[hpsdr] LTC2208 input network design

[jeff millar]

Mark...

I've done quite a bit of work using LTC family of ADC's for cellular 
applications and understand the issues in driving the ADC properly.  But I can't 
find schematic for Hermes or Mercury.  Can you point me to them?

In general, the ADC has sensitivity to signals and noise up to 1 GHz or more.  
The ADC buffer/driver puts out wide band noise and all that noise could get 
folded into the output digital stream. So, any LC network between buffer and ADC 
is designed to roll off most of the noise above the intended operating 
frequencies for the ADC to prevent the wide band noise from making ADC noise 
figure and dynamic range worse.

It's also important to keep a low output impedance of the network because the 
ADC has sample and hold transients that could kick voltage noise back into the 
input if the impedance was too high at some frequency.

An LC simply rolls off faster than an RC and so does a bit better at limiting 
buffer output noise outside the band of interest.

jeff, wa1hco

On 09/26/2012 01:37 PM, Mark Haun wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> Is there a Mercury or Hermes guru who could explain the selection of the ADC
> input network (L13, L17, and C87 in the Hermes schematic, between the
> differential amp and the ADC) and the reasoning behind it?  I'm trying to
> understand the design trade-offs, why LC instead of RC, and so on.  I tried
> searching the list archives but didn't find anything.
>
> Thanks,
>
> Mark  KJ6PC
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