Modifications to Rev 2 DOM Main Board
Local Coincidence
and
Communications Protection


Local Coincidence Mitigation

Scope:

The asymmetric neighbor signaling waveforms arise from limitations of the driver amplifier.   
Removing resistors R372 and R380 corrects the stage gain to produce symmetric waveforms.
Altering the window comparator thresholds places the expected signal in the range that will discriminate against spurious signals arising from modest cable impedance mismatches (bumps). 

Required components:

1 each  5kΩ or 4.99kΩ 1% resistor

Procedure:

Remove R372 and R380 as indicated below.  (image1)
Install 5kΩ  resistor from R373 to R381 as indicated below (image2)

Next modification

Gain Modification



Threshold shift

Communications Input Protection

Scope:

The footprint for U47, an abandoned loading option, provides the platform for the mounting of widely available ESD protective diodes.
This change assumes that diodes D1 and D2, near JP100,  are already removed...  (If they have not been removed already, please remove them at this time.)

Required Components:

4 each Low Capacitance Low Voltage Drop Diodes (Schottky).  Candidates are Vishay BAT83S, or SD101A,  (other candidates)

Procedure:

Remove U47, the MAX4145ESD, to make room for the protective diodes.

In the image below, pin 1 and 7 of the U47 footprint supply -5V.  Pins 8 and 14 of the U48 footprint supply +5V.
Install a diode, its anode to  U47 footprint pin 2,  and cathode to a +5V node.
Install a diode, its anode to  U47 footprint pin 6,  and cathode to a +5V node.
Install a diode, its cathode to  U47 footprint pin 2,  and anode to a -5V node.
Install a diode, its cathode to  U47 footprint pin 6,  and anode to a -5V node.


Protection Diodes

Other products
Candidates from Zetex can be found at http://www.zetex.com/frame.asp?page=http://www.zetex.com/3.0/b3-4.asp
however the packages are problematic.
Candidates from hp... I mean, Agelent, are at http://literature.agilent.com/litweb/pdf/5968-7181E.pdf

Any part with a capacitance of less than 2pF at a reverse voltage of 5V is probably OK. 
Products claiming larger forward current are preferred for their robustness.  It's a balancing act.

See http://literature.agilent.com/litweb/pdf/5968-7959E.pdf for information on Schottky diodes for transient protection.

Contact Gerald Przybylski.  if there are questions about these modifications.