Re: Preamp tests

["Fernando J. Barbosa" <barbosa@jlab.org>]

Hall D Electronics:

Hi Yves,

    I forgot to mention on my previous email, and in reference to your 
observations, that the preamp input has a bias voltage present (~ a few 
100 mV) at the input when powered. This bias is provided inside the ASIC 
and is a result of the circuit topology employed.

Best regards,
Fernando



Fernando J. Barbosa wrote:
> Hall D Electronics:
>
> Hi Yves,
>
>    Did you say the shields are not connected at all? That's it! The 
> cause of the oscillations is due to the fact that the cables 
> connecting the sense wires to the HV board are just antennas - their 
> shields, to be effective, need to be connected to some low impedance 
> reference point. Because these cables are RG174, I believe from a 
> previous picture, the maximum potential they can tolerate is about 900 
> V. Therefore, their shields cannot be connected to GND.
>
>    Looking at the layout of the HV board, I notice there is a resistor 
> (R50) connecting a trace to the HV and with a decoupling capacitor (at 
> the bottom of the board). Was this trace meant to connect the shields 
> of the cables to the HV reference? In this case, the potential between 
> the inner conductors and the shields is about zero. The shield has 
> then a low AC impedance to GND through the capacitor. You have to be 
> careful how you route the cables for safety but try connecting the 
> shields to this trace. You may want to the decrease R50 to 10 kOhm or so.
>
>    Alternatives? Use thicker cables that can handle 3 KV (RG58?) - 
> this may not be an option; bring the sense wire pins out of the gas 
> volume and design a HV board with short traces and good shielding. 
> There may be others that we need to look at carefully.
>
>    Also, I noticed that the decoupling capacitors are 33 pF - a bit on 
> the low side. I would change the capacitors to 220 pF, 3KV. This value 
> is not critical - just a commonly available capacitor.
>
> Best regards,
> Fernando
>
> Yves Van Haarlem wrote:
>> Hi Fernando,
>>
>> I made a report of all the tests I did today, you can find it on the 
>> wiki:
>>
>> http://www.jlab.org/Hall-D/software/wiki/index.php/CDC_19_12_2007
>>
>>
>> It should include answers of your questions and measurements you 
>> suggested.
>>
>>
>> Cheers,
>>     Yves
>>                           /--------------------------------
>>                          /   Yves Van Haarlem
>>        _--~~--_         /
>>      /~/_|  |_\~\      /      Carnegie Mellon University
>>     |____________|    /        Department of Physics
>>     |[][][][][][]|:= /          Wean Hall room 8404
>>   __| __         |__ \            Pittsburgh, PA 15213
>>  |  ||. |   ==   |  | \           USA
>> (|  ||__|   ==   |  |) \
>>  |  |[] []  ==   |  |   \           Tel.:   +1 412 268-6949
>>  |  |____________|  |    \                  +1 412 641-9252
>>  /__\            /__\     \          Fax.: +1 412 681-0648
>>   ~~              ~~       \-----------------------------------
>>
>> On Wed, 12 Dec 2007, Fernando J. Barbosa wrote:
>>
>>> Hi Yves,
>>>
>>>   I have attached the results I obtained from the tests with the 
>>> interposer + Preamp Card + cable + termination. As you can see, the 
>>> pulse response is quite nice and the noise is also quite low.
>>>
>>>   As I mentioned before, the oscillations you observe with your set 
>>> up are definitely a result of the connections to the preamp card. I 
>>> believe your tests over the last few days showed that ground loops 
>>> are not the cause.
>>>
>>> Best regards,
>>> Fernando
>>>
>