Impedance matching system for high speed digital receivers

1. A digital differential line receiver adapted to receive a data line and a data-complement line of a differential signal comprising:

a differential signal to single-ended conversion amplifier coupled to receive the data line and the data-complement line of the differential signal;

a first termination resistor coupled to the data line of the differential signal;

a second termination resistor coupled to the data-complement line of the differential signal;

a first impedance-adjusting transistor coupled between the first termination resistor and a common mode line;

a second impedance-adjusting transistor coupled between the second termination resistor and the common mode line;

a control-voltage generator coupled to sense the common mode line and provide a control voltage, the control voltage generator configured to adjust the control voltage to a voltage level such that a combined impedance of the first termination resistor, the first impedance-adjusting transistor, the second termination resistor, and the second impedance-adjusting transistor matches a specified impedance.

2. The digital differential line receiver of claim 1 wherein the specified impedance is a characteristic impedance of a transmission line providing the data line and the data complement line to the digital differential line receiver.

3. The digital differential line receiver of claim 1 wherein a combined impedance of the first termination resistor and the second termination resistor is at least eighty percent of the combined impedance of the first termination resistor, the first impedance-adjusting transistor, the second termination resistor, and the second impedance-adjusting transistor.

4. The digital differential line receiver of claim 1 wherein the control voltage generator comprises a trimmable resistor and a reference impedance further comprising a reference impedance resistor having resistance a number N times a resistance of the first termination resistor, and a reference impedance transistor having a width 1/N times a width of the first impedance-adjusting transistor; a node coupled to the reference impedance and the trimmable resistor being coupled to a inverting input of a differential amplifier coupled to provide the control voltage, and a noninverting input of the differential amplifier coupled to a voltage matching a common mode voltage of the differential signal.

5. A method of adjusting impedance at inputs of a differential line receiver comprising:

electronically trimming a reference resistor;

providing termination impedances each comprising a resistor and an impedance-adjusting transistor, the resistor and impedance-adjusting transistor coupled in series;

provide a termination impedance model comprising a resistor in series with a transistor, the reference resistor and the termination impedance model being coupled in series at a model common mode node;

driving a current through the reference resistor and the termination impedance model;

using a differential amplifier to generate a control voltage applied to the transistor of the termination impedance model so impedance of termination impedance model matches resistance of the reference resistor; and

applying the control voltage to the impedance-adjusting transistor of the termination impedances.

6. The method of claim 5 wherein the reference resistor is trimmed to a resistance N times a desired impedance of the termination impedances, where N is a number; where the resistors of each of the termination impedances has value R BASE a value less than the desired impedance of each of the termination impedances, and where the resistor of the termination impedance model has value N times R BASE , where the impedance-adjusting transistor of each termination impedance has width T TRIM , and the transistor of the termination impedance model has width T TRIM /N.