Douglas Self- New Small Signal Stage Book Imminent!

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
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
 
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
 
By total noise does that mean including the Johnson noise of the extra 100k resistor?

Yes.

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
 
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
 
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
 
robovox said:

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.
 

Latest posts

Back
Top