Re: New Preamp Tests

[Gerard Visser <gvisser@indiana.edu>]

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.
>...