Re: New Preamp Tests

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

Hi Gerard,

    I agree, the polarity is arbitrary - Simon will let us know if 
that's not the case. In my previous email, I didn't mean to imply that 
we should use the cards in the reverse of their intended application - 
just an observation. Clearly, the best approach for using all the 16 
channels and because of polarity, if needed, is to build a simple wire 
adapter on the shaper end, as you suggested earlier.

    I noticed that you referred to the channel numbers starting with 
channel 0 in your earlier email. We should stick to the convention 
adopted on the ASIC as 1-8 and carry it on, at least to the input of the 
fADC.

Regards,
Fernando



Gerard Visser wrote:
> Hi Fernando, Simon,
>     I'm confused, why does it matter that the polarity is backwards 
> from what you expect? It's arbitrary anyway. Would it not be better to 
> adjust the ADC baseline trimpot (or the baseline trimpot on the 
> receiver/shaper board) so that the signals can be used in this 
> "reverse" polarity? If we misapply the ASIC (such that it's internal 
> bias point is not at the "bottom" end of the signal range but rather 
> the "top") this will compromise its performance. [Not only a reduced 
> dynamic range but linearity and slew rates can be different.] 
> Measurements taken that way won't necessarily correspond to the real 
> performance. The receiver/shaper board and the ADC _will_ work with 
> signals of either polarity, so there is _no_ problem to have "the 
> wrong" polarity of signals there. (Of course, in Simon's analysis 
> software the sign will have to be flipped, that I presume is easy.)
>     If we _must_ have the expected polarity of signals, please cross 
> the cable wire pairs rather than crossing the ASIC input pins, I suggest.
>     Possibly the reason for the wrong polarity is that the ERNI 
> connector uses b1 on the first wire (red wire) of the cable, a1 is the 
> 2nd wire. This is a strange convention but that is the way ERNI has 
> it. I think we overlooked this in defining the ERNI connector pinout.
>
>     Gerard
>
>
> Fernando J. Barbosa wrote:
>> Hello everyone,
>>
>>     I just have a few comments regarding the latest tests that Simon and
>> I performed with the new preamp on the FDC prototype:
>>
>> Polarity: The polarity at the output of Gerard's modules (shaper boards)
>> is reversed relative to what we would expect. Because of this, we are
>> using the anode preamp card  (A inputs) on the cathodes and the cathode
>> preamp card (B inputs) on the  anode wires. It is understood that the
>> dynamic range is not well matched in this way as the A and B inputs have
>> been designed to handle the anode and cathode wires, respectively.
>> Simon, can then connect the outputs to the ADC. The polarity reversal
>> has to do with  the order  of the outputs  going into the shaper board.
>> Gerard, can you comment on this? Sometime ago you mentioned that the
>> polarity could be selected on the shaper board but maybe you were
>> referring to the final board.
>> ...
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