>For example we can estimate the current output from the photocell by
looking
>at the integration capacitor used
>and knowing the integrator output voltage swing can be no more than
>approximately 2V or so. The capacitor is
>470pF. At 120seconds exposure time this implies a photo detector
current of
>very approximately 10pA (!0^ -11 A) and hence
>total board leakage must be less than this. (dV/dt = i/C) This
implies
>board leakage resistances of hundreds of Giga Ohms and Mosfet
>gate leakage currents at pA levels. In practise for non ceramic boards
this
>is almost impossible so guard ring techniques
> and/or conformal coating is probably necessary. The OM2N has a
slightly
>larger
>capacitor (680 pF) which eases the requirement a little but not much.
>
yeah, I understand. From your description, the circuit sounds like an
electrometer type of circuit, where the input impedance of the ampifier
is extremely high in order to detect very small currents. You are
exactly correct in saying that stray capacitance and leakage through
the circuit board is critical for proper functioning of the circuit.
>The Olympus OM2 service manual for example says :
>
>" The shutter amplifier (M circuit board) requires very high
insulation
>resistance on its every part, and must be kept free from dust, smudges
etc."
Where is this board located in the OM2n? is it in the bottom of the
camera, under the cover plate? If so, I need to clean mine, since it
had a thin film of oil on it. I don't know how important this is
because oil has a very high resistance value, which is one of the
reasons it's used in power transformers, etc. What should I use to
clean it? Should I remove it from the camera in order to clean it? I
think denatured alcohol is a pretty strong degreaser, but I don't want
it to get into other parts of the camera I suppose a Q tip full of d.a.
would be ok, followed by a squirt of canned air?
Be seeing you.
Dirk Wright
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