The present invention relates to electronic circuits, and more particularly, to an input network with equalization and a wide common-mode range.
Signals transmitted through a transmission system with termination at the receiving end can be either AC (alternating current) coupled or DC (direct current) coupled. AC coupling is typically used when the common-mode voltage or power supply voltage of the transmitter is different from that of the receiver. DC coupling is typically used when the common-mode voltage or power supply voltage of the transmitter and the receiver are close to one another.
FIG. 1 illustrates a prior art transmission system that is typically used in a DC coupling environment. The transmission system has a transmitter, transmission lines, and a receiver. The transmitter includes NPN bipolar junction transistors 101-102, load resistors 103-104, and current source 105. The transmitter is coupled to receive a supply voltage VCC_TX. The transmitter is configured to receive a differential input signal at inputs IN+ and IN− and to transmit a differential output signal through two transmission lines to the inputs of input buffer 110 in the receiver. The two transmission lines each have a characteristic impedance of Z0.
Input buffer 110 is coupled to receive a second supply voltage VCC_RX. Two termination resistors 111-112 are coupled to the two input terminals of input buffer 110. Resistors 111-112 are also coupled to receive an exposed termination voltage VTERM. VTERM is applied through a pin 115 to resistors 111-112. Without VTERM, the receiver has a limited common-mode voltage range when the difference between supply voltages VCC_TX and VCC_RX is significant. Voltage VTERM is used to extend the common-mode voltage range in the receiver. However, VTERM requires an additional power supply voltage and an additional pin, which are undesirable requirements in many applications.