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

Naomi Jarvis nsj at cmu.edu
Sun Apr 17 10:07:35 EDT 2011 

The pictures are here

http://www.jlab.org/Hall-D/software/wiki/index.php/New_fADC_with_cosmics

The new fadc pic looks very like the Struck adc pic with the channel numbers scaled /2.  

Curtis was right about the dynamic range, there was a mismatch, Fernando's preamp-in-the-mail should fix that problem.

Naomi.


On Apr 17, 2011, at 3:14 AM, zihlmann at jlab.org wrote:

> 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.
>>> _______________________________________________
>>> Halld-tracking-hw mailing list
>>> Halld-tracking-hw at jlab.org
>>> https://mailman.jlab.org/mailman/listinfo/halld-tracking-hw
>>> 
>> 
>> 
>> --
>> 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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> 
> 
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