Re: Electronics Readout Characteristics

[Gerard Visser <gvisser@indiana.edu>]

Hi Fernando,
	Sorry I didn't write back earlier... I attach here below your other 
email and a combined reply, thanks!

Hi Fernando,
	The CDC and the FDC anode shapers should be identical. You have 
implemented exactly the changes listed on page 5 of 
http://argus.phys.uregina.ca/gluex/DocDB/0007/000736/003/Receiver_Shaper_Setup_for_GPC01.pdf 
for the FDC anode shaper, right? This is exactly what I did for Yves' board.
	Are both you and Yves using identical length of the signal cable? That 
will of course make a difference.
	I have some small questions on your document. In fig 2, you show the 
voltage (peak-pedestal) out of preamp as a function of input charge. 
What is the cable length (and type) when this measurement is made? Is 
the measurement made with a differential probe or w/ 2 single-ended 
probes and scope math? What is the error on charge values here, i.e., 
what is the error on the 1pF capacitance value (probably 20-30% I 
suppose... Unless you have gone to great lengths to calibrate it 
somehow.) What is the signal input transition time (should be <3 ns I 
think, or it will affect the calibration of this).
	I think your deductions on the effective gain of the shaper are 
correct. Of course, as you know it does depend on the detector pulse 
shape - so whether these numbers are exactly the right value to use in 
"gain studies" depends on your signal generator and charge injection 
network being a faithful model of the detector.
	So, whether or not there is really a disagreement w/ Yves results 
depends on exactly how he got that. I don't see the quoted effective 
gain number "3.2" anywhere on the wiki (point me to it if I missed it). 
It is possible he gets this from the figures below the caption "What 
exactly does the shaper? Well you can see this in following traces..." 
This looks like about an apparent effective gain of 2.83. However, I 
emphasize _apparent_ because evidently this measurement was made with 
the (100 Ohm input impedance) transformer-differential probe connected 
at the input of the shaper as well as the shaper's termination 
resistors. In other words, the cable is misterminated here and the gain 
will not be what it really is.
	Anyway, I completely endorse the idea that the CDC and FDC anode 
shapers boards should be identical, and that the FDC cathode shaper has 
(for now) about a factor of 5 increased gain, which is what you have 
confirmed. The ADC125 boards (superseding the shaper boards) for CDC and 
FDC will be identical, at least this is the present plan.
	If the CDC and FDC shapers _are_ equal, then perhaps the CDC HV should 
be dialed down to reduce saturation? Or else, further ASIC gain changes 
or configurability might be indicated.
	As a practical suggestion, how about if Yves were to send his shaper 
board to you and you send one of the FDC anode ones to Yves. That should 
settle this unambiguously.
	Let me know what I can do to help further,

	Gerard

p.s. Please send the partly-dead preamp board to me when you have a 
chance, I will need to make some tests again soon. Thanks!

Fernando J. Barbosa wrote:
 > Hi Gerard,
 >
 > I know you are busy with the fADC but I just want to check some info
 > with you.
 >
 > We have measured the gains for the shapers and are as follows: FDC
 > anode=2.16, FDC cathode=10.42 and CDC anode=3.2. The first two
 > measurements are outlined in my recent GlueX-doc-1065; Yves got the
 > number from his measurements posted on the wiki log.
 >
 > Do these numbers agree with your specs? Because there is some saturation
 > at the ADC on the CDC, I suggest we change the gain on that shaper to
 > the same as the FDC anode shaper.
 >
 > Regards,
 > Fernando

Fernando J. Barbosa wrote:
> Hello everyone,
> 
> I have completed systematic characterization of the readout electronics
> for the tracking chambers and I have placed a document on the portal
> (GlueX-doc-1065). It can be easily accessed through the electronics wiki
> at http://www.jlab.org/Hall-D/software/wiki/index.php/Electronics.
> 
> Gerard, could you confirm that the CDC receiver/shaper has the same
> characteristics as the FDC anode shaper?
> 
> Regards,
> Fernando