An automatic system for adjusting the output impedance of fast CMOS drivers

Claim 1

An automatic system for adjusting the output impedance of fast CMOS drivers, wherein the output impedance of a plurality of slaved drivers (DS1...DSn) is controlled by a circuit for measuring and correcting mismatch between one of the drivers (DM), taken as reference and dedicated to this aim, and the impedance at the input of a reference trans-mission line (L), equal to lines (L1...Ln) connected to the other drivers, the output impedance adjustment of the reference driver (DM) being carried out by a comparator comparing the maximum voltage of a clock signal supplied by that reference driver (DM) at the input of the reference line (L) with a reference voltage equal to half the supply voltage of the output stages of the drivers and dependant on the result of the comparison said comparator supplies a current which periodically charges or discharges a capacitor (C1), so as to obtain across its terminals a control voltage for the driver impedance, characterized in that a first clock signal is sent to the input (2, 3) of said reference driver (DM) producing a periodic signal on said reference line (L) terminated by a resistance (R) equal to its characteristic impedance and said periodic signal present at the reference line input is compared with said reference voltage in a threshold comparator (CO) in correspondence with the transitions of a second clock signal (on 6) with a frequency equal to that of the first clock signal, but in quadrature so as to carry out the comparison at the center of the half-period wherein the voltage at the input of the reference line (L) is maximum, the comparison result being used to charge or discharge the capacitor (C1), whose voltage is applied to the inputs of a plurality of buffer amplifiers, one (BU) of which controls the output impedance of the reference driver (DM) and the others (BU1...BUn) the output impedances of the slaved drivers (DS1...DSn).