« Driveway Lightning - John Dunn, Consultant, Ambertec, P.E., P.C. | Main | PWM Chip Slow CLK Fall Time - John Dunn, Consultant, Ambertec, P.E., P.C. »

September 22, 2011

Comments

John Dunn

Deliberately putting a capacitance across the two op-amp inputs is not something I would recommend. However, in the case of a photo-diode connected there, that's exactlly what can happpen. In such a case, as you point out, Atul, one would have to take remeidal measures against that diode's capacitance and make a very careful stability analysis.

Marty

It is amazing how experiences differ. I was working at Fairchild Camera and they had made thousands of miniiature cockpit cameras for the Airforce. The camera had an automatic electromechanical automatic exposure control which had a most unusual problem. The camera would work perfectly until you tapped it. The exposure control servo would run away and completely shut down the iris.Normal operation could only be resumed by resetting the input power. I was handed the problem with a two foot stack of reports written by engineers who, for many years, had tried numerous solutions. I gathered a couple of expeienced techs and we started to trace and troubleshoot the exposure control's positional and velocity closed loops. In normal operation the voltages in the op-amp's positional loop were precise. However the velocity loop's op-amp's voltages didn't 'compute'. Naturally we changed the op-amp and got the same numbers. Now we really started to look around and discovered some 10 MHz computer clock frequency at the input of that op-amp but its output was perfectly clean. After a little thought it became clear that the 1 MHz op-amp would not pass through the 10MHz and the noise was saturating the amplifier.

Here is where the problem really develops. The assignment was 'fix it but don't touch it' since thousands of these cameras had been delivered to the Military. Well, I'll shorten the story. We found a couple of parts in the area of the op-amps input which could be removed and left connections on the PC board to the inputs of the op-amp. By putting a 100 Pf cap across the inputs we found that we cleaned up the inputs and the DC levels were found to be exactly as predicted. You could hit the camera with a sledge hammer, but the exposure control loop held steady.

Marty Kanner

Michael Gambuzza

Really important with photodiode amplifers. Compensation should be worked out for minimum 45 degree phase margin. An excellent book on the subject is entitled "Photodiode Amplifiers" by Jerrold Graeme who was with Burr-Brown for a long time. It has helped me many times on these designs where photodiode capacitance can be quite large.

Frank Wiedmann

I present some methods for loop gain simulation on my webpage http://sites.google.com/site/frankwiedmann/loopgain .

Verify your Comment

Previewing your Comment

This is only a preview. Your comment has not yet been posted.

Working...
Your comment could not be posted. Error type:
Your comment has been saved. Comments are moderated and will not appear until approved by the author. Post another comment

The letters and numbers you entered did not match the image. Please try again.

As a final step before posting your comment, enter the letters and numbers you see in the image below. This prevents automated programs from posting comments.

Having trouble reading this image? View an alternate.

Working...

Post a comment

Comments are moderated, and will not appear until the author has approved them.

Your Information

(Name and email address are required. Email address will not be displayed with the comment.)

Editor

  Learn about our  

 free engineering 

consulting referral 

      service at:

 

  IEEE Consultant's 

      Network of 

     Long Island

 

We have over forty 

electrical, electronic, 

mechanical and 

software engineers 

with expertise in more 

than sixty-five categories 

of technology and business. 

All are members of the 

IEEE and adhere to the 

IEEE professional codes 

of ethics. 

No fee is charged for the

referral service. 

Each member is an 

independent consultant 

and negotiates his/her 

own consulting agreement.

 

Editor: Jerry Brown

Contributors:

John Dunn

Marty Kanner

Murray Kleiner

Dick LaRosa

David Pinkowitz

Carl Schwab

Gerry Bodner

Larry Rachman

 

Unless otherwise noted, 

reprinting or republication 

of anarticle on this blog is 

authorized by crediting the 

author and prominently 

displaying the following 

sentence at the beginning 

or end of the article,

including the hyperlink to

IEEE Consultant's Network 

of Long Island


"This report is republished 

with permission of IEEE 

Consultant's Network of 

Long Island"

 

Pages

Blog powered by Typepad

Enter your email address:

Delivered by FeedBurner