Re: question on offset DAC's for the ADC125

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

Hi Gerard,

It is a reasonable cost and I recommend that it should be implemented . 
I believe that the  impact on power should also be minimal as there are 
quite a few devices on the market that have multiple channels per chip, 
low power and low cost.

Thanks,
Fernando




Gerard Visser wrote:
> 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
begin:vcard
fn:Fernando J. Barbosa
n:Barbosa;Fernando J.
org:Jefferson Lab
adr:Suite #10, 12B3;;12000 Jefferson Ave.;Newport News;VA;23606;USA
tel;work:757-269-7433
version:2.1
end:vcard