Hi list, Can somebody explain, or point out to documentation, the effect of input bandwidth of ADC on resultant SNR i.e SNR right after ADC. For example bandwidth of my signal of interest is 2MHz, and I can apply filter A (3MHz bandwidth) filter or filter B (10MHz bandwidth) filter before the ADC with a clock of 100MHz. My questions are: I think filter A (3MHz bandwidth) filter would give me better SNR right after ADC compared to filter B (10MHz bandwidth). Is that right? Is there any relation that can tell the effect of input bandwidth of ADC on output SNR .i.e SNR after ADC ? If I use filter B (10MHz bandwidth) filter, is there any way I can improve the SNR ? What other advantages/disadvantages are there if I use filter B (10MHz bandwidth) filter ? Any suggestions would be appreciated Regards, Brad _____________________________ Posted through www.DSPRelated.com
Effect of Input ADC Bandwidth
Started by ●September 18, 2013
Reply by ●September 18, 20132013-09-18
On Wed, 18 Sep 2013 00:37:57 -0500, bradsdr wrote:> Hi list, > > Can somebody explain, or point out to documentation, the effect of input > bandwidth of ADC on resultant SNR i.e SNR right after ADC. For example > bandwidth of my signal of interest is 2MHz, and I can apply filter A > (3MHz bandwidth) filter or filter B (10MHz bandwidth) filter before the > ADC with a clock of 100MHz. My questions are: > > I think filter A (3MHz bandwidth) filter would give me better SNR right > after ADC compared to filter B (10MHz bandwidth). Is that right? > > Is there any relation that can tell the effect of input bandwidth of ADC > on output SNR .i.e SNR after ADC ? > > If I use filter B (10MHz bandwidth) filter, is there any way I can > improve the SNR ? > > What other advantages/disadvantages are there if I use filter B (10MHz > bandwidth) filter ?If by "with a clock of 100MHz" you mean the ADC is sampling at 100MHz, then the effects of either filter on plain old noise will be about the same. The 3MHz wide filter may be better at keeping out signals that may cause distortion, however. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
Reply by ●September 18, 20132013-09-18
Hi, the keyword here is "oversampling". For a narrow-band signal, the same ADC gives a higher SNR than for a wide-band signal, as you can filter away the quantization noise that falls out-of-band An intuitive explanation is that the information is not only in the quantized values, but also in the time, where the quantized signal changes. _____________________________ Posted through www.DSPRelated.com
Reply by ●September 18, 20132013-09-18
"bradsdr" <95105@dsprelated> writes:> Hi list,Hi single element,> Can somebody explain, or point out to documentation, the effect of input > bandwidth of ADC on resultant SNR i.e SNR right after ADC. For example > bandwidth of my signal of interest is 2MHz, and I can apply filter A (3MHz > bandwidth) filter or filter B (10MHz bandwidth) filter before the ADC with > a clock of 100MHz. My questions are: > > I think filter A (3MHz bandwidth) filter would give me better SNR right > after ADC compared to filter B (10MHz bandwidth). Is that right? > > Is there any relation that can tell the effect of input bandwidth of ADC on > output SNR .i.e SNR after ADC ? > > If I use filter B (10MHz bandwidth) filter, is there any way I can improve > the SNR ? > > What other advantages/disadvantages are there if I use filter B (10MHz > bandwidth) filter ? > > Any suggestions would be appreciatedProvided the input bandwidth is perfectly filtered according to Nyquist, and the ADC is ideal (e.g., perfectly linear), the (digital) output of a properly-dithered ADC is a faithful copy of the (analog) input with the exception of white (wideband) noise with a power level that depends on the number of bits. That is, the decorrelated quantization noise. If the ADC has (theoretically) infinite bits, and assuming the antialiasing filter is in place and ideal, then it makes no difference whether filtering is done before or after the ADC when that filtering that is done "inside" the digital bandwidth (0 to Fs / 2 Hz). If the ADC is practical and has N bits, and again assuming the antialiasing filter is in place and ideal, then filtering after the ADC has an advantage over filtering before the ADC since a post-ADC filter filters out the quantization noise as well as the input noise. This advantage increases as the ratio of filtered bandwidth to the digital bandwidth (Fs / 2 Hz) gets smaller because you are filtering out more and more of the quantization noise. This advantage also increases as the number of bits in the ADC decrease. If the antialiasing filter is not perfect, then we are in a gray area. It may be better to do the extra filtering before the ADC in order to reduce aliasing that might otherwise occur because of the imperfect antialiasing filter. This would have to be examined for specific cases. Note also one very basic but important characterstic of quantized signals: The amount of quantization noise power depends ONLY on the number of bits and not the sample rate. This means that, for a given number of bits, the quantization noise spectral density decreases as the sample rate increases. This is the reason why oversampling has an advantage. You can see and read about oversampling in section 3 of my presentation on a delta signal DAC here: http://www.digitalsignallabs.com/presentation.pdf I hope this helps. -- Randy Yates Digital Signal Labs http://www.digitalsignallabs.com
Reply by ●September 18, 20132013-09-18
Randy Yates <yates@digitalsignallabs.com> writes:> "bradsdr" <95105@dsprelated> writes: > >> Hi list, > > Hi single element, > >> Can somebody explain, or point out to documentation, the effect of input >> bandwidth of ADC on resultant SNR i.e SNR right after ADC. For example >> bandwidth of my signal of interest is 2MHz, and I can apply filter A (3MHz >> bandwidth) filter or filter B (10MHz bandwidth) filter before the ADC with >> a clock of 100MHz. My questions are: >> >> I think filter A (3MHz bandwidth) filter would give me better SNR right >> after ADC compared to filter B (10MHz bandwidth). Is that right? >> >> Is there any relation that can tell the effect of input bandwidth of ADC on >> output SNR .i.e SNR after ADC ? >> >> If I use filter B (10MHz bandwidth) filter, is there any way I can improve >> the SNR ? >> >> What other advantages/disadvantages are there if I use filter B (10MHz >> bandwidth) filter ? >> >> Any suggestions would be appreciatedPS: Brad, I should have stated the most general truth about filtering first: using a narrower filter is (almost?) always better than a wider filter, as long as the filter is wide enough to pass your signal-of-interest, since it filters out noise. This is true in both the analog and digital domains. However, you have the other concerns I discussed when you consider both domains. --Randy> > Provided the input bandwidth is perfectly filtered according to Nyquist, > and the ADC is ideal (e.g., perfectly linear), the (digital) output of a > properly-dithered ADC is a faithful copy of the (analog) input with the > exception of white (wideband) noise with a power level that depends on > the number of bits. That is, the decorrelated quantization noise. > > If the ADC has (theoretically) infinite bits, and assuming the > antialiasing filter is in place and ideal, then it makes no difference > whether filtering is done before or after the ADC when that filtering > that is done "inside" the digital bandwidth (0 to Fs / 2 Hz). > > If the ADC is practical and has N bits, and again assuming the > antialiasing filter is in place and ideal, then filtering after the ADC > has an advantage over filtering before the ADC since a post-ADC filter > filters out the quantization noise as well as the input noise. This > advantage increases as the ratio of filtered bandwidth to the digital > bandwidth (Fs / 2 Hz) gets smaller because you are filtering out more > and more of the quantization noise. This advantage also increases as > the number of bits in the ADC decrease. > > If the antialiasing filter is not perfect, then we are in a gray > area. It may be better to do the extra filtering before the ADC > in order to reduce aliasing that might otherwise occur because > of the imperfect antialiasing filter. This would have to be > examined for specific cases. > > Note also one very basic but important characterstic of quantized > signals: The amount of quantization noise power depends ONLY on the > number of bits and not the sample rate. This means that, for a given > number of bits, the quantization noise spectral density decreases as the > sample rate increases. This is the reason why oversampling has an > advantage. > > You can see and read about oversampling in section 3 of my presentation > on a delta signal DAC here: > > http://www.digitalsignallabs.com/presentation.pdf > > I hope this helps.-- Randy Yates Digital Signal Labs http://www.digitalsignallabs.com
Reply by ●September 19, 20132013-09-19
If you have large interfering signal components above your 2mhz signal range then it's best to remove them in the analog domain, otherwise any ADC nonlinearity could produce in-band spurs that cannot be removed by subsequent digital filtering. But if the out-of-band energy is just low-level noise then it probably won't make much difference either way. Bob
Reply by ●September 19, 20132013-09-19
radams2000@gmail.com writes:> If you have large interfering signal components above your 2mhz signal > range then it's best to remove them in the analog domain, otherwise > any ADC nonlinearity could produce in-band spurs that cannot be > removed by subsequent digital filtering. But if the out-of-band energy > is just low-level noise then it probably won't make much difference > either way.Bob, You bring up (indirectly) a point I failed to address in my response yesterday. Even if the ADC had no such nonlinearities, you still have the problem that, since the ADC's peak input is limited, such a signal would require the signal-of-interest to be greatly attenuated, which in effect throws away a good bit of your ADC resolution. If you add the possible (and practical) nonlinearity problem in, then between the two it can be a serious issue. -- Randy Yates Digital Signal Labs http://www.digitalsignallabs.com
Reply by ●September 19, 20132013-09-19
>"bradsdr" <95105@dsprelated> writes: > >> Hi list, > >Hi single element, > >> Can somebody explain, or point out to documentation, the effect ofinput>> bandwidth of ADC on resultant SNR i.e SNR right after ADC. For example >> bandwidth of my signal of interest is 2MHz, and I can apply filter A(3MHz>> bandwidth) filter or filter B (10MHz bandwidth) filter before the ADCwith>> a clock of 100MHz. My questions are: >> >> I think filter A (3MHz bandwidth) filter would give me better SNR right >> after ADC compared to filter B (10MHz bandwidth). Is that right? >> >> Is there any relation that can tell the effect of input bandwidth of ADCon>> output SNR .i.e SNR after ADC ? >> >> If I use filter B (10MHz bandwidth) filter, is there any way I canimprove>> the SNR ? >> >> What other advantages/disadvantages are there if I use filter B (10MHz >> bandwidth) filter ? >> >> Any suggestions would be appreciated > >Provided the input bandwidth is perfectly filtered according to Nyquist, >and the ADC is ideal (e.g., perfectly linear), the (digital) output of a >properly-dithered ADC is a faithful copy of the (analog) input with the >exception of white (wideband) noise with a power level that depends on >the number of bits. That is, the decorrelated quantization noise. > >If the ADC has (theoretically) infinite bits, and assuming the >antialiasing filter is in place and ideal, then it makes no difference >whether filtering is done before or after the ADC when that filtering >that is done "inside" the digital bandwidth (0 to Fs / 2 Hz). > >If the ADC is practical and has N bits, and again assuming the >antialiasing filter is in place and ideal, then filtering after the ADC >has an advantage over filtering before the ADC since a post-ADC filter >filters out the quantization noise as well as the input noise. This >advantage increases as the ratio of filtered bandwidth to the digital >bandwidth (Fs / 2 Hz) gets smaller because you are filtering out more >and more of the quantization noise. This advantage also increases as >the number of bits in the ADC decrease. > >If the antialiasing filter is not perfect, then we are in a gray >area. It may be better to do the extra filtering before the ADC >in order to reduce aliasing that might otherwise occur because >of the imperfect antialiasing filter. This would have to be >examined for specific cases. > >Note also one very basic but important characterstic of quantized >signals: The amount of quantization noise power depends ONLY on the >number of bits and not the sample rate. This means that, for a given >number of bits, the quantization noise spectral density decreases as the >sample rate increases. This is the reason why oversampling has an >advantage. > >You can see and read about oversampling in section 3 of my presentation >on a delta signal DAC here: > > http://www.digitalsignallabs.com/presentation.pdf > >I hope this helps. >-- >Randy Yates >Digital Signal Labs >http://www.digitalsignallabs.com >Hi Randy, Thanks for your response. The pdf you shared is really helpful so is your comment to understand some of the concepts. I am using ADS62944 chip with 100MHz clock its a 14 bit ADC. http://www.ti.com/product/ads62p44 Also there is an AGC before the ADC. Can you point out to me a good book which deals with detail study of such topics? Regards, Brad _____________________________ Posted through www.DSPRelated.com
Reply by ●September 19, 20132013-09-19
>radams2000@gmail.com writes: > >> If you have large interfering signal components above your 2mhz signal >> range then it's best to remove them in the analog domain, otherwise >> any ADC nonlinearity could produce in-band spurs that cannot be >> removed by subsequent digital filtering. But if the out-of-band energy >> is just low-level noise then it probably won't make much difference >> either way. > >Bob, > >You bring up (indirectly) a point I failed to address in my response >yesterday. Even if the ADC had no such nonlinearities, you still have >the problem that, since the ADC's peak input is limited, such a signal >would require the signal-of-interest to be greatly attenuated, which in >effect throws away a good bit of your ADC resolution. > >If you add the possible (and practical) nonlinearity problem in, then >between the two it can be a serious issue. >-- >Randy Yates >Digital Signal Labs >http://www.digitalsignallabs.com >Lets say my signal-of-interest is X dB, then what could be the power/strength of interfering signal that is tolerable i.e it doesn't affect my SNR? Sorry if I am asking wrong questions ADC I am going to use is quite good ads62p44: SNR(dB) 73.8 SFDR(dB) 86 ENOB (Bits) 11.8 SINAD (dB) 73.4 Input Range 2Vpp Maximum Sample Rate: 125 MSPS 14-Bit Resolution with No Missing Codes 95 dB Crosstalk 3.5 dB Coarse Gain and Programmable Fine Gain up to 6 dB for SNR/SFDR Trade-Off Amplitude Down to 400 mVPP Regards, Brad _____________________________ Posted through www.DSPRelated.com
Reply by ●September 19, 20132013-09-19
>On Wed, 18 Sep 2013 00:37:57 -0500, bradsdr wrote: > >> Hi list, >> >> Can somebody explain, or point out to documentation, the effect ofinput>> bandwidth of ADC on resultant SNR i.e SNR right after ADC. For example >> bandwidth of my signal of interest is 2MHz, and I can apply filter A >> (3MHz bandwidth) filter or filter B (10MHz bandwidth) filter before the >> ADC with a clock of 100MHz. My questions are: >> >> I think filter A (3MHz bandwidth) filter would give me better SNR right >> after ADC compared to filter B (10MHz bandwidth). Is that right? >> >> Is there any relation that can tell the effect of input bandwidth ofADC>> on output SNR .i.e SNR after ADC ? >> >> If I use filter B (10MHz bandwidth) filter, is there any way I can >> improve the SNR ? >> >> What other advantages/disadvantages are there if I use filter B (10MHz >> bandwidth) filter ? > >If by "with a clock of 100MHz" you mean the ADC is sampling at 100MHz, >then the effects of either filter on plain old noise will be about the >same.Yes that is what I meant. Thanks for your response.> >The 3MHz wide filter may be better at keeping out signals that may cause >distortion, however. > >-- >Tim Wescott >Control system and signal processing consulting >www.wescottdesign.com >_____________________________ Posted through www.DSPRelated.com
