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

[Gerard Visser]

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.