Re: FDC Tests

["Fernando J. Barbosa" <barbosa@jlab.org>]

Hi Dan,

These are all good questions. My answers follow:


Daniel S. Carman wrote:
> Fernando,
>
> I have read through your note on the FDC dynamic range and have
> a number of questions.
>
> 1. What is the difference in the determination of the gain settings
>    based on the recent 55-Fe source data vs. the extensive cosmic ray
>    data that Simon has been taking?
>
> 2. What role does the cosmic ray data play in making a final statement
>    on the dynamic range?  I was under the impression that the FDC
>    response to the 55-Fe source was quite different compared to the
>    response for the more realistic minimum ionizing particles (point
>    ionization vs. distributed ionization).  I would like a clearer
>    understanding of the chamber response for pions and protons in
>    the relevant energy range for the FDC.
>   
These two questions are related. We have not yet related these 
measurements to the cosmic ray runs. However, in conversations with 
Simon, the response to the Fe-55 source is similar to the response from 
low energy protons expected from the detector. Simon has simulated the 
response to various interactions with Garfield. Under this 
determination, the Fe-55 is used to set the upper limit given the 
present ASIC characteristics. Although the Fe-55 source has a point 
ionization characteristic, minimum ionizing particles have an extended 
source characteristic and ionization is distributed over the length of 
the track. However, there are instances when ionization clusters over 
this extended track cause a response similar to the Fe-55. Simon has 
performed some Garfield studies and can explain in more detail how this 
issue relates to pions and protons.
> 3. There are two gains in the system, one is in the ASIC chip
>    itself, and the second is in the shaping circuitry (now in Gerard's
>    shaper board, but this will ultimately be part of the FADC125). How
>    should the separate gains for these sections be set?
>   
Gerard has set the gain of the shaper (low gain) to compensate for the 
cable losses. This is arbitrary, of course, and may be set at a 
different value to match the dynamic range to the actual ADC chip. The 
important issue here is to make sure that the ASIC dynamic range of 
interest is preserved. This is relevant to take full advantage of the 
linear region.
> 4. I would ultimately like to see a single document characterizing the
>    FDC ASIC dynamic range as opposed to multiple documents.
>   
I believe I mentioned that earlier. This document should be part of the 
main document.
> 5. Your studies contain information based on looking at a single
>    cathode strip.  Doesn't it make sense to characterize multiple
>    channels on a single chip and to characterize multiple chips?
>   
We have characterized the ASIC some time ago. Both Mitch and I have 
independently measured the characteristics of the ASICs (gain 
variations, cross-talk, etc.) for all channels and ASICs. Gerard has 
also performed a considerable number of tests. All the documentation is 
on the portal.
> 6. You are specifying a nominal gain for the FDC anode wires, but
>    you have not shown any data in this report on the amplitudes for
>    a sense wire.  This seems entirely appropriate for this type of
>    study.
>   
You are correct. The emphasis on this measurements were the cathode 
strips to determine their response within the linear range of the ASIC. 
I agree we should perform similar measurements on the anode wires. 
However, the overall gain is high, even with the low gain shaper. 
Probably, we will decrease the gain on one of the shaper boards to take 
this into account. If we need to maintain the same linear response as on 
the cathode strips, the gain of the shaper will have to be decreased so 
that the overall gain will become 1.85 mV/fC/5 = 0.37 mV/fC.
> 7. You have only specified a proposed gain for the FDC ASICs, but
>    have not specified a true dynamic range for the chips.
>   
That's a good point. The ASIC dynamic range and its gain are 
inter-related. The linear range, and I presume that's what you are 
referring to, is up to 400 fC of input charge for point ionization, i.e. 
within 11 ns of peaking time and with the 3.7 mV/fC ASIC gain. For 
better linearity, about 3% or so, the linear range extends up to about 
300 fC. I  recall Gerard suggested something like  280 fC for about 1% 
linearity.
    The dynamic range of the chip, independent of the linear range, is a 
function of the power supply, biasing and gain. Because the output 
offset bias is set at 1.25 V, this is the maximum amplitude we can get 
out of the ASIC (+2.5 V supply). In this sense, the output dynamic range 
is fixed and bounded by the power supply. The CMOS process used on the 
ASIC dictates the power supply operation.
> 8. I would like to reiterate my statement from the other day.  I
>    think we should pay a bit more attention to the gain setting
>    of the anode circuitry.  I would not like to give up a fully
>    linear response for the anode wires, even though the plans are
>    to use discrimators/TDCs on the wires.
>   
See answer to question 6. On the word document I sent, I suggested the 
lower gain of 2 mV/fC for a possible difference between the FDC cathodes 
with the CDC anode wires. Both of these require operation within the 
linear range. Remember that we only have two gain settings on the ASIC.
> Anyway, these are the questions that came to me after reading.  I
> think it is fine to circulate the document in its given form to
> the group for discussion at today's FDC/CDC meeting.
>   
Generally speaking, we need to agree on the scaling factors based on 
Garfield, short of performing additional tests on a test beam with the 
characteristics of interest. We also need to perform additional tests 
and here is a tentative plan:

1. Perform tests with Fe-55, cathodes and low gain shaper. I propose 
repeating these measurements with a better scope that I have with 
histogram capabilities (just convenient).

2. Change shaper gain and perform similar tests on anode wires.

3. Perform long term tests with cosmics and DAQ.

4. Scale Fe-55 and cosmics responses to particles of interest in Hall D.

5. Write a single document.

This is just a tentative plan but we need to perform these tests within 
a week or so. I will circulate this email and the document so that 
Simon, Gerard, Curtis and Yves can also comment and agree on the action 
plan.

Regards,
Fernando



>
> 				Regards,
>
> 					Daniel
>
>
> **********************************************************************
> *                                                                    *
> * Dr. Daniel S. Carman                   e-mail : carman@jlab.org    *
> * Staff Scientist                        office : (757)-269-5586     *
> * Jefferson Laboratory                   web: www.jlab.org/~carman   *
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