Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Digital Line Level DACs, Digital Crossovers, Equalizers, etc.

OPA based Ultra Low noise VREF for DAC
OPA based Ultra Low noise VREF for DAC
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 3rd June 2018, 03:28 PM   #1
babolcs is offline babolcs  Hungary
diyAudio Member
 
Join Date: Dec 2011
Default OPA based Ultra Low noise VREF for DAC

The intention of this thread to be able to provide a High quality (low noise)VREF mainly for DACs based on the starting circuit.
(in the Thread of ES9038Q2M a discussion started for AVCC Voltage source - battery based)

In this thread I would like to discuss an OPA based circuit.
The very base circuit is not a completed one, naturally filtering capacitors, (or against oscillation), component selection, etc is in front of us)
It should be modelled and built and measured too.
If someone willing to participate feel free to comment and modify.

I have to mention that I merged 3 ideas from the following sources:
1) OPA as power supply
Frans de Wit from diyaudio:
Balanced input all DC coupled RIAA preamp (the 4th picture)

2) Walt Jung who used LED in an OPA circuit as VREF
GLED431: An Ultra Low Noise LED Reference Cell | Walt's Blog 2014

3) And JFET (2SK170) as low noise current source from the SALAS shunt circuits.

(Likely these ideas originally derives from someone else, but I have seen at these sources and though it is correct if I mention them here.)

Szabolcs (babolcs)
Attached Images
File Type: png AVCC.PNG (368.1 KB, 539 views)

Last edited by babolcs; 3rd June 2018 at 03:31 PM.
  Reply With Quote
Old 3rd June 2018, 09:23 PM   #2
Markw4 is offline Markw4  United States
diyAudio Member
 
Join Date: Jul 2016
Location: California
Maybe helpful to take a careful look at this: http://www.waltjung.org/PDFs/Walts_B...14_GLED431.pdf
and this: Mr. Jung's ultra-low noise VREF - the GLED431

Also, for clarity I will copy the "4th picture down," as referenced in the first post by the OP. As can be seen, opamps are used directly as power supply regulators, similar to the ESS recommended opamp VCCA circuit only with 1uf output capacitance instead of 47uf (or 10uf), and a little less loop gain.
Attached Images
File Type: jpg Power Clean.jpg (255.4 KB, 515 views)

Last edited by Markw4; 3rd June 2018 at 09:39 PM.
  Reply With Quote
Old 3rd June 2018, 09:44 PM   #3
babolcs is offline babolcs  Hungary
diyAudio Member
 
Join Date: Dec 2011
Quote:
Originally Posted by Markw4 View Post
Maybe helpful to take a careful look at this: http://www.waltjung.org/PDFs/Walts_B...14_GLED431.pdf
and this: Mr. Jung's ultra-low noise VREF - the GLED431

Also, for clarity I will copy the "4th picture down," as referenced in the first post by the OP to this thread. As can be seen, opamps are used directly as power supply regulators, similar to the ESS recommended opamp VCCA circuit only with 1uf output capacitance instead of 47uf (or 10uf), and less loop gain.
Dear Mark, many thanks for the clarification. Yes this article from Walt was one of the starting Pont :-). I think application of OPA can have some great advantages. The PSRR is extremely high. For this model roughly 120dB. If we can manage the operation in this circuit definitely not this will be the main source of noise. The other can be the input noise, which is mainly eliminated with the OPAa output sourcing. I mean that neither the resistors nor the led is powered from the unfiltered VCC. Tomorrow evening I intend to do a test build with 5534. Sz.
  Reply With Quote
Old 3rd June 2018, 09:50 PM   #4
babolcs is offline babolcs  Hungary
diyAudio Member
 
Join Date: Dec 2011
Ps Why better to apply FET instead of bipolar? The FET there is no PN junction which works revers BC junction. This is why it has much lower noise. Especially the 2sk170 which has a special geometry which decrease the noise furthermore.
  Reply With Quote
Old 3rd June 2018, 10:05 PM   #5
Markw4 is offline Markw4  United States
diyAudio Member
 
Join Date: Jul 2016
Location: California
You have some resistors in the circuit, one in series with the fet and one at the output of the 2nd opamp. How would you quantify their effects on noise?

Last edited by Markw4; 3rd June 2018 at 10:21 PM.
  Reply With Quote
Old 3rd June 2018, 10:33 PM   #6
Markw4 is offline Markw4  United States
diyAudio Member
 
Join Date: Jul 2016
Location: California
Thinking about it a little more, fet or bipolar, series resistor with the transistor, all that, are not the point at all from the perspective of their noise contribution. As noise sources they are effectively in parallel with the LED and its low incremental resistance should should pretty much define noise there. They can't lower it much more at least to the extent we are talking about thermal noise. What the transistors and resistors up there around the transistor are there for is to make a simple current source. The question would then be which current source of the two offers the best regulation performance, no? Also, from Jung's perspective the B-E junction could be used to help achieve a particular output voltage.

Aside: Don't know about in this case but sometimes series elements if carefully chosen can help with temperature compensation, something that often used done with diodes and zeners. The were run at a series current where their tempcos happened to cancel out. Those were the old days, oh well.

Okay then, I provided some info about one of the resistors I asked you to think about. What about the other resistor?

Last edited by Markw4; 3rd June 2018 at 10:43 PM.
  Reply With Quote
Old 4th June 2018, 01:36 AM   #7
abraxalito is offline abraxalito  United Kingdom
diyAudio Member
 
abraxalito's Avatar
 
Join Date: Sep 2007
Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
Send a message via Yahoo to abraxalito
Quote:
Originally Posted by Markw4 View Post
You have some resistors in the circuit, one in series with the fet and one at the output of the 2nd opamp. How would you quantify their effects on noise?

@baboics - have you considered using an inductor rather than a resistor in the output filter/stabilizing network? It'll for sure contribute less noise than a resistor.
__________________
I know you think you understand what you thought I said but I'm not sure you realize that what you heard is not what I meant - Alan Greenspan
  Reply With Quote
Old 4th June 2018, 02:20 AM   #8
chris719 is offline chris719  United States
diyAudio Member
 
Join Date: Jun 2004
Location: Connecticut
OPA based Ultra Low noise VREF for DAC
You could consider an older circuit that Walt also designed:

http://waltjung.org/PDFs/Build_Ultra..._Reference.pdf
  Reply With Quote
Old 4th June 2018, 04:57 AM   #9
babolcs is offline babolcs  Hungary
diyAudio Member
 
Join Date: Dec 2011
Quote:
Originally Posted by abraxalito View Post
@baboics - have you considered using an inductor rather than a resistor in the output filter/stabilizing network? It'll for sure contribute less noise than a resistor.
The output resistor thermal noise definitely counts. For 50ohm it is 1nv/sqrt(Hz). For 100 it is 1.4 roughly.
Since a capacitor is used this will be devided, varying with the frequency. So a pink noise is generated by the output. Definitely an L could be better. But this R also limits the max current. Of the OPA. (At Starting it is high).

Also the input part can be lowered to a certain value. The ad797 has quite high output current. 50mA. For OPA1 the resistors +(FET +LED) +the 2nd OPA base current+ 2nd OPA output current are the load. 14+2+8 are the first three. So for output roughly 10-15mA can be calculated. It would be also possible that the 2nd OPA is a different type which operates from 2.5+2.5V. And in that case the VCC for 2nd OPA can be reduce to 7V. (Ad797 minimum is 10) but here the PSRR is decreasing and other properties too. (Can’t find diagram for the Ad797, just for LME49720)

BTW In the original circuit the BE has higher thermal effect then the FET sourced LED.
  Reply With Quote
Old 4th June 2018, 05:20 AM   #10
babolcs is offline babolcs  Hungary
diyAudio Member
 
Join Date: Dec 2011
Besides these I have a big question mark referring to the whole circuit:
Will it start at all?
I modelled the DC operation in Spice and it showed normal expeted values. But if at starting time the OPA1's output is at 0V than it never will start. (this was not discussed in the original Walt article).
depends on the particularly bought OPA1's offset (positive or Negative)

But in this circuit the situation is even worth since the FET source will not work under 4V. so likely a parallel resistor/Cap is needed there. ??

I considered a pull up resistor, but it could be done only from the unfiltered VCC which would dramatically increase the noise.
Mark you were right in the ES9038 thread that it has to be bulit :-) - Spice doens't answer all the questions.
  Reply With Quote

Reply


OPA based Ultra Low noise VREF for DACHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
FS: „Ultra Low Noise“ Power Supply, LT3045 based PCB’s Stammheim Vendor's Bazaar 276 17th July 2018 09:22 AM
Mr. Jung's ultra-low noise VREF - the GLED431 jackinnj Power Supplies 48 7th May 2018 06:44 PM
Ultra Low Noise - Ultra Low Impedance - Ultra big Problems! Stefan0815 Analog Line Level 2 14th January 2014 06:01 AM
what is Vref+ and Vref- in class H amplifier sch attached in the thread.. rhythmsandy Solid State 4 21st November 2013 02:35 PM
AD580 2.5 V very low noise VRef jackinnj Swap Meet 0 2nd September 2002 02:11 PM


New To Site? Need Help?

All times are GMT. The time now is 03:52 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki