Test for Phase Shift & Lag ? (opamp+buffer)
Mar 7, 2007 at 5:07 AM Thread Starter Post #1 of 4

Freq Band

New Head-Fier
Joined
Jan 1, 2007
Posts
41
Likes
0
I am curious how to test for these measurements, being that they help determine compensation caps' values.
(and "overshoot" ?)

(I am a novice, and this is about advanced as my knowledge allows ....so forgive me if my terminology is not correct.)

Or, are these answered on the opamp's datasheet? If so, how does one determine a value when the opamp is paired with a buffer?

Is the compensation cap value determined by the opamp's needs only, regardless of a buffer put on it's output?

(Buffers I use the LH0033, LH0002, and BUF634...in building my amps.)
---------------------------
You can look at my current amp here:
Example, the 18pf cap is there a better/best value?
http://www.head-fi.org/forums/showthread.php?t=223073
---------------------------

I have a 4ch O'scope, and a Funct Generator, and a DMM.

=FB=
 
Mar 7, 2007 at 7:05 AM Post #2 of 4

Pars

Can Jam '10 Organizer
Joined
Oct 28, 2003
Posts
4,123
Likes
51
Amb or someone with a more R&D background might pipe up, but I think you could just run a square wave (clean) thru the amp and look at it on a scope. You'll see any over/undershoot there and see what effects compensation has on it. Also, running RMAA on the amp might prove valuable as well. And of course, listening to it
biggrin.gif
.

Chris
 
Mar 7, 2007 at 8:17 AM Post #4 of 4

error401

1000+ Head-Fier
Joined
Oct 11, 2006
Posts
1,244
Likes
11
Quote:

Originally Posted by Pars /img/forum/go_quote.gif
Amb or someone with a more R&D background might pipe up, but I think you could just run a square wave (clean) thru the amp and look at it on a scope. You'll see any over/undershoot there and see what effects compensation has on it. Also, running RMAA on the amp might prove valuable as well. And of course, listening to it
biggrin.gif
.

Chris



A sine wave would probably be more useful to take a look at phase discrepancies, but I don't think there's really a good way to analyze this on a 'scope. I'd probably use a dual trace scope with a sine wave at a few different frequencies. Compare the input to the output, you should be able to see any phase difference easily. The problem is that the amp must have some phase lag, we care that it's linear across the frequency domain, not that it exists. Comparing the 'baseline' (say 1khz) lag to the frequency you're looking at might be difficult; I suspect that at most frequencies the lag through the amp will absolutely dwarf any nonlinearity, making it hard to measure.

RMAA is a good idea, though it doesn't give any (direct) information on this. It might be enough to tune the compensation network though. Not sure...
 

Users who are viewing this thread

Top