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From: Tim Wescott on 26 May 2010 01:32
On 05/25/2010 06:21 PM, Phil Allison wrote:
> "Zico"
>
> I'm wondering if there's some circuit configuration (presumably
> an op-amp based design) that would subtract two signals (say,
> two audio signals) in a way that is *100% and absolutely
> unaffected* by components tolerance.
>
>
> ** A transformer can be used to subtract one signal from another IF the
> ends of the primary are connected to each signal source - due to the fact
> that windings respond only to the difference between the ends.

I have one word: "plastics".

Oops -- wrong world.

I have two words: "capacitive coupling".

You may get good enough, you may get pretty darn good with a specially
constructed transformer with careful shielding between primary and
secondary, but you won't get "100% perfect".

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: Zico on 26 May 2010 10:12

> > So, my question:  what about for a circuit that subtracts two
> > signals?  Or, equivalently (and even better for audio signals),
> > a circuit that adds two signals + an inverting circuit with gain 1 ?
> > (the standard solutions I know for these two rely on components
> > precision/tolerance)
>
> Just accept that until the last trump sounds, you are stuck with living
> in a world where all circuits depend on their components, and even
> op-amps don't live up to their Platonic ideals.

Though at this point I'm convinced that there is no circuit
configuration
like what I was looking for, I will clarify something (you know, at
the
risk of continuing to squeeze to death a pointless discussion :-) )

Nicely put, this idea that "even op-amps don't live up to their
platonic
ideas" :-) (and no, I'm not being sarcastic!) ... I should have
expressed
better what I intended with the analogy: two parts: (1) the
tolerance
and imperfections of an op-amp are below my threshold of what
would be considered "perfect" and "absolutely precise" (maybe there
is a linguistic self-contradiction here, I don't know) ... That is:
the
errors introduced by the op-amp and the resistance of the wires and
traces do not count.

(2) Perhaps more importantly: the op-amp will be part of the circuit
anyways, so, by placing resistors, we're *adding* to the
imprecisions;
not only that, but compensating for those requires trimmers, which at
the very least is an extra/house-keeping part of the design and an
extra source of trouble (both at manufacture time and at operations
time)

So, this was sort of the main intent of the example/analogy ...

Now, perhaps an interesting question that comes to mind now: are
the errors introduced by the imperfections of a reasonably good
audio-signals op-amp (say, an LF353?) worse than a 0.1% tolerance
in the resistors? (doesn't sound like --- 0.1% tolerance in a
configuration with linear dependence on the resistor value gives
an error 60dB below the signal ... An op-amp does better than
that ... right?

Thanks,
-Zico

From: Jasen Betts on 26 May 2010 08:11
On 2010-05-25, Zico <zico(a)mailinator.com> wrote:
> Hi,
>
> I'm wondering if there's some circuit configuration (presumably
> an op-amp based design) that would subtract two signals (say,
> two audio signals) in a way that is *100% and absolutely
> unaffected* by components tolerance.

If you need precise ratios use a transformer


--- news://freenews.netfront.net/ - complaints: news(a)netfront.net ---
From: Bob Myers on 26 May 2010 11:21
On 5/26/2010 8:12 AM, Zico wrote:
>
> Nicely put, this idea that "even op-amps don't live up to their
> platonic
> ideas" :-) (and no, I'm not being sarcastic!) ... I should have
> expressed
> better what I intended with the analogy: two parts: (1) the
> tolerance
> and imperfections of an op-amp are below my threshold of what
> would be considered "perfect" and "absolutely precise" (maybe there
> is a linguistic self-contradiction here, I don't know) ... That is:
> the
> errors introduced by the op-amp and the resistance of the wires and
> traces do not count.
>

Well, there's a REALLY big part of your problem so far, in terms
of getting an answer here - all you've ever said is "perfect," or words
to that effect, and still have let anyone in on just what would be
considered your "threshold." In short, tell us what you're really trying
to achieve, and you'll be a whole lot more likely to get some helpful
responses.

Bob M.

From: John Larkin on 26 May 2010 11:21
On Tue, 25 May 2010 16:14:05 -0700 (PDT), Zico <zico(a)mailinator.com>
wrote:

>Hi,
>
>I'm wondering if there's some circuit configuration (presumably
>an op-amp based design) that would subtract two signals (say,
>two audio signals) in a way that is *100% and absolutely
>unaffected* by components tolerance.
>
>I do not mean using 0.1% tolerance resistors, or matched resistor
>networks, or adding a potentiometer for fine adjustment. Those
>solutions *minimize* the effect of components tolerance.
>
>For example (more like an analogy): if I need a non-inverting
>buffer/amplifier with absolutely precise gain, say 2, then I could
>try the standard op-amp circuit, and use two identical resistors,
>for a gain of 2. Problem is, gain *can not* be exactly two (well,
>it can not be *expected* to be exactly two); I *can* obtain an
>absolutely precise gain of *1* ... Connect output terminal
>*directly* to the inverting input, and voil� --- this will give, from
>any conceivable point of view (at least for every practical
>purposes), an *absolutely exact* gain of 1; where I'm trying
>to get is: the solution in this example goes beyond the highest
>available precision components; it goes beyond the most
>expensive and most precise matched pairs of resistors, etc.
>
>So, my question: what about for a circuit that subtracts two
>signals? Or, equivalently (and even better for audio signals),
>a circuit that adds two signals + an inverting circuit with gain 1 ?
>(the standard solutions I know for these two rely on components
>precision/tolerance)
>
>Thanks,
>-Zico

A flying capacitor circuit comes very close.

John

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