what does the Q2 do in this CCD preamplifier?

[[email protected]]

http://instrumentation.obs.carnegiescience.edu/ccd/gcam/images/PRE_SCH.jpg

looks like Q2 is a clamp reset to zero voltage reference ,i.e. the ground.
However, the ccd signal is changing rapidly , for example, within 1us the ccd has a signal like ILX511B (http://www.oceanoptics.com/technical/ILX511B(E).pdf) and within this time constant the
capacitor didn't ground due to the long time constant.

so this clamp reset doesn't work for 1us ccd?

 

[speff]

Looks to me like a "black level clamp" to set the DC level prior to each scan line. That make sense? The CCD data sheets may offer more clues.

--sp

 

[George Herold]

http://instrumentation.obs.carnegiescience.edu/ccd/gcam/images/PRE_SC...

looks like Q2 is a clamp reset to zero voltage reference ,i.e. the ground..
However, the ccd signal is changing rapidly , for example, within 1us theccd has a signal like ILX511B (http://www.oceanoptics.com/technical/ILX511B(E).pdf) and within this time constant the
capacitor didn't ground due to the long time constant.

so this clamp reset doesn't work for 1us ccd?

I don't know much about CCd's. Do you need to reset the cap before
reading each pixel, or just before a whole scan? (I think that's what
Speff said.)

George H.

 

[Tauno Voipio]

I don't know much about CCd's. Do you need to reset the cap before
reading each pixel, or just before a whole scan? (I think that's what
Speff said.)

George H.

Once for the scan, if the time constant is long enough,
or, for a multiline scan, once for each line.

The point is that a CCD has a pretty large and drifting
DC baseline, compared to the video signal. The purpose
is to charge a reference DC level into the cap during
the interval between the scans and hope that it stays
well enough to give good video out.

 

[Jamie]

http://instrumentation.obs.carnegiescience.edu/ccd/gcam/images/PRE_SCH.jpg

looks like Q2 is a clamp reset to zero voltage reference ,i.e. the ground.
However, the ccd signal is changing rapidly , for example, within 1us the ccd has a signal like ILX511B (http://www.oceanoptics.com/technical/ILX511B(E).pdf) and within this time constant the
capacitor didn't ground due to the long time constant.

so this clamp reset doesn't work for 1us ccd?

That looks like the makings of a chopper amplifier, used in cases where
supper sensitivity is required. The CCD reference most likely will
include the pulses, but at a rate faster than expect and thus can be
filtered later down the line. Looking at C2, it looks like the
integrator needed to remove this or reduced BW.

That's just a guess...

Jamie

 

[[email protected]]

http://instrumentation.obs.carnegiescience.edu/ccd/gcam/images/PRE_SCH.jpg



looks like Q2 is a clamp reset to zero voltage reference ,i.e. the ground.

However, the ccd signal is changing rapidly , for example, within 1us the ccd has a signal like ILX511B (http://www.oceanoptics.com/technical/ILX511B(E).pdf) and within this time constant the

capacitor didn't ground due to the long time constant.



so this clamp reset doesn't work for 1us ccd?

If the circuit is driven by an unbuffered CCD shift register then because the output impedance is very large, Q2 does effectively GND the amplifier input. It is not necessary to charge C1.

 

[speff]

The point is that a CCD has a pretty large and drift
DC baseline, compared to the video signal. The purpose
is to charge a reference DC level into the cap during
the interval between the scans and hope that it stays
well enough to give good video out.

The time constant with the switch 'off' is 1 second (so it
won't change much in 100 or 200msec). With the switch 'on'
it is about 40usec (so it will reset pretty well in a few
hundred usec).

Here's the original article, which mentions the part under
discussion in passing as a "DC restore".

http://instrumentation.obs.carnegiescience.edu/ccd/gcam/preamp.html

There are actually two "reset" switches-- one is Q2 and the other
is inside the CCD chip. The latter switches during each pixel time
between a reference voltage (perhaps several volts from ground) and the
video voltage (offset from the reference voltage). Q2, in conjunction
with the 100n capacitor, gets rid of the several volts offset mentioned above
(with its attendant drift and effect on signal to noise in subsequent
processing).

--sp