[Halld-tracking-hw] HVB - FINAL VERSION - and fADC

[zihlmann at jlab.org]

Hi Curtis,
what is the relative width of the minimum ionizing peak for the two cases.
The reason I ask is that if it is the same then both cases will give the
same result in terms of the efficiency at the lower limit. The peak position
alone does not say much the width is important as well.

cheers,
Beni

> Hi Eugene -
>
>     there is a broad dynamic range in the signals from the CDC, so we have
> been very careful to watch that as we sent along. What we are seeing now
> is that the minimum ionizing peak is at about 60 counts above pedestal
> with
> the new FADC125, but about 120 counts above pedestal with the older
> Struck. This is perhaps too close to the noise for us. What it looks like
> is
> that the least significant bit is a factor of 2 larger in the FADC125
> relative
> to the Struck.  This also has implications on the high end as the output
> of the ASIC was tuned to match the maximum of the Struck. We are concerned
> that there may be a mismatch causing us to loose a factor of two in our
> dynamic
> range.
>
>     Curtis
>
>
> On 4/12/11 10:47 AM, Eugene Chudakov wrote:
>> Hi,
>>
>> I may be missing something here - I do not see what a factor of 2 in
>> the FADC gain would improve. I would think that the going from 12 bits
>> to 11 bits in FADC is not terribly important, in particular for gas
>> chambers (the signal fluctuations>10%). One may argue that for the
>> FDC these fluctuations cancel out and we need to see small amplitudes
>> from the side bands. On the top of the fluctuations there is a noise.
>> The RMS of the noise from FDC strips is about 40 FADC bins. This noise
>> is semi-periodic and probably can be reduced by a factor of 4 or so
>> (Lubomir was doing it by looking at the channels nearby).  Still, the
>> noise is much wider than one FADC bin. Is the noise from the straws
>> much better?
>>
>> Eugene
>>
>>
>> On Tue, 12 Apr 2011, Fernando J. Barbosa wrote:
>>
>>> Hi Curtis and Gerard,
>>>
>>> My apologies for the confusion. It has been some time since I looked at
>>> this.
>>> The response of the shaper+Struck should be ~ fADC125.
>>>
>>> The ASIC on the FDC cathodes has a gain setting of ~4 higher than the
>>> gain on
>>> the CDC anodes (with unmodified HVB or ~2 w/ modified HVB). The ASIC
>>> dynamic
>>> range (at the same level of linearity) is, however, quite different for
>>> different gain settings (GlueX-doc-1364). This is, Qin x Gain is not
>>> constant
>>> (@5% peak amplitude linearity: Vout max @ high gain ~ 200 mV; Vout max
>>> @ low
>>> gain ~ 350 mV). We are happy with fADC125 settings for the FDC but the
>>> FDC
>>> would benefit from a slight increase in gain as well to fully utilize
>>> the
>>> full dynamic range of the fADC125.
>>>
>>> I agree with Gerard that doubling the gain of the fADC125 will not
>>> change the
>>> S/N that much and perhaps a slight increase of, say 20%, would be OK
>>> for both
>>> the CDC and the FDC. The following would require one fADC125 assembly:
>>>
>>> FDC - ASIC with current gain setting and increase fADC125 gain by 20%
>>> CDC - ASIC with current low gain setting (lowest possible), modified
>>> HVB and
>>> increase fADC125 gain by 20%.
>>>
>>> Best regards,
>>> Fernando
>>>
>>>
>>>
>>>
>>>
>>>
>>> Gerard Visser wrote:
>>>> Hi Fernando&  all,
>>>>      Please see below -->
>>>>
>>>>      - Gerard
>>>>
>>>> On 4/11/2011 9:29 PM, Fernando J. Barbosa wrote:
>>>>> Hi Curtis,
>>>>>
>>>>> My understanding was that your previous results were obtained with
>>>>> the
>>>>> fADC125.
>>>>> The Struck ADC does not have any shaping to speak of (other than the
>>>>> resulting
>>>>> convolution of the input pulse with its response) so it does not
>>>>> provide
>>>>> for a
>>>>> direct comparison of results.
>>>> I think we've always meant "shaper board + Struck ADC" when saying
>>>> "Struck
>>>> ADC" in this context...
>>>>
>>>>> The fADC125's dynamic range or "gain" cannot be adjusted via firmware
>>>>> to
>>>>> make
>>>>> full use of its dynamic range but requires changing a number of
>>>>> resistors
>>>>> and
>>>>> capacitors per channel (I believe a total of 2 of each). Is this
>>>>> correct,
>>>>> Gerard?
>>>> At it's most basic level, gain is set by just one resistor (the
>>>> feedback
>>>> resistor on the AD8129). However, in practice it's not so simple, the
>>>> current value is 3.09k and ~doubling that to double the gain is
>>>> probably
>>>> not a good idea, the value is getting too large and undesirable
>>>> parasitic
>>>> effects probably will happen. Doubling the gain instead would
>>>> requiring
>>>> halving the gain setting impedance. Easy enough, but it is made up of
>>>> three
>>>> resistors and two capacitors since it provides also the cable
>>>> frequency
>>>> response equalization.
>>>>
>>>> A strategy for prototyping probably would be to change one channel
>>>> each way
>>>> and measure the differences in frequency response. If not too bad, for
>>>> prototyping just change the gain with the feedback resistor and for
>>>> production do it "the right way". If it is significantly different,
>>>> then we
>>>> have to change 360 chip components. That is still feasible, just
>>>> tedious.
>>>> Probably could be done in 2 days or so of effort. There's minimal risk
>>>> of
>>>> breaking anything if a skilled tech does this work.
>>>>
>>>>> Changing these components is time consuming (72 channels) but should
>>>>> be
>>>>> done for
>>>>> testing the full readout chain prior to committing to the final
>>>>> assembly
>>>>> version. My concern is that this change may be incompatible with the
>>>>> FDC
>>>>> requirements, necessitating two assembly versions (OK), and will
>>>>> impact
>>>>> the
>>>>> procurement schedules. I believe that the FDC can accommodate a
>>>>> change in
>>>>> the
>>>>> dynamic range but perhaps not to the magnitude needed by the CDC.
>>>>> Correct,
>>>>> Lubomir?
>>>> I have to say I am fairly confused how there could be a significant
>>>> discrepancy between the FDC and CDC as far as the ADC12 input range is
>>>> concerned. The good linear output range from the GAS-II preamp should
>>>> be
>>>> basically the same in all cases. (Isn't that right, Fernando?)
>>>> Certainly we
>>>> want the good linear output range from the GAS-II, multiplied by the
>>>> loss
>>>> factor from the cable, to be about the input range of the ADC125. I.e.
>>>> that
>>>> the system GAS-II/cable/ADC125 should have a linearity which is just
>>>> about
>>>> starting to be unacceptable (due to GAS-II linearity effects) at the
>>>> top
>>>> limit of the ADC125 range. (Assuming of course that the pedestal is
>>>> set
>>>> reasonably like ~100 or so on the scale of 4095.)
>>>>
>>>> If the FDC is 'happy with' the ADC125 gain and the CDC is getting the
>>>> same
>>>> "good linear" output range from the preamp, then if we double the
>>>> ADC125
>>>> gain we will be seeing only half of the "good linear" output range of
>>>> the
>>>> preamp on the CDC. This probably doesn't help anything at all.
>>>>
>>>> I think we need to discuss that.
>>>>
>>>> Isn't Naomi's plot
>>>> http://www.jlab.org/Hall-D/software/wiki/images/4/4d/Run_31628_amp6.png
>>>> showing a pretty good signal/noise ratio for (I assume fairly
>>>> perpendicular) mips? Maybe I don't understand exactly what is plotted
>>>> here
>>>> ("Max amplitude" in the title)? It doesn't seem though that doubling
>>>> the
>>>> ADC125 gain will make a significant benefit. But I must be missing
>>>> something.
>>>>
>>>>> We need to discuss these issues at an upcoming tracking meeting with
>>>>> the
>>>>> CDC and
>>>>> the FDC groups and Gerard.
>>>> Yes, I will join in if we can discuss on wednesday...
>>>>
>>>>> Another topic that Gerard mentioned to me recently and that needs
>>>>> checking
>>>>> is
>>>>> the buffer size.
>>>>>
>>>>> Best regards,
>>>>> Fernando
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> Curtis A. Meyer wrote:
>>>>>> Hi Fernando
>>>>>>
>>>>>> the Struck FADC with the modified HVB is fine. It appears that the
>>>>>> FADC125
>>>>>> has a factor of two lower gain than the Struck. Perhaps there is a
>>>>>> way to
>>>>>> adjust this
>>>>>> in the firmware. Our original conclusions based on the data that we
>>>>>> collected
>>>>>> with the old struck are still valid, the factor of two is between
>>>>>> the two
>>>>>> FADCs
>>>>>> which has caught us a little by surprise.
>>>>>>
>>>>>> Curtis
>>>>>> On 4/11/11 5:43 PM, Fernando J. Barbosa wrote:
>>>>>>> Hi Naomi,
>>>>>>>
>>>>>>> The gain was halved from the original on the modified HVB to fit
>>>>>>> the
>>>>>>> amplitudes within the fADC range. From your histograms, it seems
>>>>>>> that
>>>>>>> you
>>>>>>> need to double the range of the fADC with the original HVB.
>>>>>>> Correct?
>>>>>>>
>>>>>>> What caused the dramatic change? Gas? Leaks?
>>>>>>>
>>>>>>> Certainly we need to discuss this as there will most likely be an
>>>>>>> impact
>>>>>>> on
>>>>>>> the FDC, if we are to have one fADC for both the CDC and the FDC.
>>>>>>>
>>>>>>> Best regards,
>>>>>>> Fernando
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Naomi Jarvis wrote:
>>>>>>>> Hi Fernando and Gerard,
>>>>>>>>
>>>>>>>> I have some results with cosmics and the new fADC online here:
>>>>>>>> http://www.jlab.org/Hall-D/software/wiki/index.php/New_fADC_with_cosmics
>>>>>>>>
>>>>>>>> This is still a work in progress, we can see (top row of
>>>>>>>> histograms)
>>>>>>>> that
>>>>>>>> the gain in the new fADC is about 1/2 of that of the Struck fADC
>>>>>>>> (&shaper).
>>>>>>>>
>>>>>>>> Is it possible to change the full scale range of the new fADC?
>>>>>>>>
>>>>>>>> > From previous work with the Struck fADCs, we know that the
>>>>>>>> preamp
>>>>>>>> output
>>>>>>>> saturates at approx 0.5V (measured after the shaper), so we would
>>>>>>>> only
>>>>>>>> be
>>>>>>>> using half the range of the ADC if we use the unmodified HVB.
>>>>>>>>
>>>>>>>> We would like to put this on the agenda for the tracking meeting
>>>>>>>> on
>>>>>>>> Wednesday.
>>>>>>>>
>>>>>>>> Naomi and Curtis.
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>
>
> --
> Prof. Curtis A. Meyer		Department of Physics
> Phone:	(412) 268-2745		Carnegie Mellon University
> Fax:	(412) 681-0648		Pittsburgh PA 15213-3890
> cmeyer at ernest.phys.cmu.edu	http://www.curtismeyer.com/
>
>
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