A/D converter for wideband digital communication

#This invention relates to analog-to-digital (“A/D”) signal conversion in wideband digital communication. Specifically, the invention relates to improving the dynamic range of A/D converters (“ADCs”).

#Currently, in many areas of high-speed digital communication, analog signals are processed and converted into digital data by being downconverted, filtered, and amplified to a particular analog baseband frequency prior to A/D conversion. As a result, such signals are confined to certain frequency boundaries and therefore make ADCs with uniform quantization (i.e., equal granularity or step sizes between increments) an acceptable solution for such applications.

#However, the recent advent of digital cable television and cellular phone technologies such as CDMA (Carrier Division Multiple Access) has brought about an increase in the use of wideband digital signals. These signals, which may typically be approximated by a Gaussian probability distribution function, are generally small in amplitude with extremely large amplitude spikes. As a result, the dynamic range of the previous uniform ADCs are no longer adequate, since they would result in either the sacrificing of small-scale resolution or else would clip the large-amplitude signals due to their limited range.

#FIG. 1 is a simplified block diagram of an illustrative embodiment of A/D conversion circuitry in accordance with the present invention; and

#FIG. 2 is an illustrative circuit diagram of a nonlinear amplifier portion of the A/D conversion circuitry in accordance with the present invention.

#FIG. 1 shows an illustrative block diagram of the circuitry of the present invention. The circuitry in FIG. 1 includes a first nonlinear amplifier 10, ADC 12, and inverse transfer function circuitry 14. Analog input signals are first transmitted to nonlinear amplifier 10. Nonlinear amplifier 10 amplifies the input signals according to a nonlinear output function ƒ(x). The amplified signals are then passed on to ADC 12. ADC 12 digitizes the amplified incoming analog signals, and subsequently outputs the converted digital signals to inverse transfer function circuitry 14. Inverse transfer function circuitry 14 has an output function that is the inverse (i.e., ƒ−1(x)) of that of nonlinear amplifier 10. Circuitry 14 performs a nonlinear mapping to the received digital signals that results in the desired linear relationship between the analog input and the final digital output signals.

#FIG. 2 is an illustrative circuit diagram of nonlinear amplifier 10 in accordance with the present invention. In particular, nonlinear amplifier 20 in FIG. 2 includes amplifier 22, resistors 24 and 26, and matched diodes 28 and 30. Amplifier 22 may be for instance an operational amplifier (“op-amp”). Diodes 28 and 30 are connected in parallel facing opposite directions and are matched to each other so as to cause nonlinear amplifier 20 to produce an output that has zero-input symmetry. In other words, the output of nonlinear amplifier 20 is dependent only on the magnitude of the input.

#The gain of nonlinear amplifier 20 is determined by the electrical characteristics of resistors 24 and 26 and of diodes 28 and 30. Specifically, the amount of current I across diodes 28 and 30 for a given voltage drop V across the diodes is (the same equation applies to both diodes): I=Io×((exp(V/Vo)−1) where Io and Vo are constant values that depend on the characteristic of each corresponding diode. Plugging this expression into the equation for the impedance Zd for each diode yields: Zd=V/I=V/(Io×((exp(V/Vo)−1)