sep 29, 8:03 am, Jerry Avins <j...@ieee.org> wrote: >> On 9/29/2011 9:31 AM, me0...@yahoo.com wrote: >> >> >> >> >> >> > On Sep 29, 3:47 am, "glias"<glias37@n_o_s_p_a_m.hotmail.com> wrote: >> >> Hello all, >> >> I have IR preamp with a gain of 64dB, the output noise is about 50mV >> >> peak to peak for a signal of 4V peak peak (max) and the bandwidth is >> >> 350kHz. I have a AD7626 (16 bits 10MSPS ADC) which is largly >> >> suffisient for my aplpication. >> >> I would want to know what would be the best method to filter the >> >> noise (which is white noise and 1/f noise which come from the IR >> >> detector) in my band of interest ...? is it possible ? Does the >> >> oversampling could help me to reduce it ? or does it only improves >> >> the quantification noise of the ADC ? >> >> Does it exist another (digital) way to help me to reduce the noise ? >> >> Does the averaging could help me ? >> >> >> I hope that you could help me. >> >> Regards >> >> > The way to approach this problem is to solve for system NF of the >> > system. >> >> > Once you get that number you can solve for system dynamic range that >> > can be used to calculate the noise floor. >> >> > No = (KTBGF) >> >> > F = ((s/n)i)/((s/n)o) >> >> > The dynamic range of the A/D can be calculated using: >> >> > 6db*N + 10*log(Fs) 10*log(Bwa) 1.25db >> >> > Where: KT is boltzman constant, B is analog bandwidth, G is amp gain, >> > F is noise figure, N is A/D bits, Fs is sample rate and Bwa is the >> > final analysis bandwidth. >> >> > So calculate the NF of the amp, NF of the A/D and everything falls >> > out. >> >> > Regards, >> > Hope this helps >> >> Hardly! He asked how to filter. (Not that the question has an answer >> with the information given.) >> >> Jerry >> -- >> Engineering is the art of making what you want from things you can >> get.- Hide quoted text - >> >> - Show quoted text - > > > I have a hard time telling if this is a homework problem or not.Usually the homework problems are worded as such: all the information will be there, there will be distracting information included, but clumsily, the cited application will be trivial, far-fetched, generic, or any combination of the above, the level of specificity of things like voltage ranges or processors will be too high for the stage in the project at which you'd be asking the question, etc. In other words, homework questions spoon-feed a problem to the student to test them on the knowledge that he's learning in the class; if you've done real-world problems of that ilk you can generally see the spoon when you read the homework problem. Sometimes posters will do such a good job of condensing a real-world problem down to the essentials that it looks like homework -- then they get jumped on, and have to defend themselves. -- www.wescottdesign.com
Reply by ●September 29, 20112011-09-29
>On Sep 29, 8:03=A0am, Jerry Avins <j...@ieee.org> wrote: >> On 9/29/2011 9:31 AM, me0...@yahoo.com wrote: >> >> >> >> >> >> > On Sep 29, 3:47 am, "glias"<glias37@n_o_s_p_a_m.hotmail.com>=A0wrote:>> >> Hello all, >> >> I have IR preamp with a gain of 64dB, the output noise is about 50mVp=>eak >> >> to peak for a signal of 4V peak peak (max) and the bandwidth is350kHz=>. >> >> I have a AD7626 (16 bits 10MSPS ADC) which is largly suffisient formy>> >> aplpication. >> >> I would want to know what would be the best method to filter thenoise>> >> (which is white noise and 1/f noise which come from the IR detector)i=>n my >> >> band of interest ...? is it possible ? Does the oversampling couldhel=>p me >> >> to reduce it ? or does it only improves the quantification noise ofth=>e ADC >> >> ? >> >> Does it exist another (digital) way to help me to reduce the noise ?D=>oes >> >> the averaging could help me ? >> >> >> I hope that you could help me. >> >> Regards >> >> > The way to approach this problem is to solve for system NF of the >> > system. >> >> > Once you get that number you can solve for system dynamic range that >> > can be used to calculate the noise floor. >> >> > No =3D (KTBGF) >> >> > F =3D ((s/n)i)/((s/n)o) >> >> > The dynamic range of the A/D can be calculated using: >> >> > 6db*N + 10*log(Fs) 10*log(Bwa) 1.25db >> >> > Where: KT is boltzman constant, B is analog bandwidth, G is amp gain, >> > F is noise figure, N is A/D bits, Fs is sample rate and Bwa is the >> > final analysis bandwidth. >> >> > So calculate the NF of the amp, NF of the A/D and everything falls >> > out. >> >> > Regards, >> > Hope this helps >> >> Hardly! He asked how to filter. (Not that the question has an answer >> with the information given.) >> >> Jerry >> -- >> Engineering is the art of making what you want from things you can get.-=>Hide quoted text - >> >> - Show quoted text - > > >I have a hard time telling if this is a homework problem or not. > >I was attempting to stimulate some thought re dynamic range and how he >might use the concept of noise to solve his problem. > >It looks to me like he=92s taking a measurement and reading the fuzz on >a scope trace. But I don=92t know. > >An interested student would ask where those equations come from and >how he could use them to solve his problem. > >The concept of how to design a gain lineup, whether that be a op-amp >and A/D or a downconvert with a A/D behind and solve for noise is all >the same. > >Simple concepts such as where to place that narrowband filter and why >elude many. > >Never did I say that he supplied enough information =85 but he does have >access to the solution. He just needs to know what to solve for. >Hello all, Thanks a lot for your replies. First, the application is not a homework, it's a real application and a real problem (for me). The bandwidth of the analog front end (bias and pre amp of the detector is have a high pass filter (a simple first order ac coupling) with 1,5Hz for the high pass filter and 350kHz 3rd order Bessel type low pass filter. I'm sorry for the measurement I don't have access to spectrum analyzer so Yes, the measurement of the "noise" is just a reading from a scope. But it gives me an idea. I'm not a specialist and just wanted to know what could be done to reduce my noise (if it is possible). The oversampling technique permit to reduce the noise from the quantification noise from the ADC. (correct me if I'm wrong !). But it doesn't reduce the noise which come from my analog signal chain, isn't it ? I guess that the only solution to my problem is to improve the analog chain .. ? So using the digital filter just permit to reduce the analog filter (anti aliasing) ?
Reply by ●September 29, 20112011-09-29
On Thu, 29 Sep 2011 11:29:38 -0500, glias wrote:>>On Sep 29, 8:03=A0am, Jerry Avins <j...@ieee.org> wrote: >>> On 9/29/2011 9:31 AM, me0...@yahoo.com wrote: >>> >>> >>> >>> >>> >>> > On Sep 29, 3:47 am, "glias"<glias37@n_o_s_p_a_m.hotmail.com> > =A0wrote: >>> >> Hello all, >>> >> I have IR preamp with a gain of 64dB, the output noise is about >>> >> 50mV > p= >>eak >>> >> to peak for a signal of 4V peak peak (max) and the bandwidth is > 350kHz= >>. >>> >> I have a AD7626 (16 bits 10MSPS ADC) which is largly suffisient for > my >>> >> aplpication. >>> >> I would want to know what would be the best method to filter the > noise >>> >> (which is white noise and 1/f noise which come from the IR >>> >> detector) > i= >>n my >>> >> band of interest ...? is it possible ? Does the oversampling could > hel= >>p me >>> >> to reduce it ? or does it only improves the quantification noise of > th= >>e ADC >>> >> ? >>> >> Does it exist another (digital) way to help me to reduce the noise >>> >> ? > D= >>oes >>> >> the averaging could help me ? >>> >>> >> I hope that you could help me. >>> >> Regards >>> >>> > The way to approach this problem is to solve for system NF of the >>> > system. >>> >>> > Once you get that number you can solve for system dynamic range that >>> > can be used to calculate the noise floor. >>> >>> > No =3D (KTBGF) >>> >>> > F =3D ((s/n)i)/((s/n)o) >>> >>> > The dynamic range of the A/D can be calculated using: >>> >>> > 6db*N + 10*log(Fs) 10*log(Bwa) 1.25db >>> >>> > Where: KT is boltzman constant, B is analog bandwidth, G is amp >>> > gain, F is noise figure, N is A/D bits, Fs is sample rate and Bwa is >>> > the final analysis bandwidth. >>> >>> > So calculate the NF of the amp, NF of the A/D and everything falls >>> > out. >>> >>> > Regards, >>> > Hope this helps >>> >>> Hardly! He asked how to filter. (Not that the question has an answer >>> with the information given.) >>> >>> Jerry >>> -- >>> Engineering is the art of making what you want from things you can >>> get.- > = >>Hide quoted text - >>> >>> - Show quoted text - >> >> >>I have a hard time telling if this is a homework problem or not. >> >>I was attempting to stimulate some thought re dynamic range and how he >>might use the concept of noise to solve his problem. >> >>It looks to me like he=92s taking a measurement and reading the fuzz on >>a scope trace. But I don=92t know. >> >>An interested student would ask where those equations come from and how >>he could use them to solve his problem. >> >>The concept of how to design a gain lineup, whether that be a op-amp and >>A/D or a downconvert with a A/D behind and solve for noise is all the >>same. >> >>Simple concepts such as where to place that narrowband filter and why >>elude many. >> >>Never did I say that he supplied enough information =85 but he does have >>access to the solution. He just needs to know what to solve for. >> >> > Hello all, > Thanks a lot for your replies. > First, the application is not a homework, it's a real application and a > real problem (for me). > The bandwidth of the analog front end (bias and pre amp of the detector > is have a high pass filter (a simple first order ac coupling) with 1,5Hz > for the high pass filter and 350kHz 3rd order Bessel type low pass > filter. I'm sorry for the measurement I don't have access to spectrum > analyzer soSo the bandwidth of your _filter_ is 350kHz, fin. But what is the useful bandwidth of your _signal_?> Yes, the measurement of the "noise" is just a reading from a scope. But > it gives me an idea. > I'm not a specialist and just wanted to know what could be done to > reduce my noise (if it is possible). The oversampling technique permit > to reduce the noise from the quantification noise from the ADC. (correct > me if I'm wrong !).More or less correct, yes. _If_ the ADC has enough intrinsic noise that it shows up in the ADC output, then yes, oversampling and averaging can help you to overcome quantization noise.> But it doesn't reduce the noise which come from my > analog signal chain, isn't it ?Correct. It can't do a thing about that.> I guess that the only solution to my problem is to improve the analog > chain . ?Unless your analog low-pass filter is wider than necessary, or your noise is being introduced after it, yes.> So using the digital filter just permit to reduce the analog filter > (anti aliasing) ?Using a filter in digital-land offers a multitude of advantages, about the only thing it _doesn't_ do is undo the effects of aliasing -- that would be like a blender that can un-scramble eggs. -- www.wescottdesign.com
Reply by ●September 29, 20112011-09-29
On Sep 29, 10:41�am, Tim Wescott <t...@seemywebsite.com> wrote:> On Thu, 29 Sep 2011 11:29:38 -0500, glias wrote: > >>On Sep 29, 8:03=A0am, Jerry Avins <j...@ieee.org> wrote: > >>> On 9/29/2011 9:31 AM, me0...@yahoo.com wrote: > > >>> > On Sep 29, 3:47 am, "glias"<glias37@n_o_s_p_a_m.hotmail.com> > > =A0wrote: > >>> >> Hello all, > >>> >> I have IR preamp with a gain of 64dB, the output noise is about > >>> >> 50mV > > p= > >>eak > >>> >> to peak for a signal of 4V peak peak (max) and the bandwidth is > > 350kHz= > >>. > >>> >> I have a AD7626 (16 bits 10MSPS ADC) which is largly suffisient for > > my > >>> >> aplpication. > >>> >> I would want to know what would be the best method to filter the > > noise > >>> >> (which is white noise and 1/f noise which come from the IR > >>> >> detector) > > i= > >>n my > >>> >> band of interest ...? is it possible ? Does the oversampling could > > hel= > >>p me > >>> >> to reduce it ? or does it only improves the quantification noise of > > th= > >>e ADC > >>> >> ? > >>> >> Does it exist another (digital) way to help me to reduce the noise > >>> >> ? > > D= > >>oes > >>> >> the averaging could help me ? > > >>> >> I hope that you could help me. > >>> >> Regards > > >>> > The way to approach this problem is to solve for system NF of the > >>> > system. > > >>> > Once you get that number you can solve for system dynamic range that > >>> > can be used to calculate the noise floor. > > >>> > No =3D (KTBGF) > > >>> > F =3D ((s/n)i)/((s/n)o) > > >>> > The dynamic range of the A/D can be calculated using: > > >>> > 6db*N + 10*log(Fs) 10*log(Bwa) 1.25db > > >>> > Where: KT is boltzman constant, B is analog bandwidth, G is amp > >>> > gain, F is noise figure, N is A/D bits, Fs is sample rate and Bwa is > >>> > the final analysis bandwidth. > > >>> > So calculate the NF of the amp, NF of the A/D and everything falls > >>> > out. > > >>> > Regards, > >>> > Hope this helps > > >>> Hardly! He asked how to filter. (Not that the question has an answer > >>> with the information given.) > > >>> Jerry > >>> -- > >>> Engineering is the art of making what you want from things you can > >>> get.- > > = > >>Hide quoted text - > > >>> - Show quoted text - > > >>I have a hard time telling if this is a homework problem or not. > > >>I was attempting to stimulate some thought re dynamic range and how he > >>might use the concept of noise to solve his problem. > > >>It looks to me like he=92s taking a measurement and reading the fuzz on > >>a scope trace. �But I don=92t know. > > >>An interested student would ask where those equations come from and how > >>he could use them to solve his problem. > > >>The concept of how to design a gain lineup, whether that be a op-amp and > >>A/D or a downconvert with a A/D behind and solve for noise is all the > >>same. > > >>Simple concepts such as where to place that narrowband filter and why > >>elude many. > > >>Never did I say that he supplied enough information =85 but he does have > >>access to the solution. �He just needs to know what to solve for. > > > Hello all, > > Thanks a lot for your replies. > > First, the application is not a homework, it's a real application and a > > real problem (for me). > > The bandwidth of the analog front end (bias and pre amp of the detector > > is have a high pass filter (a simple first order ac coupling) with 1,5Hz > > for the high pass filter and 350kHz 3rd order Bessel type low pass > > filter. I'm sorry for the measurement I don't have access to spectrum > > analyzer so > > So the bandwidth of your _filter_ is 350kHz, fin. �But what is the useful > bandwidth of your _signal_? > > > Yes, the measurement of the "noise" is just a reading from a scope. But > > it gives me an idea. > > I'm not a specialist and just wanted to know what could be done to > > reduce my noise (if it is possible). The oversampling technique permit > > to reduce the noise from the quantification noise from the ADC. (correct > > me if I'm wrong !). > > More or less correct, yes. �_If_ the ADC has enough intrinsic noise that > it shows up in the ADC output, then yes, oversampling and averaging can > help you to overcome quantization noise. > > > But it doesn't reduce the noise which come from my > > analog signal chain, isn't it ? > > Correct. �It can't do a thing about that. > > > I guess that the only solution to my problem is to improve the analog > > chain . ? > > Unless your analog low-pass filter is wider than necessary, or your noise > is being introduced after it, yes. > > > So using the digital filter just permit to reduce the analog filter > > (anti aliasing) ? > > Using a filter in digital-land offers a multitude of advantages, about > the only thing it _doesn't_ do is undo the effects of aliasing -- that > would be like a blender that can un-scramble eggs. > > --www.wescottdesign.com- Hide quoted text - > > - Show quoted text -Which topology are you using: Sensor--->Filter--->op-amp--->A/D or Sensor---> Op-Amp--->Filter--->A/D For any amp string the noise performance of the system, i.e., the noise floor will be set by the first amp. It can never be better than that. Another way to say it is, if you create noise in your amp string, you can�t see a signal from your input source better than that. Any lossy components, (i.e. resistors, non-ideal filters), in front of an amplifier adds to the noise. You will never be able to see below the noise floor set by your system NF. What process gain and averaging allow you to do, is get down to the noise floor of your gain chain. So you want your narrowest filter to be the digital filter. Noise Out of your gain stage = KTBGF. Consider your A/D as just another gain stage with a NF. There is hope that things could get better than what you see on your scope. Especially, if your topology is the first one shown above. I suspect you are making the noise measurement with a scope bandwidth wider than your filter bandwidth. Correct? If true, that adds to the fuzz on the trace because of the increased bandwidth. Also, are you making the measurement with the sensor connected? Let your digital filter set your acquisition bandwidth. If you aren�t already, use the second topology above. Terminate the input to your amp/filter with the appropriate impedance and make the measurement with your A/D and digital filter to determine your true system noise floor. If it goes up when you add your sensor, well you know. It should be with a few dB of your calculation or you�ve got other problems. You could also sweep your system using a sine source at the input to verify measurement bandwidth. Just suck the data into MatLab if you don�t have a canned FFT. Painful, but it works. All of the above applies to thermal noise and not the 1/f stuff. As Tim said, the digital filter will not fix any aliased stuff � you have to stop that before it gets to the converter. Your analog filter should do that. However, beware, depending on the self-resonance of your filter components, you could very well have re-entrant modes at Nyquest. So use your scope and a signal generator to sweep the A/D pre-filter. Hope this helps.
