Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
Re: Douglas Self- New Small Signal Stage Book Imminent!
« Reply #20 on: April 30, 2010, 12:06:39 PM »
I quickly measured differential and common-mode current noise density of an AD797 and LT1115. The AD797 gave 1.4 pA/sqrt(Hz) and 2.1 pA/sqrt(Hz) while the LT1115 showed 1 pA/sqrt(Hz) and 2.3 pA/sqrt(Hz). Hence it is clear that both opamps (and not just the LT1115) show drastic amounts of common-mode current noise. Which one is quieter depends on the application; for the LT1115 the funny situation occurs that with a 100k resistor in series with *both* inputs total noise is actually lower than with just one resistor in series with one input.

Samuel


JohnRoberts

Re: Douglas Self- New Small Signal Stage Book Imminent!
« Reply #21 on: April 30, 2010, 01:05:13 PM »
By total noise does that mean including the Johnson noise of the extra 100k resistor? (I guess I could do the math myself).

This suggests a return to the old design practice of presenting the same impedance to + and - inputs needed to mitigate higher input bias currents in older IC designs causing DC errors.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
Re: Douglas Self- New Small Signal Stage Book Imminent!
« Reply #22 on: May 01, 2010, 04:28:17 AM »
Quote
By total noise does that mean including the Johnson noise of the extra 100k resistor?

Yes.

Quote
This suggests a return to the old design practice of presenting the same impedance to + and - inputs needed to mitigate higher input bias currents in older IC designs causing DC errors.

Of course my example is basically a theoretical one. No one would use these amplifiers with such a high source impedance--a JFET part would offer much lower noise figure. But I might once derive the math which calculates the optimum cancellation resistor given the resistance at the other port--that'd be a useful thing to have.

Samuel

Hairball Audio

Re: Douglas Self- New Small Signal Stage Book Imminent!
« Reply #23 on: May 07, 2010, 12:33:38 PM »
I picked up this book last week on the recommendation of another forum member.

I'm someone who has a lot to learn and I've bought a ton of books.  So far I must say this one is by far the best.  I'm really enjoying it and I highly recommend it.

Mike
Hairball Audio, LLC
DIY Parts and Kits
https://hairballaudio.com

Top secret stuff and upcoming releases:
http://instagram.com/Hairballaudio
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volker

Re: Douglas Self- New Small Signal Stage Book Imminent!
« Reply #24 on: August 23, 2010, 08:57:29 AM »
Does anybody know what's up with his site? It has been down for quite some time now.

Is there a new page somewhere I couldn't find?

robovox

I have an electric grandpa bed that talks to me while I sleep.
                                                              --Dean Venture

ruairioflaherty

Re: Douglas Self- New Small Signal Stage Book Imminent!
« Reply #26 on: August 23, 2010, 04:53:12 PM »
Been meaning to post in this thread for a while. This book is fantastic, if you are on the fence about spending the money just go ahead and do it, definitely the most enjoyable book I've read on the subject (my skill level is low).

Cheers,
Ruairi

volker

Re: Douglas Self- New Small Signal Stage Book Imminent!
« Reply #27 on: August 23, 2010, 06:19:15 PM »
Try http://www.aqpl43.dsl.pipex.com/index.htm, volker.

Thanks! I found this here afterwards.




Yes Ruairi, most definitely. In some places I'd wish for more "why" or detail instead of "do it this way", but it's really great anyhow. And even in that aspect it's still way beyond all of the other books I've read on specific audio topics, that are too often meant for real electronics beginners and therefore cover all the same stuff again and again.


 

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