High common mode relay multiplexer

DETAILED DESCRIPTION I claim: 1.
In a multiplexer having a plurality of signal channels, each of said channels having a plurality of signal lines, said lines including a high signal line, a low signal line and a guard line, a relay for each of said channels, said relays each having contacts connecting the signal input terminals of each line to its signal output terminal, each of said relays including a coil for actuating said contacts, the combination comprising: a floating voltage supply; an electronic switch connecting said supply across each of said coils; channel selecting means for closing a selected switch; the signal output side of each of said guard lines being interconnected; a connection between one side of said supply and the output side of said interconnected guard lines; the signal output side of each of said high signal lines being interconnected; and the signal output side of each of the said low signal lines being interconnected.
2.
The invention as defined in claim 1 wherein each of said switches is an optical coupler.
3.
The invention as defined in claim 2 wherein said optical coupler includes a light sensitive transistor.
4.
The invention as defined in claim 3 wherein said channel selecting means comprises a light-emitting diode in optical proximity to said light sensitive transistor.




Description:
BACKGROUND OF THE INVENTION One of the problems associated with relay multiplexing of low level analog signals is that the capacitance from the relay contacts to the coils tends to degrade the AC common mode rejection of the multiplexer.
In a typical multi-channel prior art relay multiplexer, each channel has high and low signal input lines and a guard line.
Normally the contacts of the relay are electrostatically shielded from the relay coil.
The function of the guard line is to provide an electrostatic shield for the high and low signal inputs.
When the electrostatic shield is driven by the guard, the effective capacitance between the relay contacts and associated coil is minimized