[Halld-tracking-hw] HVB - FINAL VERSION - and fADC
Eugene Chudakov
gen at jlab.org
Tue Apr 12 12:54:14 EDT 2011
Hi Curtis,
what FADC pedestal width do you observe?
Thanks,
Eugene
On Tue, 12 Apr 2011, Curtis A. Meyer wrote:
> 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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