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question on offset DAC's for the ADC125
Hall D Electronics:
Hi Fernando, Elke,
I have a question concerning the desired specification for offset DAC's
for the ADC125.
As you know the fADC250 has an individual offset control for each channel.
On the other hand, for the ADC125, the existing specification
GlueX-doc-855-v2 has an offset DAC in groups of 12 channels. [The way
this would be implemented, the 6 such groups of 12 channels will be the
even channels and the odd channels of each of the three input
connectors. That's because the even channels are handled on the main
board, the odd channels on the mezzanine board; we'd have 3 DAC's per
board.]
However, as I am getting this portion of the design settled in detail,
I'm finding that it would not be so difficult to provide an individual
offset DAC for each channel at reasonable cost. I have identified
several candidate parts that could provide this function, while holding
to the other specifications (no detrimental effect on noise level, +/-
55% offset range).
The added cost of the individual offset scheme versus the original
6-blocks-of-12 scheme is about $83 per module. So the total cost
increase for the production run of 200 modules is $16,600.
I would recommend that we do this, it will be a more flexible solution
and it could increase the available dynamic range of the ADC in
application by a few percent, in case the GAS-1 (or other input source)
pedestals are not perfectly matched.
A decision should be taken within next few weeks, please. Thanks,
Gerard