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Entry  Tue May 18 09:24:02 2010, Stefan Ritt, Reference design for DRS4 active input buffer ac.pngac_bw.pngdc.pngdc_bw.pngDRS4_ft_V3.jpg
    Reply  Tue Oct 12 03:53:37 2010, Jinhong Wang, Reference design for DRS4 active input buffer 
       Reply  Tue Nov 16 16:38:06 2010, Stefan Ritt, Reference design for DRS4 active input buffer 
Message ID: 111     Entry time: Tue Nov 16 16:38:06 2010     In reply to: 110
Author: Stefan Ritt 
Subject: Reference design for DRS4 active input buffer 

Jinhong Wang wrote:

 

 Hi, stefan,

 

     In the DC coupled version of the analog drivers for DRS4 input in Eval. Board V3, you mentioned that CMOFS of THS4508 was set to 1.8V to match the input range of DRS4, however, will this clash with the requirements of  DRS4 input voltage between 0.1 V ~1.5V ? The output of THS4508 can easily rise beyond 1.5V for CMOFS=1.8V. I also noticed the resister paris R13/R15, R14/R16 was added among the output of THS4508 and the inputs of DRS4, were these resister pairs were used to attenuate the level of THS4508 output signal (a half ? ) to match the input requirements of DRS4? Maybe I have some misunderstanding about it.

 

No you are right about that. The THS4508 has a gain of +2, and by using the resistor pairs we do

1) Reduce the gain back to unity

2) Reduce the input DC level from 1.8V to 0.9V to match the input range

3) Terminate the signals at the input of the DRS chip to minimize reflections

I know this is not so obvious from the schematic, so thanks for asking.

- Stefan 

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