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[time-freq] After staring at te TADD-1 schematic for a while ....

John Ackermann N8UR jra at febo.com
Fri Sep 8 14:01:07 UTC 2006

Thanks for the thoughts, Chris.  I'm becoming convinced that going to an 
inverting amplifier, at least for the first stage, might be the right 
thing to do for the next rev of the board (by the way, we've sold the 
first 50 units we kitted, and are now starting to kit up the remaining 
~50 boards; when those are gone we'll be able to make modest revisions 
in the design).

Frankly, the current circuit is an evolved copy of one I found 
elsewhere, and the non-inverting configuration worked well enough in my 
initial tests.  When changing to an inverting circuit was proposed 
later, it would have required fairly significant changes to the board 
layout, and at that time we were struggling with some other stability 
problems, so we elected not to do the redesign then.

So, we'll definitely revisit this prior to the next spin of the boards.



Christopher Hoover wrote:
> Some more thoughts on the TADD-1 design ....
> (I should preface this by saying my analog skills are not strong ... I'm
> a digital guy and I'm out of my element in the analog world.  I might be
> way off base ...)
> It seems the design might be better, given the single-sided supply and
> the need for a virtual ground, if the first and final stage were
> inverting, resulting in ultimately the same non-inverted output.
> IIRC, an inverting amplifer config is preferred when a virtual ground is
> needed as the current into the non-inverting input from the virtual
> ground node is (approx.) zero.  This makes practical the use of a
> resistive divider to produce the potential for the virtual ground node.
> The other issue is that performance of  an opamp is better when any
> common mode is held constant.  That falls out of the inverting case with
> a virtual ground on the non-inverting input.
> Here's what I am suggesting (the x1 and x2 stages could be swapped):
> wdyt?
> -ch
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