LVDS CM Termination

Paul D. Aloisi 603-429-8687 aloisi at
Thu Oct 19 07:06:36 PDT 1995


The common mode termination is to terminate common mode reflections.
The system allows for common mode voltage differences of +/- 0.5 Volts
DC and over +/- 1 Volt AC. 

If the common mode resistance is zero the common mode voltage is dropp
accross the 60 ohm termination resistors. The 1 volt AC difference allowed
is 4 times the signal level. The 17 mA common mode current without common 
mode resistors vs 3 mA with the common mode resistor. 

The 60 ohm terminators have a 5% tolerance, the common mode current with
the maximum difference in resistor  at 17 mA worst case current difference
|from the common mode between the lines is 1.7 mA. The signal is only 2.2 mA
going through the same termination resistor versus 0.3 mA with the 150
ohm termination. Common mode current difference has distroyed your signal.

The second reason is to closer match the impedance of the cable common mode.
The common mode current should be terminated, not reflected by a short.

LVDS takes the best features of high powered SCSI and converts it to
a power range that requires external transceivers to a low power application
that allows the transceivers to be integrated into the controllers at
a cost only slightly higher than single ended. The termination for proposed
for LVDS is a low power version of the terminator used for the standard
differential which effectively has the same 150 common mode termination,
the 2 330 ohms to the supplies. The addition features in the proposed
terminator leaves the maximum signal margin when all the circuits are
mapped out.

The 150 ohm Common Mode termination needs to be there.

Paul D. Aloisi

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