Another I/V Op-amp - diyAudio
Go Back   Home > Forums > Source & Line > Digital Line Level

Digital Line Level DACs, Digital Crossovers, Equalizers, 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 7th February 2012, 04:41 PM   #1
Arius is offline Arius  United States
diyAudio Member
 
Join Date: Mar 2004
Location: California
Default Another I/V Op-amp

Sorry to start a thread about I/V op-amps. Too many threads about them plus I'm not really trying to revive any specific discussion.

Just FYI for the readers:

Stereophile Feb 2012 reviewed the Bricasti Design M1 D/A Processor.

DAC is AD1955 (with DSP implementing external filter) in mono differential mode. I/V opamps are AD843. Buffers are discrete transistor based.

Results: "Class A+", "State-of-the-art measured performance"

The I/V opamp itself isn't fancy but was carefully chosen and voiced.
AD843 | 34 MHz, CBFET Fast Settling Op Amp | Operational Amplifiers (Op Amps) | All Operational Amplifiers | Analog Devices

Slew rate: 250V/us
Input noise: 19nV/rt Hz
Settling time: 135ns to 0.01%

Please let this be informational only - if you feel strongly about one method of I/V over another (resistor, common-base transistor/FET, your fav opamp), there is no reason to force anyone else to like your taste. As for the technical merits of each method, they have already been debated far and wide.

Thanks.

PS: I have no affliliation whatsoever with any of the above mentioned companies.

Last edited by Arius; 7th February 2012 at 04:42 PM. Reason: Added PS
  Reply With Quote
Old 7th February 2012, 06:05 PM   #2
diyAudio Member
 
Join Date: Jan 2008
Location: Virginia
I am playing now with a few OpAmps (for the I/V stage). My next test subject looks promising:
AD8099. Stable at Gain +2.
Noise: 0.95nV/vHz, 2.6pA/vHz
Slew rate: 475V/s
Settling time: 18ns to 0.1%, 30ns to 0.01%
Distortion: 2nd Harmonic -92dB @ 10MHz; 3rd Harmonic -105dB @ 10MHz
-3dB bandwidth: 700MHz (G=+2)

I have on my bench a ADA4897-1 that is stable at Gain +1:
noise: 1 nV/√Hz, 2.8 pA/√Hz
distortion: −115 dBc @ 100 kHz, VOUT = 2 V p-p
−3 dB bandwidth (G = +1): 230 MHz
slew rate: 120 V/μs
settling time: 45 ns to 0.1%; 90ns to 0.01%

I did try before to "make" my own discrete OpAmp stage (BJT, JFET, combination) and I realized that I am not capable of even getting closer of values like above ones.

Last edited by SoNic_real_one; 7th February 2012 at 06:22 PM.
  Reply With Quote
Old 7th February 2012, 07:47 PM   #3
jcx is offline jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
I realize the app notes for instrumentation application of high speed DACs do emphasize settling time but it really isn't quite the same importance for digital audio reproduction

in audio reproduction we want/need low pass filtered output to reject the "steps", image frequencies, any noise shaping high frequency modulation

if you take the current technology "sweet spot" as ~96 k sample rate converters with substantial internal upsampling then ~40 kHz low pass is required with order depending on upsampling ratio and any noise shaping

even doubling the sample rate and low pass frequency still gives ~ 2 uS time constants - nS settling time really is irrelevant as a prime I/V performance spec for audio reproduction

like slew rate, settling time can be an indirect indicator of certain op amp qualities that may be helpful - but the actual requirements of 40 kHz or even 80 kHz low pass filtered digital audio reproduction at 2 Vrms consumer line levels doesn't require SOTA max slew rate or settling time per se

what is really needed for audio I/V conversion is Linear treatment of the DAC Iout while performing the required low pass filtering

there a 2 parts of the linear requirement:

low delta V at the DAC out - depending on internal details Iout DAC have limited output compliance V, bigger V causes nonlinear loss of the modulated current in protection diodes, switch on-resistance modulation, collector/drain nonlinear Zout - often not rigorously speced but Order of 10 mV seems a good target without a tighter spec or measurements indicating otherwise

and Linear treatment of the DAC Iout in the I/V circuit - in the face of high current slew rates and tight input V spec - incorporating low pass filtering in the op amp I/V design gives better linearity

even "the best" op amps can't meet both requirements with just R feedback - feedback C greatly reduces dynamics at the I/V input - simple sim does show this clearly

additional topology improvements are available "noise gain" C to gnd at the I/V input is a start- again sims illustrate the principles even if not detailed enough for strictly predictive results


The ADA489x, AD8099 are something new - the "highly linear input" stage properties are poorly advertised but his is a real breakthrough for BJT input linearity with large input Vdiff - just what we should want for DAC I/V

Last edited by jcx; 7th February 2012 at 07:50 PM.
  Reply With Quote
Old 7th February 2012, 08:50 PM   #4
diyAudio Member
 
Join Date: Jan 2008
Location: Virginia
IMO Linearity is derived principally from a fast feedback loop. A slow loop will induce errors in following the signal and intermodulation products.
Also, limiting the SR at the output of a DAC with capacitors (even if they are in feedback loop) will trow off the compliance voltage. If the DAC current source was ideal, sure you can do that safely. But it's not.

Last edited by SoNic_real_one; 7th February 2012 at 08:53 PM.
  Reply With Quote
Old 7th February 2012, 09:01 PM   #5
jcx is offline jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
you really are just "shooting from the hip" aren't you - you haven't read reference I gave in the other thread, thought hard about the problem, the points I've made, tried sims to illuminate the issues
  Reply With Quote
Old 7th February 2012, 09:45 PM   #6
diyAudio Member
 
Join Date: Jan 2008
Location: Virginia
So let say that you are right and it doesn't matter how fast or slow are the OpAmps.
Why people bother to try tubes, resistor I/V? Or why even the DAC manufactures mention the slew rate and settling time in the aplication notes on datasheets?
Just dump a capacitor on outputs and you are good to go...
PS: I don't belive too much in spice simulations for anything more complex than a couple of transistors.

Last edited by SoNic_real_one; 7th February 2012 at 09:56 PM.
  Reply With Quote
Old 7th February 2012, 10:10 PM   #7
jcx is offline jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
now you're posturing - I never said slow op amp are good for I/V - you have also blown off suggestions, guidance from Thorsten on DAC I/V - clearly you aren't interested in the learning opportunities here

you may consider any further of my comments in threads you’re participating in as directed to the general readership
  Reply With Quote
Old 8th February 2012, 12:01 AM   #8
diyAudio Member
 
Join Date: Jan 2008
Location: Virginia
Thorsten is on my ignore list. Because there is nothing to learn from him: resistive I/V and tubes rules. Selling to a niche. Thanks but no thanks, I don't sacrifice noise for speed!

I don't understand what you want, you just attacked the ideea of fast OpAmp for I/V without providing anything in place (besides "the capacitors are great"). You say that 2uS is "enough", that might be fine with you... I don't want nothing that is below listed current settling time for a given DAC.
Maybe I need to learn, but how about engineers that design DAC's - they need to learn too?
Tell all those guys here that are looing into discrete stages that they are wrong too... And even your example above - why do you think is looking for tubes and resistive I/V?

http://www.ti.com/lit/ds/sbas097/sbas097.pdf - page 9
http://www.ti.com/lit/ds/sles117a/sles117a.pdf - page 19

Last edited by SoNic_real_one; 8th February 2012 at 12:08 AM.
  Reply With Quote
Old 8th February 2012, 02:08 AM   #9
diyAudio Member
 
abraxalito's Avatar
 
Join Date: Sep 2007
Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
Blog Entries: 109
Send a message via MSN to abraxalito Send a message via Yahoo to abraxalito Send a message via Skype™ to abraxalito
Quote:
Originally Posted by SoNic_real_one View Post
AD8099. Stable at Gain +2.
It looks a jolly nice part until you look at its settling transient. Guess that's messy in large part because of the gain peaking (+3dB) at G=+2.

You're going to need a series RC on the DAC output to keep the noise gain high, preferably higher than +2 to mitigate the peaking.

Curious to know how it sounds
__________________
Seek not the favour of the multitude...rather the testimony of few. And number not voices, but weigh them. - Kant
The capacity for impartial observation is commonly called 'cynicism' by those who lack it.
  Reply With Quote
Old 8th February 2012, 03:39 PM   #10
diyAudio Member
 
b1o2r3i4s5's Avatar
 
Join Date: Feb 2012
Location: HK
Quote:
Originally Posted by SoNic_real_one View Post
I did try before to "make" my own discrete OpAmp stage (BJT, JFET, combination) and I realized that I am not capable of even getting closer of values like above ones.
I too have played with many OP's but moved on to discretes learning in no circumstance will any OP sound better than discretes even if the specs are way off. but maybe i just haven't tried a good OP, so far the best i've tried is the AD8066 which i prefer over my AD843's.
The specs seem to have little to do with how they sound.

I'd advice you to give discretes another try.

Last edited by b1o2r3i4s5; 8th February 2012 at 03:48 PM.
  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
2x Gain pure op-amp headphone amp idea Dxvideo Headphone Systems 30 10th September 2011 03:05 AM
op-amp + op-amp buffer combinations eccdbb Chip Amps 2 22nd July 2011 05:13 AM
Poor power output, op amp based headphone amp. dannybeckett Solid State 13 27th September 2010 09:31 PM
Who has tried a current boosted op-amp (with a transistor) as a chip-amp buffer? sardonx Chip Amps 18 19th March 2005 01:23 PM
New OP-AMP for Music Man HD-130 bass amp Kevin McCready Solid State 0 8th September 2002 12:35 AM


New To Site? Need Help?

All times are GMT. The time now is 04:34 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