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Message ID: 139     Entry time: Mon Dec 12 16:43:04 2011
Author: Stefan Ritt 
Subject: DC coupled DRS4 input stage 

In the attachement you will find a working DC-coupled input stage to the DRS4 chip. The bandwidth of this design is about 700 MHz, the gain is 1.

The upper version does not have an additional input buffer. This is not a very "clean" design, since the differential driver has an input impedance of 150 Ohm, which together with the 75 Ohm termination resistor gives about 50 Ohm termination. 

The lover version has an additional input buffer, which nicely decouples the input from the differential driver, so a proper 50 Ohm termination can be used at the cost of additional power for that buffer.

The COM common mode voltage and the OFS voltage have to be set according to the required input range, so that ranges such as 0V...1V, -0.5V...+0.5V, -1V...0V can be used. The COM voltage can be high impedance (simple resistor divider), while the OFS voltage needs to be low impedance (fast active buffer). The analog switch ADG918 can be used to digitize a precise calibration voltage CAL+, which can then be used for precise gain calibration of the DRS4 sampling cells.

Attachment 1: DRS4_front_end_DC.pdf  47 kB  | Hide | Hide all
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