[Hps-ecal] HV calibration

[Nathan Baltzell]

Hi Everyone,

We used a new cable to easily measure the HV independently of the CAEN’s
readback in order to try to correct the miscalibration of the modules (and/or
filter boxes) that we saw from high-cosmic-gain HV groups.

One complication is that the 10 MOhm resistance of the multimeter is a couple
orders of magnitude less than the ECal, and the wiring in the filter boxes
creates a systematic current-dependence to the voltage.  This means what
we measure with the multimeter is not what the ECal sees.

But it looks like we can subtract off that effect, since all the non-high-cosmic-gain
HV groups read the same offset, assuming their gains were already set correctly.

Here’s the numbers:
http://www.jlab.org/~baltzell/HPS/ecal/ecalhvcal.txt

It is worth noting that Ben predicted this global 3V offset based on the filter
box’s schematic and the difference between currents with the multimeter (35uA)
and the ecal (~1uA).

Another interesting point is that we had some low-gain groups from cosmics.
And those same groups were ones with high current before we replaced preamps
last week.  This also has the correct sign and agrees with the current dependence
of the voltage.  

So if we have changes in HV current (e.g. due to preamps), the gain of the ecal
is changing due to the filter boxes.   We can put the currents from EPICs into the
data stream.

The two recalibrated HV modules were shipped yesterday.  Meanwhile, we can
run cosmics with these new voltage settings for the high gain channels and
see what happens.  And possibly also get a big resistor to repeat the measurement
better.  I’ve also heard the idea to change the wiring of the filter boxes so the
current-dependence of voltage goes away.

-Nathan