ADA4637 Opamp IV Stage Simulation - PCM1794 (some help and wisdom needed)

[OneManDIY]

I don't have a problem with TI's NE5534 - did you add the comp pins, add the 22 pF comp C?

.asy symbol with comp pins has to be added where you can find it - edit off the .txt extension

also had extra character at end of TI's model file that I deleted when pasting the text into Ltspice' "Spice directive" box (far right tool bar button)


the AN138 PMI op amp modeling paper is great - too bad AD doesn't actually use it now - some AD project engineers/spice model authors don't understand it

jcx your model works like a charm! What the heck is going on!!!

I noticed the extra character at the end of the text file and deleted it and auto generated another part file but nothing :/ I get floating nodes!!!

I used your spice directive to create a new part:
-I pasted your spice directive into a txt file and named it .cir
-opened the .cir file and right clicked on .SUBCKT and than Create Part
-opened the part editor moved around the pins
-saved the part, plugged into the circuit and nothing. I get floating node warning (yes the comp pins and the 22 pF is there) but nothing.

I redid that couple of times by changing pins... etc and the autogenerated part just wont work!

On the other hand the spice directive method works every time by using your generic opampC2.asy and typing the NE5533 in the Value field

I guess the autogenerated parts in LTSpice arent working very well. That would also explain why the ADA4627 ain't working since it gives the same errors :/

The ADA4637 part was autogenerated and it seems to be working well but I will do your approach and use a spice directive instead and see if there is a change in the simulation.

attached are my files from which I auto generate the NE5534.

Thanks a lot for the help tho!!!!!!

 

[OneManDIY]

How about doing the i/v conversion on single chip sharing positive and negative supplies plus steering the current from + and - iout of the DAC? Seems to be a perfect job for fully-balanced opamps as OPA1632. Let the harmonics/glitches cancel out not in the summation circuit, but in the i/v opamp itself...
Here - DAC ????????????? ?????? v1.2 - ????????? ??????? - PCM1794A Picardian chord S/PDIF WM8804 - Overture audio

Proper filtering is essential as no opamp circuit could deal nicely with all the noise of DS DACs.

very good idea!!! I'll sim that too and try to understand it and see which one can work better!

Thanks for the Link, looks like an excellent approach

 

[jcx]

someone resurrected a thread with a link I had lost to a great technical work on Adudio DAC internals - understanding how the DAC works helps understand what the demands of the I/V op amp really are

http://web.archive.org/web/20070118.../~ivarlo/files/School/PhD/Report_audiodac.pdf

 

[OneManDIY]

someone resurrected a thread with a link I had lost to a great technical work on Adudio DAC internals - understanding how the DAC works helps understand what the demands of the I/V op amp really are

http://web.archive.org/web/20070118.../~ivarlo/files/School/PhD/Report_audiodac.pdf

Very thorough reference! Thanks! Let see when I get to read it fully

I also found this app note on: Design for a Wideband, Differential Transimpedance DAC Output
I will try and simulate that some of those suggestions as well but they are basically the same as what I am simulating now.

Here is a short list of Op amps that I've looked up for the IV Stage.
ADA4898
AD797
AD829
OPA827
LME49600
LME49990
..... it seems almost impossible to pick the "best" IV opamp. There are plenty of transimpedance op amps :/


I have also selected a couple of AD op amps to test with ADA4898 looking as the best candidate on paper with some impressive specs but the good old AD797 is ahead in a couple of regards :/


My simulation on the TI's reference, from the PCM1794 datasheet, with NE5534 and the LT1028 is attached below. I have also simulated ADA4896/7 with LT1028 for comparison.

The ADA4896/7 with LT1028 seems to have a better noise floor(compared to NE5534/LT1028) but worse 3rd harmonic by a couple of DB. The remaining harmonics are lower.

The Vsupply for the ADA4896 is +-5V but it has an awesome CMRR and PSRR. The ADA797 has just as good specs and better in terms of CMRR, THD, and PSRR but lower slew and GW..... need to run that sim next.

The weird thing is that my FFT analysis is not simulating well below 90dB... I have gone down to 110db noise floor but with significant harmonic distortion ripples going up to -80 to -70dB sometimes but not lower.

 

[s3tup]

Add AD811 to classic I/V opamps list, these are good.

 

[susemein]

hello,
have you a ltspice model for LME49990, I have search but I found Tina Model and that is nor running,
many thanks