[Halld-tracking-hw] HVB - FINAL VERSION - and fADC
Simon Taylor
staylor at jlab.org
Tue Apr 12 11:44:35 EDT 2011
Hi.
After some clarification from Gerrard as to how the data in each channel
are stored for 12 bit vs 14 bit ADCs, it looks
like the value for the RMS pedestal noise of 40 quoted below by Eugene
needs to be revised down by a factor of 4 to about 10. We have 12 bit
adcs in our setup.
Simon
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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