The present invention generally relates to integrated circuit devices and, more particularly, to a reduced form factor complementary metal oxide semiconductor comparator exhibiting high speed operating characteristics.
In presently available digital video display systems, an analog input signal, representing an image is converted into a digital signal for later presentation to a user on a suitable display device (i.e., computer monitor, LCD, flat panel display, etc.). To provide such functionality, the analog input signal is first converted into a digital signal, appropriately filtered, and then the filtered digital signal is transmitted to the display device for presentation to the user.
The transmission of the digital signal from the converting and filtering device(s) to the display device is performed through the use of an interface. In particular, presently available digital display systems employ a digital video interface (DVI) interface which operates, for example, according to the standard promulgated by the Digital Display Working Group (DDWG). The DVI standard requires the use of transition-minimized differential-signaling (TMDS) to transmit graphic and other complementary data from a computing device, which performs the aforementioned conversion and filtering operations, to the display device. TMDS requires the use of a TMDS transmitter and a corresponding TMDS receiver.
The TMDS receiver may include comparison circuitry and other operational components that provide the receiver""s functionality. In order to meet the DVI standard, the receiver, in addition to other specifications, must be able to operate at frequencies up to about 1.65 GHz with 3-times oversampling. Consequently, any underlying comparator must also be able to operate at frequencies up to about 1.65 GHz with 3-times over sampling.
The aforementioned and related drawbacks associated with conventional comparators and their application in digital systems are substantially reduced or eliminated by the present invention. The present invention is directed to a complementary metal oxide semiconductor (CMOS) comparator capable of operating at frequencies of about 5.0 GHz, having a 0.35 xcexcm form factor layout. Thus, any device employing the comparator of the present invention will be in compliance with, and can be used, in conjunction with the DVI and other digital interfaces.
The comparator of the present invention includes a pair of cross-coupled inverters having a pair of outputs and a pair of input transistors, each having a gate, source and drain, each having a gate for receiving an input voltage signal, and a source and drain. Further included is a means, responsive to a first signal, for connecting together the outputs of the cross-coupled inverters so that the inverter outputs have a nearly equal voltage (about half the supply voltage) and for supplying a bias current to the input transistors so as to charge the drains of the input transistors based on input voltage signals present at the gates of the input transistors, a means, responsive to a second signal, for applying the charged drains of the input transistors to the outputs of the cross-coupled inverters to establish, on the outputs of the cross-coupled inverters, a voltage difference representative of a voltage difference at the inputs and for supplying a source current, from a supply voltage, to the bias current means, and a means for establishing an RC time-constant at the drains of the input transistors. By maintaining the output of the comparator at one-half the maximum output level, the switching characteristics (i.e. the speed in which the inverters provide an output signal) is enhanced as the comparator does not have to be pulled from a low voltage to a high voltage at high frequencies after each comparison. In other words, the comparator does not have to traverse a full voltage swing of the underlying components in order to perform a comparison between the corresponding input signals.
An advantage provided by the present invention is that the comparator can operate at high frequencies.
Another advantage provided by the present invention is that it requires less real estate than conventional comparators.
Yet another advantage provided by the present invention is that it exhibits a small peak-to-peak output voltage swing.
A feature of the present invention is that it operates over a wide input bandwidth range.