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

Tue Apr 12 02:27:23 EDT 2011

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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