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Shalin Mehta on
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Dear list,
We want to add external PMT to FV300 for having non-descanned path. We have old PMT from electron tubes (9798B with QL30 housing). We are a bit puzzled with its behaviour... will be great help if more experienced people could throw some 'light' :-)
We want to match the output of our PMT to ADCs of olympus so that we can interface the PMT using intermediate pre-amp circuit.
So we tested the PMT on the inverted confocal (Olympus FV300)
by placing it right on top of the sample and blocking the laser by
putting two filters before PMT window. We connected the signal connector (BNC) of the PMT housing to CRO (DC coupling 1 Meg impedance). We simultaneously compared the
output of olympus PMT on CRO. The sample were fluorescent beads.
Now we observe this:
1. Output from olympus PMT+ pre-amp circuit was in range
of 0-4 V when images appeared under and over saturated on fluoview software. So we concluded
that olympus ADCs have a dynamic range of 0-4V.
Output of our PMT is negative voltage (as expected due to electrons flowing from anode to ground).
2. Our PMT does follow increase in fluorescence as sample is
scanned at the same speed as olympus PMT (when we look at signals on
CRO).
But our PMT cannot follow the decrease in fluorescence!!
Our PMT outputs negative pulse and olympus PMT outputs
positive pulse. We see that our PMT 'peaks up' in negative direction exactly in same
time as the olympus PMT. But on the opposite path the circuit is slow.
To me it seems the problem is with way we are measuring output of PMT.
Perhpas the thing is that current sees low time constant circuit
when charging but long time constant circuit on the opposite path.
I could send the image and pics if someone thinks will help in making judgement.
could someone please comment on following questions?
1.
I think PMT is a current source with internal capacitance and
internal resistance connected in parallel. When we connect the CRO (1 Meg) to
the housing we are putting a large resistance in parallel. Is that
right to assume?
2. If above is the case, the current from sees low time
constant path (low internal reistance (from cathode to anode to ground)
times capacitance). But on the return path the internal resistance is
high (I believe PMT is quite resistant to electron flow from anode to
cathode). Since this leaves no return path with small time constant the
circuit doesn't come to zero quickly.
3.
Perhaps we can design a simple op-amp interving amplifier to provide
isolation from CRO and match resistances correctly. If we have
equivalent electrical circuit of the PMT we can do it quickly.
Anyone has 'typical' internal resistance and capacitance values for QL30 housing? I searched Electron tubes website to no avail.
Any other thoughts on matching external PMT to the ADC of confocal?
As always, thanks for all the inputs,
Shalin
--
My co-ordinates:
Shalin Mehta, Graduate student
Graduate Programme in Bioengineering, NUS, Singapore
Email: shalin {dot} mehta {at} gmail {dot} com
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