Lostgallifreyan wrote in
:

What's the source impedance driving the opamp?


Good question, and one I've yet to follow up, my understanding of these
things has only just reached that bit.. I learned that two input noise
figures can be divided one by the other to find out the ideal source
impedance to feed a given input with, but I only read that last night,
these things take time to explore... So far I've always used the basic
logic that is usually applied to avoid precise impedance matching: make
sure the source is very low, and the input very high. This is apparently
fine for readio reception and most audio couplings, so I assumed I could
do it too. I took the idea further, I assumed that if I keep the adapter
as simple as possible I can reduce noise more than it will rise due to
thermal noise in large resistor values, hence I used a passive adder
with 1Meg resistors. I can change this to 100K perhaps, at risk of
drawing more power. This method already works fine in my power meter
design so I guess it's ok here too.


I just found this via Google:
"Typical low-noise work with bipolar input transistors depends upon having a
low-impedance signal source to shunt and thus reduce noise developed in the
input transistor. The transistor noise flows out the preamp input into the
signal source output. That means the impedance between the signal source and
the preamp op amp must be low at the frequency of interest, say, a tenth of
the target impedance."

I guess that means my idea falls foul of ideal practise because the OPA2277
is a bipolar input device, and I usually do that passive adder trick with
high values and a JFET input amp. The laser power meter showed no apparent
noise problems despit using a differentiator in its design that might amplify
any noise problem that did exist, but even so, that can't be expected to hold
true for audio up to 24 KHz I guess. I still think it will work ok though,
I'll just not be soldering any IC's in till I've tested stuff.

There's an upgrade to the LF412 called AD712 that I liked in principle, but
its only ideally aimed at 12 bit systems, so while it will be fine for my
logging intents, it won't preservew the audio specs of ther device I'm
modifying..

I'll look up the OPA134 and OPA164 amps, but I think knowing more about the
limits imposed by context will tell me more about what I need to know that
looking up yet more op-amps.