Patent Publication Number: US-2002003485-A1

Title: Method of performing A/D conversion, and an A/D converter

Description:
FIELD OF THE INVENTION  
       [0001] The invention relates to A/D conversion using a sigma delta modulator.  
       BACKGROUND OF THE INVENTION  
       [0002] Conversion of analogue signals into a digital form makes both the transmission and processing of signals easier and more efficient. A/D conversions are important in mobile phone technology, for example. One of such A/D converter arrangements is a cascaded second-order sigma-delta converter. The idea with a second-order sigma-delta converter is that the analogue signal to be converted is sampled by means of one second-order sigma-delta converter and the quantization error resulting from the conversion is sampled by means of another sigma-delta converter, the quantization error being subtracted from the samples of the signal. Such an arrangement is described in more detail in U.S. Pat. No. 5061928, for example, herein incorporated by reference.  
       [0003] There are drawbacks in an A/D converter like this. A sigma-delta converter consumes a lot of current and power, because the settling time of the sampler and the amplifier is only half of the length of the sampling period, which consists of one or more clock cycles.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0004] An object of the invention is to improve the A/D conversion and the A/D converter implementing the method in order to decrease the current and power consumption. This is achieved by a method of performing A/D conversion, in which method the A/D conversion is carried out by using at least two sigma-delta converters connected in cascade, the sigma-delta converters operating at a double sampling rate.  
       [0005] An object of the invention is also an A/D converter comprising at least two sigma-delta converters connected in cascade, the sigma-delta converters being arranged to operate at a double sampling rate.  
       [0006] Preferred embodiments of the invention are disclosed in the dependent claims.  
       [0007] The invention is based on the idea that the A/D conversion is carried out by means of at least two cascaded sigma-delta converters, in which a double sampling rate is used. In the converter, the analogue input signal is converted by at least one sigma-delta converter, the quantization error of the converted signal is converted by means of at least one other sigma-delta converter, and the quantization error converted into a digital form is subtracted from the converted digital signal.  
       [0008] Several advantages are achieved by the method according to the invention. The current and power consumption of the sigma-delta converter will be decreased, which enables the use of high sampling rates. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009] The invention will be described in greater detail in connection with preferred embodiments, with reference to the attached drawings, in which  
     [0010]FIG. 1 illustrates the sampling and forming of residue during the sampling period;  
     [0011]FIG. 2 illustrates a block diagram of a receiver;  
     [0012]FIG. 3A illustrates two cascaded sigma-delta converters FIG. 3B illustrates a fourth-order sigma-delta converter; and  
     [0013]FIG. 4 illustrates an embodiment of forming a double sampling rate. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0014] An A/D converter according to the invention is applicable to a radio system receiver but not limited thereto. Radio systems include CDMA (Code Division Multiple Access), GSM (Global System for Mobile Communication), WCDMA (Wide Band CDMA) and TDMA (Time Division Multiple Access) systems.  
     [0015]FIG. 1 further clarifies conventional sampling. During the first half  2  of the sampling period a sample is taken, and during the second half  4  of the sampling period integration is carried out. Conventionally, the sample is taken into one or more capacitors present in the sampling and the integration is carried out by means of capacitors and amplifiers present in the integration. The sampling period, during which the sample is taken (or correspondingly, during which the sample is integrated), is conventionally one clock cycle controlling the operation of the sampling and integrating capacitors. In the arrangement of the invention, however, the sampling is performed in the half  2  of the sampling period and in the half  4  of the sampling period separately; in other words, two samplings are carried out during the sampling period. Correspondingly, the integration is effected during the half  2  and the half  4  of the sampling period separately. Hereby, a new sample is taken during the integration of the sample already taken, such a sampling rate being called a double sampling rate. The current and power consumption will, however, remain the same as at a single sampling rate.  
     [0016]FIG. 2 illustrates a block diagram of a receiver. A radio-frequency signal received by means of an antenna  10  of the receiver proceeds to radio-frequency means  12 , in which the received signal is converted into a baseband signal. This is usually carried out in such a way that the radio-frequency signal is multiplied by a radio-frequency signal of a local oscillator and the input signal formed is low-pass-filtered. The baseband signal is converted into a digital form in an A/D converter  14 , after which the digital signal is processed in signal processing means  16 . The operation of the receiver is controlled by a control block  18 .  
     [0017]FIG. 3A shows a block diagram of the cascade connection of two sigma-delta converters. The performance numbers of the sigma-delta converters may be the same or differ from each other. An analogue signal enters a first sigma-delta converter  50 , the outputs of which are formed by a one-or multi-bit output signal  56  and a quantization error signal  58 . The quantization error signal  58  is fed into a second sigma-delta converter  52 , in which the quantization error is converted into a digital error signal  60 . Both the output signal  56  and the error signal  60  are fed into a digital processing block  54 , in which the quantization error is subtracted from the converted signal in a manner known per se. Although FIG. 3A presents a cascade connection of only two sigma-delta converters, it will be obvious to a person skilled in the art that it is possible to connect several sigma-delta converters in cascade.  
     [0018]FIG. 3B illustrates one embodiment for an A/D converter as a block diagram. The A/D converter disclosed comprises a fourth-order sigma-delta converter, which has been formed by connecting two second-order sigma-delta converters in cascade. The analogue signal to be converted enters an amplifier  100 , proceeding further to a delay element  102  and an adder  106 . These blocks  100  to  104  form a first integrator, in which a sample is taken from the signal at a double sampling rate into capacitive units, and the sample charged in the capacitive units is amplified (sampling at a double sampling rate is described in more detail in connection with FIG. 4). The integrated sample signal proceeds further to the next adder  106 , in which a first feedback signal is subtracted from the sample signal. The difference signal is integrated in an integrator illustrated by means of an adder  108 , a delay element  110  and an amplifier  112 . In the integrator, a sample is taken from the signal at a double sampling rate. Subsequently, a second feedback signal is subtracted from the sample signal in an adder  114 . The difference signal is integrated by an integrator illustrated by means of an adder  116  and a delay element  118 . The integrated signal receives its digital value as the sign is determined in a sign element  120 , which is a comparator, for example. By means of the sign, the value of the bit is determined either as −1 or 1, which correspond to the bit values 1 and 0. After the sign element  120 , the signal branches into two feedback loops connected to different locations and into two digital signal-processing parts.  
     [0019] In the feedback, the quantized value is scaled in a scaling element  122  to be suitable for the feedback. The first feedback signal is further integrated in an integrator illustrated by an amplifier  126 , a delay element  128  and an adder  130 . Also in this integration, a double sampling rate is used. The integrated signal is subtracted in the adder  106 . The second feedback signal is connected to the adder  114  through a sampling amplifier  124 . The sampling in the amplifier  124  is performed at a double sampling rate. These functions in the blocks  100 - 124  are included in a first second-order sigma-delta amplifier  216 .  
     [0020] A second sigma-delta converter  218  connected in cascade to the first sigma-delta converter operates in the same way as the first sigma-delta converter  216 . The output signal of the first sigma-delta converter  216  and the input signal of the sign element  120  function as the input of the second sigma-delta converter  218 . The difference of these two, formed by amplifiers  150  and  152  and an adder  154 , corresponds to the quantization error. The difference signal is integrated in an integrator illustrated by a delay element  156  and an adder  158 . The first subtraction of the feedback signal is carried out in an adder  160 . Samples are taken from the difference signal at a double sampling rate, and the difference signal is integrated in an integrator illustrated by an adder  162 , a delay element  164  and an amplifier  166 . The second feedback signal is subtracted from the sample signal in an adder  168  and the difference signal is integrated in an integrator illustrated by an adder  170  and a delay element  172 . The digital value of the quantization error is formed in a sign element  174 . After the sign element  174  the signal branches into two feedback loops connected to different locations and into two digital signal-processing parts.  
     [0021] In the feedback, the value of the bit is scaled in a scaling element  176 . The first feedback signal is further integrated in an integrator illustrated by an amplifier  180 , a delay element  182  and an adder  184 . Also in this integration, a double sampling rate is used. The integrated signal is subtracted in the adder  160 . The second feedback signal is connected to the adder  168  through a sampling amplifier  178 . The sampling in the amplifier  178  is performed at a double sampling rate. These functions in blocks  150 - 178  are included in the second-order sigma-delta converter  218 .  
     [0022] The digital output signal of the first sigma-delta converter  216  proceeds to two delay elements  200  and  202 . The delays correspond to the processing phases of the second sigma-delta converter  218  and keep the signals of the first and second sigma-delta converters synchronized. The digital output signal of the second sigma-delta converter  218  proceeds to a differentiator unit comprising two differentiators. The first differentiator comprises a delay element  204  as well as an adder  206 , in which the delayed signal is subtracted from the output signal. The second differentiator comprises a delay element  208  as well as an adder  210 , in which the delayed output signal of the adder  206  is subtracted from the output signal of the adder  206 . The amplifier  212  scales the difference signal, which is added in the adder  214  to the delayed output signal of the first sigma-delta converter  216 . In this way, a digital signal is converted into an analogue signal by using two cascaded second-order sigma-delta converters at a double sampling rate.  
     [0023]FIG. 4 shows one embodiment of a second-order sigma-delta converter, corresponding essentially to the second-order sigma-delta converter disclosed in EP patent 901233. In the converter, a double sampling rate is used. A double sampling rate is achieved by a pair of capacitors. The arrangement in FIG. 4 represents a bilinear converter, in which the amplifier functions as a differential amplifier and the disturbance that is caused by the mismatch resulting from possible different values of the sampling capacitors is connected to the common mode ground so as not to disturb the operation. The integration is carried out by exploiting common mode ground. As the operation of the converter is known per se, it will not be explained in detail here. The input of the converter is formed by a positive and a negative input voltage. When taking a sample during one sampling period consisting of one or more clock cycles, capacitors  220 ,  228  are connected to the positive input voltage and common mode ground by means of switches  238 ,  242 ,  254  and  258 . Correspondingly, capacitors  222 ,  230  are connected to the negative input voltage by means of switches  240  and  256 , being, however, connected to different input poles of an amplifier  236  by means of switches  244  and  260 . As the sampling and integration can be performed during the whole sampling period, the sampling is called &#39;sampling at a double sampling rate&#39;. Also feedback capacitors  246  and  226  are connected to the input poles. During the following sampling period, the capacitor connections are changed by means of switches. During each sampling phase, the amplifier  236  performs the integration of the sample that is charged in the capacitors connected to the input poles by means of capacitors  232  and  234 . This corresponds to the blocks  108 ,  110  and  112  (or  162 ,  164  and  166 ) in FIG. 2. The second integration phase corresponding to the integrators  116  and  118  (or  170  and  172 ) in FIG. 2 comprises capacitors  262  to  272 , switches  278  to  292  and an amplifier  274 . Here, too, the double sampling rate is achieved by pairs of capacitors, which are alternately connected to function as sampling capacitors. The values of the capacitors  220  to  230  are four times higher than those of the capacitors  232  and  234 . The values of the capacitors  270  and  272  are twice as high as those of the capacitors  262  to  268 . A positive or a negative input voltage is selected by means of switches  294  and  296 .  
     [0024] The arrangement according to the invention is applicable to one-or multi-bit conversions. In multi-bit conversion, the quantization is carried out on several levels (e.g. block  120 , 174 ). Hereby, the output of one converter is formed by several bits at each conversion period.  
     [0025] Furthermore, the A/D converter may comprise sigma-delta converters of various orders in cascade instead of two second-order sigma-delta converters. For instance, the cascade may be formed by one second-order sigma-delta converter and one or two first-order sigma-delta converters. It is also possible to use a sigma-delta converter of a higher order. One or more sigma-delta converters may, in a cascade connection, perform multi-bit conversion instead of one-bit conversion.  
     [0026] While the invention has been described with reference to the example according to the appended drawings, it is obvious that the invention is not limited thereto but may be modified in many ways within the inventive idea defined in the appended claims.