
Home  Forums  Rules  Articles  The diyAudio Store  Gallery  Blogs  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
Solid State Talk all about solid state amplification. 

Please consider donating to help us continue to serve you.
Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving 

Thread Tools  Search this Thread 
15th January 2014, 12:08 AM  #1 
diyAudio Member
Join Date: Dec 2009
Location: Md

SNR from noise density
A bit if a gap in my expertise here.
OK, My amp has an output of 22V RMS Spice tells me my noise density is 112 nV/sqrt Hz Divide 112 by sqrt of 20K or 141, I get .794nV Here is where I go wrong. I am off by a factor of 1000. If I plug this into 20 log (22/.794 ee9) I get 208. Of I do 20 Log (22/.794 ee6) I get 148 dB which is the believable value. I tried to follow Cordell in his spice section and I believe his example is off by 10, so even more confused. Maybe I am totally confused. 
15th January 2014, 01:36 AM  #2 
diyAudio Member
Join Date: Sep 2007
Location: Hangzhou  Marco Polo's 'most beautiful city'. 700yrs is a long time though...
Blog Entries: 103

Where you went wrong is on the 4th line. You're supposed to multiply by sqrt(20k), not divide by it. The noise spec is nanovolts per rootHertz. So to get to nanovolts you must multiply by the denominator units.
__________________
I have the advantage of having found out how hard it is to get to really know something... how easy it is to make mistakes and fool yourself.  Richard Feynman 
15th January 2014, 10:45 PM  #3 
diyAudio Member
Join Date: Dec 2009
Location: Md

OK following this.
Spice gives the output noise density. I need to reference the input, so divide spice supplied number by the system's gain. 112nVperHz / 27 is 4.148nVper Hz Multiply by the number of sqrt increments, or 141 gives me 585nV 20 log Vout / Vnoise or 20 log (22/585 ee9 is 151 dB Do I have this? Makes more sense today. 
15th January 2014, 11:26 PM  #4 
diyAudio Member
Join Date: Sep 2007
Location: Hangzhou  Marco Polo's 'most beautiful city'. 700yrs is a long time though...
Blog Entries: 103

151dB seems impressively high for an SNR but yep you nailed the calculation ISTM.
__________________
I have the advantage of having found out how hard it is to get to really know something... how easy it is to make mistakes and fool yourself.  Richard Feynman 
16th January 2014, 01:16 AM  #5 
diyAudio Member
Join Date: Nov 2009
Location: Cape Town

Nope, still wrong.
If you want to calculate the signal to noise ratio, you need to either: a) Compare output signal voltage to output noise or: b) Compare input signal voltage to inputreferred noise You compare output signal voltage to inputreferred noise. 
16th January 2014, 01:57 AM  #6 
diyAudio Member
Join Date: Sep 2007
Location: Hangzhou  Marco Polo's 'most beautiful city'. 700yrs is a long time though...
Blog Entries: 103

Ha good point  I'd missed that  it explains why the 151dB looks too impressive. So subtract 20log(27) from 151 to get the correct answer. 27 being the system gain.
__________________
I have the advantage of having found out how hard it is to get to really know something... how easy it is to make mistakes and fool yourself.  Richard Feynman 
16th January 2014, 01:11 PM  #7 
diyAudio Member
Join Date: Oct 2007

Even that might be optimistic. You need to add the voltage noise component, the current noise component (in, pA per rt Hz, times Rs gives volts), and the Johnson noise of the source resistance itself. All 3 components add RMS. If the source resistance is low (like 1020 ohms) the last two aren't really big players, but get it up into the k ohms and it can doiminate.

16th January 2014, 01:42 PM  #8 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

4.something is about the noise level of a good low noise opamp.
4nanovolts/root Hertz is the medium frequency and high frequency levels for a 5334. Expect a low noise discrete Power Amp to be in the range 8nV to 20nV per root Hertz. Excluding avoidable Hum and buzz. A chipamp is of the order of 20nV to 100nV per root Hertz. Including avoidable Hum and Buzz can make those Power Amplifier number tens of times worse. That's why I keep asking for Numbers when Builders claim their new builds have no Hum. To me 1mVac of hum is NOT = no Hum. <0.1mVac is my definition of no Hum. ~0.3mVac is hum, but almost inaudible. 20nV/root Hz is 0.08mVac in a 27 gain Power Amplifier over the 20Hz to 20kHz frequency range
__________________
regards Andrew T. Last edited by AndrewT; 16th January 2014 at 01:50 PM. 
16th January 2014, 02:28 PM  #9 
diyAudio Member
Join Date: Oct 2007

I've seen as low as 2nv/rt Hz for state of the art op amps, and 1.7nv/rt Hz is the theoretical limit for a bipolar transistor. Something like that on >100 ohm source resistance (inclusing the feedback of course) is d*** quiet. Stick your ear up to the tweeter and hear NOTHING kind of quiet.

16th January 2014, 02:58 PM  #10 
diyAudio Member
Join Date: Apr 2011
Location: Pretoria

In LTspice, you specify a noise simulation. Run it, click on the output of your circuit and a graph of noise versus frequency will appear. It will most likely rise significantly at lower frequency, say below 100Hz. Now control click on the graph label. A little window will appear. Type in the stop and start frequencies. LTspice will then calculate the total RMS noise voltage for you.
As an example, I get 25.234uV for a circuit I am working on, for 20Hz to 20kHz. Peak output voltage is 34.6217V, or 24.28V RMS. S/N ratio is 20 * log(24.28/25.23E6) = 119dB. Not bad
__________________
Electronic equipment is filled with magic smoke that makes it function. When it escapes, the device no longer works. 
Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Reduce Output Noise and International SNR standard  adityaonline  Power Supplies  9  17th April 2012 10:24 AM 
Noise Spectral Density and Fouier Analysis  kbeist  Software Tools  12  8th April 2012 12:50 PM 
MC Stepup Xfmr and SNR  analog_guy  Analogue Source  13  13th January 2010 04:30 AM 
Penultimate Zen SNR?  amo  Pass Labs  6  15th July 2003 07:21 PM 
PCM1792 and DSD1792 with 132dB SNR  sonnya  Digital Source  68  25th May 2003 05:19 PM 
New To Site?  Need Help? 