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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