Abstract:
A multi-stage high-speed analog-to-digital converter with gain and offset adjustment. A programmable amplifier consisting of two capacitor arrays, and digitally controlled switching devices to manipulate an inputted analog signal. A four stage pipeline ADC provides an 8 bit digital output with redundancy and error correction.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of U.S. provisional application titled “PROGRAMMABLE GAIN ANALOG-TO-DIGITAL CONVERTER” filed on May 11, 2001, Ser. No. 60/290,427. All disclosures of this application is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the invention: 
     The present invention relates to an analog-to-digital converter, and more particularly, to a pipelined 4-stage ADC with digital gain, and offset control. 
     2. Description of related art 
     Analog-to-digital converters are circuits commonly used in communications, instrumentation, and consumer devices. A preferred embodiment of the invention utilizes its design in a video-graphics conversion circuit of a flat-panel monitor. Currently most personal computers utilize video cards that convert digital signals into RGB analog signals for displaying graphics on CRT monitors. It is therefore necessary for flat-panel displays to be able to interface with current video graphics systems. FIG. 1 shows a typical flat-panel video graphics conversion circuit. The flat-panel display requires an analog interface  16  to change the analog RGB signals from the RAMDACs  14  into the digital signals required by the graphics controller  18 . The RAMDACs  14  are configured at a pixel rate based on the resolution of the screen to convert the digital signals from the graphics processing engine  12  in the PC graphics card  10  into an analog 256-level pulse amplitude modulated signal which is transmitted along with timing signals to the analog interface  16  of the flat panel display. The analog interface  16 , which requires high-speed ADCs to convert the data into a digital format for processing, receives the analog graphics data. During the ADC conversion it is also necessary to alter the offset and gain of the signal for adjusting image quality. In a display device, increasing the gain setting results in an image with more contrast. The offset control is independent for the red, green, and blue channels and serves to shift the entire input range, resulting in a change in image brightness. Therefore a need exists for a high-speed, high resolution ADC with offset and gain control, that has low power consumption. 
     SUMMARY OF THE INVENTION 
     It is therefore, an object of the present invention to provide a pipelined analog-to-digital converter with offset and gain control. It is also an object to provide a pipelined analog-to-digital converter that has reduced power consumption. 
     To accomplish these and other objects of the present invention, a 4 stage pipelined analog-to-digital converter with a programmable gain stage is provided. The programmable gain stage comprises two capacitor arrays that serve as digital-to-analog converters. Each capacitor array is composed of a binary-weighted section that is further composed of two stages capacitively coupled together to reduce the capacitor ratios. An input analog signal is inputted to a programmable gain stage wherein the offset and gain of the signal can be manipulated using a digital control. A fully differential signal is then sent to the first stage of the pipeline ADC. The first stage outputs a 1.5 bit digital output and provides an extra gain of 2 to the signal. The extra gain allows the programmable gain stage to use less power, and therefore less power consumption in the overall circuit. The second and third stages each provide 2.5 bit digital outputs each. The final stage provides a 3 bit digital output. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, 
     FIG. 1 is a block diagram of the video graphics conversion circuit of the present invention according to a preferred embodiment. 
     FIG. 2 is a schematic diagram of the multi-stage gain ADC of the present invention according to a preferred embodiment. 
     FIG. 3 is a circuit diagram of the programmable gain stage of the present invention according to a preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific preferred embodiments in which the invention may be practiced. The preferred embodiments are described in sufficient detail to enable these skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
     FIG. 2 shows a preferred embodiment of the invention. An analog signal is inputted into a programmable gain stage  20 . The programmable gain stage  20  outputs a fully differential signal, which is fed into a 4 stage 8-bit pipeline ADC. The 1 st  stage  22  of the ADC pipeline provides a 1.5 bit digital output. The 2 nd  stage  24  and 3 rd  stage  26  each provides a 2.5 bit digital output. The 4 th  stage  28  provides a 3 bit digital output. The extra 0.5 bit in each of the first 3 stages are used for redundancy and error correction. The bits in the 4 different stages are summed together to give a total result of 8 bits. Each stage of the pipelined architecture must also provide a dc gain sufficiently high to ensure that the final settled value of the amplified residue is accurate. The dc gain requirement of the first stage is determined by the resolution of the pipeline ADC. The first stage used in the invention provides an extra gain of 2, reducing power usage in the programmable gain stage  20 , therefore reducing overall power consumption. 
     FIG. 3 shows the programmable gain stage  20  of the invention that consists of 2 capacitor arrays that serve as digital-to-analog converters. Each capacitor array is composed of a binary-weighted section along with a few additional capacitors. The binary-weighted section is further composed of 2 stages capacitively coupled together by capacitors  34  and  35  to reduce the capacitor ratios. Referring to FIG. 3, only a portion of the capacitor arrays will be discussed in details as each of the subsequent following portions is substantially the same. An upper capacitor  31  and lower capacitor  33  are connected by a switching device  32 . The switching device is inputted with an analog signal, a high voltage level, and a low voltage level (also can be a reference ground). The switching device is also inputted with a plurality of digital input signals controlling the internal switches. The internal switches (not shown) serve to connect the upper capacitors  31  and lower capacitors  33  with the input signals. By using appropriately sized capacitors within the array, a proper digital-to-analog conversion is achieved. This allows the control of the gain and offset of the input signal which in a preferred embodiment provides 8 bits or 256 steps of adjustment. The size of the least-significant-bit in offset adjustment is proportional to the full-scale output range which is not affected when the gain is adjusted. The programmable gain stage also provides good common mode rejection by switching the bottom plates of the corresponding capacitors in the two capacitor arrays together during an integration phase. The charges sampled by the capacitor arrays are integrated to capacitors  37  with the help of an amplifier  36  and switching devices  38  that are controlled by a plurality of digital input signals. In this way, the gain stage can sample a single-ended signal and convert it into a differential signal with a prescribed common mode value. The amplifier serves to provide the required gain to the signal. 
     In another embodiment of the invention, a number of the ADCs described hereinabove are interleaved together to give a higher throughput rate. 
     Various additional modifications may be made to the illustrated embodiments without departing from the spirit and scope of the invention. Therefore, the invention lies in the claims hereinafter appended.