Using the AD844 as an I/V - Page 16 - diyAudio
Go Back   Home > Forums > Source & Line > Digital Source

Digital Source Digital Players and Recorders: CD , SACD , Tape, Memory Card, 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 21st January 2013, 10:01 PM   #151
diyAudio Member
 
georgehifi's Avatar
 
Join Date: Sep 2003
Location: Manly Australia (Jewel of the Pacific)
Send a message via MSN to georgehifi
I still think there maybe still more room for improvement.
I have done some more study of the AD844 in open loop mode with it's own buffer as the output, and the data sheet says the buffer as it is, is good only for driving up to 100pf of capacitive load, up to 1000pf with the feed forward network they suggest, which if I read right limits the current output drivability if used.
I measured the capacitance of what I'm driving interconnects and inputs and I think it's around 800pf.
I have two options one to use the feed forward network AD suggests which I believe comprimises the current output of the 844.
Or to use the buffer (attached), but this buffer is an unknown one and I wonder if it's output is able to drive the 800pf load it will see.
My next question is I have no idea how to sim circuits, can anyone sim this buffer I have (attached) which is availible on ebay for $10 to see if it can take up to 1000pf load on it's output without ringing or oscillating?

Cheers George
Attached Images
File Type: jpg fetbuffer 2 + dc offset adjust.jpg (42.4 KB, 358 views)
__________________
Avatar : Production Lightspeed Attenuator
www.lightspeedattenuator.com

Last edited by georgehifi; 21st January 2013 at 10:04 PM.
  Reply With Quote
Old 21st January 2013, 10:46 PM   #152
jcx is online now jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
typically the C load limit is for closed loop operation - the C slows down the buffer, adds phase shift

can usally be stabilized with a series dcoupling R or ferrite bead - since the conrer frequency can be MHz there is little ausio consequence
  Reply With Quote
Old 21st January 2013, 11:23 PM   #153
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
You could use a BUF634 or an LME49600 instead. They'll drive just about anything.
  Reply With Quote
Old 22nd January 2013, 12:14 AM   #154
diyAudio Member
 
georgehifi's Avatar
 
Join Date: Sep 2003
Location: Manly Australia (Jewel of the Pacific)
Send a message via MSN to georgehifi
Quote:
Originally Posted by jcx View Post
typically the C load limit is for closed loop operation - the C slows down the buffer, adds phase shift

can usally be stabilized with a series dcoupling R or ferrite bead - since the conrer frequency can be MHz there is little ausio consequence
So are you saying jcx that in open loop operation which as you know is the way it's being used that the C load can be much higher than the 100pf stated without stability feedforward network?

Cheers George
__________________
Avatar : Production Lightspeed Attenuator
www.lightspeedattenuator.com

Last edited by georgehifi; 22nd January 2013 at 12:17 AM.
  Reply With Quote
Old 22nd January 2013, 02:23 AM   #155
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: 95
Send a message via MSN to abraxalito Send a message via Yahoo to abraxalito Send a message via Skype™ to abraxalito
Yes that's what jcx is saying. The capacitive load limit is not due to the current capability of the buffer so much as the phase shift that's introduced. In open loop operation you do not care about phase shift very much, if at all because it has no consequence.
__________________
It doesn't have to take the form of a conspiracy, rather a consensus... James H Kunstler
  Reply With Quote
Old 22nd January 2013, 03:49 AM   #156
diyAudio Member
 
georgehifi's Avatar
 
Join Date: Sep 2003
Location: Manly Australia (Jewel of the Pacific)
Send a message via MSN to georgehifi
Quote:
Originally Posted by abraxalito View Post
Yes that's what jcx is saying. The capacitive load limit is not due to the current capability of the buffer so much as the phase shift that's introduced. In open loop operation you do not care about phase shift very much, if at all because it has no consequence.
So then because it's got short circuit protection, I can remove the series 47ohm I put in for safety sake, and let the output buffer drive direct from it's 15ohm source, even into a 800pf load without ringing or oscillation problems?
I know there's not much difference between 15ohm or 47ohm output impedance, but I always like the lower number if no problems arise.

Cheers George
__________________
Avatar : Production Lightspeed Attenuator
www.lightspeedattenuator.com
  Reply With Quote
Old 22nd January 2013, 04:06 AM   #157
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: 95
Send a message via MSN to abraxalito Send a message via Yahoo to abraxalito Send a message via Skype™ to abraxalito
There won't be oscillation from the loop feedback no, but there's always the possibility of parasitics (much higher frequency oscillations) where emitter followers are concerned so if I were you, I'd isolate with an inductor (or ferrite bead) with appropriate damping resistor. This will maintain the low output impedance at audio and give some protection against HF parasitics. To be even more conservative, add a snubber (series RC) at the buffer output as is done with power amps.
__________________
It doesn't have to take the form of a conspiracy, rather a consensus... James H Kunstler
  Reply With Quote
Old 22nd January 2013, 05:08 AM   #158
diyAudio Member
 
georgehifi's Avatar
 
Join Date: Sep 2003
Location: Manly Australia (Jewel of the Pacific)
Send a message via MSN to georgehifi
Quote:
Originally Posted by abraxalito View Post
but there's always the possibility of parasitics (much higher frequency oscillations) where emitter followers are concerned so if I were you, I'd isolate with an inductor (or ferrite bead) with appropriate damping resistor. This will maintain the low output impedance at audio and give some protection against HF parasitics.
Like you explained way back post 15 Using the AD844 as an I/V but this time in series on the buffer output.

Cheers George
__________________
Avatar : Production Lightspeed Attenuator
www.lightspeedattenuator.com
  Reply With Quote
Old 22nd January 2013, 05:30 AM   #159
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: 95
Send a message via MSN to abraxalito Send a message via Yahoo to abraxalito Send a message via Skype™ to abraxalito
Yes, or here's a more explicit example in the schematic I've put up for an ESS output filter - Build thread for Diyinhk ES9018 DAC on Ebay

In this example the opamp isn't open loop so more inductors are needed for stability. In your case probably just one or two is all that's required.
__________________
It doesn't have to take the form of a conspiracy, rather a consensus... James H Kunstler
  Reply With Quote
Old 23rd January 2013, 12:04 AM   #160
diyAudio Member
 
georgehifi's Avatar
 
Join Date: Sep 2003
Location: Manly Australia (Jewel of the Pacific)
Send a message via MSN to georgehifi
Here are some screen shots from my 200mhz Tek scope, sorry no flash too much reflection. PCM1704 to AD844, TZ to ground via 2.7kohm with 500pf across it 844's output buffer straight to output rca's which were loaded with 50kohms
The funny one for me is the spiral oscillation on the 20khz sine wave anyone seen anything like this?,this noise dissappers at 12khz sine waves. Then after I put away all the gear I though maybe it was my smp bench light that was doing it. Next time I'll see again.

1khz square wave 2v
20khz sine wave
Noise on output .1mV per division 20mhz scope filter bandwidth
Noise on output .1mv per division 200mhz scope bandwidth

Cheers George
Attached Images
File Type: jpg 1khz square wave 2v.JPG (341.4 KB, 307 views)
File Type: jpg 20kzh sine wave.JPG (355.7 KB, 297 views)
File Type: jpg Noise on output .1mV per division 20mhz BW.JPG (339.5 KB, 282 views)
File Type: jpg Noise on output .1mv per division 200mhz BW.JPG (346.3 KB, 269 views)
__________________
Avatar : Production Lightspeed Attenuator
www.lightspeedattenuator.com
  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
AD844 as a common base stage in the I/V converter Pedja Digital Source 42 25th March 2013 07:56 AM
WTB AD844 Opamps apassgear Swap Meet 6 31st January 2007 02:57 PM


New To Site? Need Help?

All times are GMT. The time now is 05:36 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