18 years ago
each of several signal sources. Most of the cards will differ only in the
bandpass filter at the input, perhaps 3 - 4 different types (at first).
Each card will contain an input buffer amplifier (possibly also implementing
a 1st order filter), an additional bandpass filter, and a gain stage [in no
special order and two or more of these could be combined into a single stage
if advantageous]. Output of each card goes to the backplane when enabled
via digital selection lines. Each card's output impedance is such that more
than one card can be enabled at a time and their outputs are summed.
Further down the line are additional signal conditioning cards -- master
gain, filtering, monitoring, etc -- and then a final distribution amplifier
(or 2-3 such).
While there are a variety of signal sources for this (sub)system, most will
remain connected for long terms, and while it's tempting, I don't want to
create a specific card for each source, but a very few generic cards which
differ mostly in the bandpass filtering. This means that each card has to
have a variable attenuation to equalize the input which will be adjusted
when an input source is changed. I've envisioned this being done via a
multi-turn pot mounted at the front edge of the card. There are several
places in the circuit I can see putting this attenuator:
1. As a voltage divider at the input connected directly to the input
capacitor and ground with the wiper going to a unity gain voltage follower.
2. As a variable resistance -- in series with a fixed resistor -- going
into the - input of an inverting op amp serving as the input buffer with a
fixed feedback resistor to provide variable gain
3. As a variable resistance -- in series with a fixed resistor -- in the
feedback path of an inverting op amp serving as the input buffer with a
fixed input resistor to provide variable gain
4. As a voltage divider providing the input and feedback resistance for
inverting op amp serving as the input buffer to provide variable gain.
5. As a voltage divider providing the input and feedback resistance for
a non-inverting op amp serving as the input buffer to provide variable gain.
6. As a voltage divider after a unity gain voltage follower at the input
feeding the next stage
[Phew -- have I missed any configurations?]
Anyway, there's obviously a large number of places it can be put. Assuming
that the range of adjustment is relatively small so that the equivalent
Thevenin resistance seen by the op amp to which it's connected doesn't
change too much, I can't see where any of these configurations has any great
advantage or disadvantage relative to any other -- but I'm sure I'm missing
something here. Please comment on these alternatives and tell me which
ones, if any, are significantly better or worse than the others.