How to reduce measured noise floor in my DAC/opamp circuit - diyAudio
Go Back   Home > Forums > Source & Line > Analog Line Level

Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

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
Reply
 
Thread Tools Search this Thread
Old 18th February 2014, 03:42 PM   #1
maxw is offline maxw  United Kingdom
diyAudio Member
 
maxw's Avatar
 
Join Date: Jun 2004
Location: London
Default How to reduce measured noise floor in my DAC/opamp circuit

I've designed and tested an attenuator based on a multiplying DAC and an opamp. It works really well but I'd like to know if there is any changes I could make in order to improve performance, mainly noise and dynamic range.

Using my E-MU 0404 for measurements, I think THD is below what I can measure but see the attached RMAA graphs for noise and dynamic range. I have three items on the graphs:
- Loop back = WHITE
- Attenuator with no gain (R2 and R3 removed) = GREEN
- Attenuator with 12dB gain = LIGHT BLUE

I'd like to improve the attenuator with 12dB gain, the schematic and layout are attached.

Any suggestions??? I'm just not quite sure where to start...
Better PSU?
0805 components and shorter traces?
Mount components on both sides for shorter traces?
A lower noise opamp?

Click the image to open in full size.

Click the image to open in full size.
Attached Images
File Type: jpg mdac-rmaa-comparison-noise.jpg (306.9 KB, 120 views)
File Type: jpg mdac-rmaa-comparison-dyn-range.jpg (304.5 KB, 112 views)
File Type: jpg mdac-layout.jpg (535.7 KB, 109 views)
  Reply With Quote
Old 19th February 2014, 08:39 AM   #2
diyAudio Member
 
Join Date: Nov 2004
Location: Germany
If I'm not mistaken you have a big fat ground loop in the board (plus a small one) that you should break somewhere. Those are the perils of ground fills.

Other than that, where's your absolute noise level at currently? Doesn't look too bad tbh. An AD8599 is a low-noise part but I'm not sure what sort of impedances it sees here.
  Reply With Quote
Old 19th February 2014, 09:04 AM   #3
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
H.Ott tells us that fast boards should have all the Inputs and all the Outputs located at one (compact) edge of the board.
I see "clk". Does that mean this is a fast board?
__________________
regards Andrew T.
  Reply With Quote
Old 19th February 2014, 09:24 AM   #4
maxw is offline maxw  United Kingdom
diyAudio Member
 
maxw's Avatar
 
Join Date: Jun 2004
Location: London
Quote:
Originally Posted by sgrossklass View Post
If I'm not mistaken you have a big fat ground loop in the board (plus a small one) that you should break somewhere. Those are the perils of ground fills.
Can you point out where? That IC 'U1' is an SPI isolator which isolates the microcontroller from the DAC, 'U2'. There is supposed to be a gap there.

Quote:
Originally Posted by sgrossklass View Post
Other than that, where's your absolute noise level at currently?
Not sure what you mean by that? Is that THD+N? Or just the noise level figure that RMAA gives? RMAA loopback says about -113dB where my attenuator measure -107dB. Does that make sense?

Quote:
Originally Posted by sgrossklass View Post
Doesn't look too bad tbh. An AD8599 is a low-noise part but I'm not sure what sort of impedances it sees here.
Sorry, I forgot to say, I actually used an LME49720 opamp. Do you think switching to a AD8599 would make a measurable difference? It is 1.1nV/√Hz compared to the LME49720 at 2.7nV/√Hz.
  Reply With Quote
Old 19th February 2014, 09:26 AM   #5
maxw is offline maxw  United Kingdom
diyAudio Member
 
maxw's Avatar
 
Join Date: Jun 2004
Location: London
Quote:
Originally Posted by AndrewT View Post
H.Ott tells us that fast boards should have all the Inputs and all the Outputs located at one (compact) edge of the board.
I see "clk". Does that mean this is a fast board?
CLK is the SPI control clock signal. The connectors on the isolated left side of the PCB are all for digital control of the DAC IC (using SPI).
  Reply With Quote
Old 19th February 2014, 07:59 PM   #6
maxw is offline maxw  United Kingdom
diyAudio Member
 
maxw's Avatar
 
Join Date: Jun 2004
Location: London
Wait....I guess if I can only measure XdB noise level via loopback then if I'm trying to measure an attenuator with +12dB gain then it's likely I'll measure X + 12dB no matter how good my attenuator is!

Perhaps I need better measuring equipment in this case.
Attached Images
File Type: jpg MDAC-vs-loopback.jpg (134.2 KB, 32 views)
  Reply With Quote
Old 20th February 2014, 08:38 AM   #7
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
create a digital sound file that is recorded at very low levels. But high enough to tell the DAC to stay switched ON.

Replay that very low level signal through your system and read the level of noise in the signal.

Compare changes in your DAC and it's supply by comparing the relative noise to signal levels.
__________________
regards Andrew T.
  Reply With Quote
Old 20th February 2014, 09:28 AM   #8
maxw is offline maxw  United Kingdom
diyAudio Member
 
maxw's Avatar
 
Join Date: Jun 2004
Location: London
Quote:
Originally Posted by AndrewT View Post
create a digital sound file that is recorded at very low levels. But high enough to tell the DAC to stay switched ON.

Replay that very low level signal through your system and read the level of noise in the signal.

Compare changes in your DAC and it's supply by comparing the relative noise to signal levels.
Why would this be any different to just using RMAA or ARTA to measure noise?
  Reply With Quote
Old 20th February 2014, 09:40 AM   #9
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
I don't have RMAA nor ARTA.
I'm a computer noob.

I tried using the Forum suggested file to turn on my sound card.
Now my sound card does not work at all. I can't get it back for any sound.
__________________
regards Andrew T.
  Reply With Quote
Old 20th February 2014, 09:53 AM   #10
maxw is offline maxw  United Kingdom
diyAudio Member
 
maxw's Avatar
 
Join Date: Jun 2004
Location: London
Quote:
Originally Posted by AndrewT View Post
I don't have RMAA nor ARTA.
I'm a computer noob.
Ah OK, no worries

What you are describing would be logically the same as what these programs do. They play a sound via the DAC that then goes into the device you are testing and then into an ADC and back in to the computer. The program then analyses the differences to produce the graphs.

Quote:
Originally Posted by AndrewT View Post
I tried using the Forum suggested file to turn on my sound card.
Now my sound card does not work at all. I can't get it back for any sound.
That's not good!
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Help needed for modifying DAC-opamp-tube circuit brianhauris Tubes / Valves 4 8th October 2013 02:22 PM
noise floor analyser vincent_brient Vendor's Bazaar 6 19th September 2011 08:57 AM
tripath noise floor. lbstyling Class D 10 23rd September 2010 04:54 AM
noise floor problem johndiy Solid State 12 31st August 2006 01:32 AM
Who's measured the noise on their RIAA preamp? CeramicMan Analogue Source 20 19th February 2004 06:35 PM


New To Site? Need Help?

All times are GMT. The time now is 04:21 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2