The pcm1794a datasheet I/V converter and how to improve it. - Page 13 - 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 2nd September 2012, 03:10 PM   #121
diyAudio Member
 
Joachim Gerhard's Avatar
 
Join Date: Mar 2009
The OPA1641 should work too. I think it has a GB of 10MHz.
  Reply With Quote
Old 3rd September 2012, 01:31 PM   #122
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
OK. I am unable to model -6.2mA on both the + and - pins of the current generator in Tina. So, I just modeled this part of the schematic and set the generator to 7.8mA sine wave with a -6.2mA DC offset. Performance is very good but there's a DC offset of -1.55V on the output. I don't know if that's a big deal. Distortion, noise and bandwidth are all excellent. S/N is 140dB. I'm impressed. The NE5534 performs extremely well. Input noise is 13picoV/Hz^0.5. Gain is just right at about 50dB. Of course, I adjusted the feedback resistor to achieve that. It seems to be fine at low resistor values. The outputs are running at 23mA. I didn't spend much time optimizing it.
Attached Images
File Type: jpg NE5534 + buffer IV.JPG (36.1 KB, 164 views)
  Reply With Quote
Old 3rd September 2012, 01:59 PM   #123
diyAudio Member
 
Join Date: Apr 2003
Location: Eastern Pennsylvania
Quote:
Originally Posted by dirkwright View Post
OK. I am unable to model -6.2mA on both the + and - pins of the current generator in Tina. So, I just modeled this part of the schematic and set the generator to 7.8mA sine wave with a -6.2mA DC offset. Performance is very good but there's a DC offset of -1.55V on the output
Dirk,

That -1.55V offset is due to the -6.2mA bias current, and is correct for the the 250R feedback resistor used in your circuit. -6.2mA x 250R = -1.55V. No matter what value feedback resistor you use there will be a negative output voltage offset generated by the -6.2mA bias current. Which means that this circuit, as is, cannot be D.C. coupled and will require an output coupling cap.

You could implement a D.C. servo, which would need sink the entire -6.2mA bias current directly at the DAC's output pin. The main issue I foresee there is that the servo output would likely not have a very high output impedance, which means that it might contribute distortion to the signal. A more elegant solution for D.C. coupling would be to have an 6.2mA active current-sink (such as a JFET or an LM334) attached to the DAC's output pin, but obtaining zero would require manual adjustment. These solutions would require implementing a negative power supply.
__________________
Ken

Last edited by Ken Newton; 3rd September 2012 at 02:14 PM.
  Reply With Quote
Old 3rd September 2012, 03:27 PM   #124
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
Quote:
Originally Posted by Ken Newton View Post
Dirk,

That -1.55V offset is due to the -6.2mA bias current, and is correct for the the 250R feedback resistor used in your circuit. -6.2mA x 250R = -1.55V. No matter what value feedback resistor you use there will be a negative output voltage offset generated by the -6.2mA bias current. Which means that this circuit, as is, cannot be D.C. coupled and will require an output coupling cap.

You could implement a D.C. servo, which would need sink the entire -6.2mA bias current directly at the DAC's output pin. The main issue I foresee there is that the servo output would likely not have a very high output impedance, which means that it might contribute distortion to the signal. A more elegant solution for D.C. coupling would be to have an 6.2mA active current-sink (such as a JFET or an LM334) attached to the DAC's output pin, but obtaining zero would require manual adjustment. These solutions would require implementing a negative power supply.
Thanks Ken, I appreciate your comments. What I modeled was only part of the entire TI circuit, since Tina doesn't allow me to have 2 signal sources in the same circuit, and I can't model -6.2mA on both pins of a single current source. What's missing from the circuit I posted is another IV converter identical to this one, and a balanced-to-unbalanced following opamp. I don't know what will be the result of -1.55V on both the + and - pins of the following opamp, mostly because I can't figure that out in my head. I guess it won't be a big deal but I can't do the math in my head. I was asked to model simms73's circuit earlier (post #3) and I finally got around to it now. I have software limitations that prevent me from modeling the entire TI circuit.
  Reply With Quote
Old 3rd September 2012, 03:31 PM   #125
diyAudio Member
 
Avro Arrow's Avatar
 
Join Date: Sep 2010
Location: Toronto, eh
Use a separate CCS to model the 6.2mA center current.
Use IG to model the 7.8mA signal current.

Maybe that will work.


Quote:
Originally Posted by dirkwright View Post
OK. I am unable to model -6.2mA on both the + and - pins of the current generator in Tina. So, I just modeled this part of the schematic and set the generator to 7.8mA sine wave with a -6.2mA DC offset. Performance is very good but there's a DC offset of -1.55V on the output. I don't know if that's a big deal. Distortion, noise and bandwidth are all excellent. S/N is 140dB. I'm impressed. The NE5534 performs extremely well. Input noise is 13picoV/Hz^0.5. Gain is just right at about 50dB. Of course, I adjusted the feedback resistor to achieve that. It seems to be fine at low resistor values. The outputs are running at 23mA. I didn't spend much time optimizing it.
  Reply With Quote
Old 3rd September 2012, 03:36 PM   #126
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
Quote:
Originally Posted by Avro Arrow View Post
Use a separate CCS to model the 6.2mA center current.
Use IG to model the 7.8mA signal current.

Maybe that will work.
I think I tried something like that and the software got confused, or I got strange results. I can try again, thanks for the suggestion.
  Reply With Quote
Old 3rd September 2012, 03:41 PM   #127
diyAudio Member
 
Avro Arrow's Avatar
 
Join Date: Sep 2010
Location: Toronto, eh
I have been trying to follow this thread and learn more about I/V and just want to make sure I am getting my facts straight.

The Iout+ on pcm1794 put out between -2.3mA and -10.1mA centered on -6.2mA.

The Iout- put out between -2.3mA and -10.1mA 180 degrees out of phase with the Iout+ pins.

Is that correct?
  Reply With Quote
Old 3rd September 2012, 03:45 PM   #128
diyAudio Member
 
Joachim Gerhard's Avatar
 
Join Date: Mar 2009
Dirk, you can try to put a 220 Ohm resistor between base and emitter of T2. Has nothing to do with the offset.
  Reply With Quote
Old 3rd September 2012, 04:23 PM   #129
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
Quote:
Originally Posted by Avro Arrow View Post
I have been trying to follow this thread and learn more about I/V and just want to make sure I am getting my facts straight.

The Iout+ on pcm1794 put out between -2.3mA and -10.1mA centered on -6.2mA.

The Iout- put out between -2.3mA and -10.1mA 180 degrees out of phase with the Iout+ pins.

Is that correct?
Yup.
  Reply With Quote
Old 3rd September 2012, 04:26 PM   #130
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
Quote:
Originally Posted by Joachim Gerhard View Post
Dirk, you can try to put a 220 Ohm resistor between base and emitter of T2. Has nothing to do with the offset.
Did I do it right? Doesn't seem to make a difference.
Attached Images
File Type: jpg simms73 IV.JPG (36.7 KB, 142 views)
  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
Comments on PCM1794A DAC design novec Digital Source 54 4th June 2012 07:58 PM
Micro DIR9001 + PCM1794A DAC theAnonymous1 Digital Source 88 14th June 2010 09:01 AM
FS: Twistedpear PCM1794A DAC Board pftrvlr Swap Meet 2 18th April 2010 03:52 PM
FS: 25pcs each PCM1794A and DIR9001 theAnonymous1 Swap Meet 1 5th December 2009 02:11 PM
Anyone used a PCM1794A with balanced output? hifimaker Digital Source 11 18th March 2006 05:36 PM


New To Site? Need Help?

All times are GMT. The time now is 07:23 AM.


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