Audio Banter - View Single Post - Instrumentation op-amp for DC-coupling to audio input?

**Lostgallifreyan**

George Herold wrote in news:c9ea2d68-021e-4387-8389-
:

Hi, I usually reserve the name 'instrumentation amp' for those
differential input three opamp things that I use for bridge circuits.
That said, I do like the OPA277 for DC measurments. Do you need the
10uV offset? The OPA227 is a bit faster, but has a bit more DC offset
voltage.

It makes life easier if I have that low offset.

I'd also strongly prefer
to use what I have, it means I can buy more cheaply and try to encourage
makers to persist in making and selling certain devices by staying with
those. I also like dual-amp IC's a lot, I find them very practical to lay out
compact boards for them.

I see what you mean about the three-amp devices, with no compromise on input
resistance between the two inputs of a single amp. I guess I use the term
'instrumentation' fairly loosely, based on intended purpose rather than the
device itself.

Have you looked at the OPA134? There is also a newer version... I
think the number is OPA164??, (I couldn't get the TI website to work)
smaller voltage noise than the 134 but more cuurent noise and large
capacitance on the input. 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.