METHOD AND APPARATUS FOR CONTROLLING STAGES OF A MULTI-STAGE CIRCUIT

Abstract: A control mechanism that can be used to control a SIGMA DELTA to provide the required level of performance while reducing power consumption. The SIGMA DELTA ADC is designe dwith multiple stage (i.e., loops or sections), and provides improved performance (e.g., higher dynamic range) as more stages are enabled. The control mechanism selectively enables a sufficient number of stages to provide the required performance and disables remaining stages to conserve power. The control mechanism achieves this by measuring one or more characteristics (e.g., signal level) of the ADC input signal through a SIGMA DELTA ADC that is similar to the SIGMA DELTA ADC on the signal path, comparing the measured characteristic(s) to particular threshold level(s), and controlling the stages such that the desired objectives are achieved. In one implementation, the control circuit includes one or more detector stages, a conditioning circuit, and a signal processor. The detector stage(s) receive the input signal and provide a detected signal. The conditioning circuit receives the detected signal and provides conditined samples. The signal processor receives the conditioned samples and provides a control signal that selectively disables zero or more SIGMA DELTA stages in the SIGMA DELTA ADC.

Description: METHOD AND APPARATUS FOR CONTROLLING STAGES OF A MULTI-STAGE CIRCUIT BACKGROUND OF THE INVENTION I. Field of the Invention The present invention relates to electronic circuits. More particularly, the present invention relates to a novel and improved method and apparatus for controlling stages of a multi-stage circuit such as a sigma-delta analog-to- digital converter (ΣΔ ADC). II. Description of the Related Art An analog-to-digital converter (ADC) is an important component in many electronic circuits, and is especially important in digital communication systems. An ADC converts a continuous analog waveform into discrete samples at evenly spaced time intervals. The samples can subsequently be processed by other digital signal processing blocks to provide enhancement, compression, and/or error detection /correction of the sampled data. Exemplary applications which require ADCs are code division multiple access (CDMA) communication system and high-definition television (HDTV). Some important performance parameters of an ADC include linearity, DC offset, and signal-to-noise ratio (SNR). Suboptimal values for these parameters can cause degradation in the performance of a communication system. Linearity relates to the difference between an actual transfer curve (digital output versus analog input) and the ideal transfer curve. For a flash ADC, good linearity is more difficult to obtain as the number of bits in the ADC increases. The DC offset can degrade the acquisition and tracking performance of phase locked loops and the error detection/correction capability of the decoder, such as the Viterbi decoder. SNR can affect the bit-error-rate (BER) performance of the communication system because the quantization and circuit noise from the ADC results in degradation of the sampled data. In many communication systems, the received RF signal is downconverted to baseband before quantization. Typically, the received signal is downconverted from a RF frequency to an intermediate frequency (IF) in the first downconversion stage. The first downconversion allows the receiver to downconvert signals at various RF frequencies to a fixed IF frequency where signal processing can be performed. For example, the fixed IF frequency allows for a fixed bandpass filter, such as a surface acoustic wave (SAW) filter, to remove undesirable images and spurious responses from the IF signal before the second downconversion stage. The IF signal is then downconverted to baseband where sampling is performed to provide the digitized baseband samples. In most communication applications, an ADC is required at the receiver. In some applications, the receiver is a commercial unit where cost and reliability are important design criteria because of the number of units produced. Furthermore, in some applications, such as a CDMA mobile communication system, power consumption is critical because of the remote/portable nature of the receiver. In the prior art, a flash ADC or a successive approximation ADC is used to sample the received signal. In the flash ADC, the input signal is compared against L-l reference voltages, which are generated by a resistive ladder, by L-l comparators. Flash ADCs are bulky and consume large amount of power because L-l comparators and L resistors are required. Furthermore, flash ADCs can have poor linearity and poor DC offset characteristics, if the L resistors in the resistive ladder are not matched. However, flash ADCs are popular because of their high speed. Successive approximation ADCs are also often used in communication systems. These ADCs minimize complexity by performing approximations of the input signal over two or more stages. However, these ADCs can also exhibit the same poor linearity and poor DC offset characteristics as exhibited by the flash ADCs. Therefore, successive approximation ADCs as well as flash ' ADCs are not ideal candidates for use in many communication applications. For some applications, improved data conversion performance can be achieved with a sigma-delta ADC (ΣΔ ADC). SUMMARY OF THE INVENTION The present invention provides a control mechanism that can be used to control a ΣΔ ADC to provide the required level of performance while reducing power consumption. The ΣΔ ADC is designed with multiple stages (i.e., loops or sections), and provides improved performance (e.g., higher dynamic range) as more stages are enabled. The control mechanism selectively enables a sufficient number of stages to provide the required performance and disables remaining stages to conserve power. The control mechanism achieves this by measuring one or more characteristics (e.g., signal level) of the ADC input signal through a ΣΔ ADC that is similar to the ΣΔ ADC on the signal path, comparing the measured characteristic(s) to particular threshold level(s), and controlling the stages such that the desired objectives are achieved. An embodiment of the invention provides a data conversion circuit that includes a ΣΔ ADC coupled to a control circuit. The ΣΔ ADC includes a number of ΣΔ stages coupled in cascade that receives an input signal and provides data samples. The control circuit includes one or more detector stages, a conditioning circuit, and a signal processor. The detector stage(s) receive the input signal and provide a detected signal. The conditioning circuit couples to the detector stage(s), receives the detected signal, and provides conditioned samples. The signal processor couples to the conditioning circuit, receives the conditioned samples, and provides a control signal that selectively disables zero or more ΣΔ stages. In an embodiment, at least one detector stage is implemented as a replica of one of the ΣΔ stages, and can be shrunken and /or biased with less current than the ΣΔ stage it replicates. The detected signal can be indicative of an amplitude of the input signal. The ΣΔ stages can be disabled based on the detected signal amplitude, and possibly based on the relative locations of the ΣΔ stages within the ΣΔ ADC. In a specific implementation, the ΣΔ ADC includes two fourth-order bandpass stages or two second-order lowpass stages. The data conversion circuit is advantageously used in a cellular (e.g., CDMA) receiver. Another embodiment of the invention provides an electronic circuit that includes a multi-stage circuit coupled to a control circuit. The multi-stage circuit includes N signal stages coupled in a particular configuration (e.g., cascade, parallel, and others). The control circuit provides a control signal that selectively disables zero or more signal stages. The control circuit includes one or more detector stages, a conditioning circuit, and a signal processor that can be configured and operated as described above. The detector stage(s) can be implemented as replica(s) of the signal stage(s), and can be shrunken and /or biased with less current. The signal stages can be selectively disabled based on, for example, the amplitude of the input signal. Yet another embodiment of the invention provides a control circuit for controlling a multi-stage circuit that includes a number of signal stages. The control circuit includes one or more detector stages, a conditioning circuit, and a signal processor. At least one detector stage is implemented as a replica of one of the signal stages. The detector stage(s), conditioning circuit, and signal processor can be configured and operated as described above. The control circuit is advantageously used to control a ΣΔ ADC. Yet another embodiment of the invention provides a method for controlling ΣΔ stages in a ΣΔ ADC. In accordance with the method, a characteristic of an ADC input signal is detected using one or more detector stages. At least one detector stage is implemented as a replica of one of the ΣΔ stages. The detected characteristic is compared against a comparison level. A control signal is generated based, in part, on the comparison. Zero or more ΣΔ stages are then selectively disabled in accordance with the control signal. The characteristic being detected can be an amplitude of the input signal, and the ΣΔ stages can be disabled based on the detected signal amplitude. BRIEF DESCRIPTION OF THE DRAWINGS The features, nature, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: FIG. 1 is a block diagram of an exemplary single-loop sigma-delta analog-to-digital converter (ΣΔ ADC) architecture; FIG. 2 is a block diagram of an exemplary MASH ΣΔ ADC architecture; FIGS. 3A-3D are pole-zero diagram and frequency response plot of an integrator and pole-zero diagram and frequency response plot of a bandpass resonator, respectively; FIG. 4 is a block diagram of an exemplary two loop bandpass MASH ΣΔ ADC of the present invention; FIGS. 5A-5E are block diagrams of a resonator within the bandpass MASH ΣΔ ADC and implementations of the resonator with a delay cell resonator, a lossless discrete integrator resonator, a Forward-Euler resonator, and a two-path interleaved resonator, respectively; FIGS. 6A-6B are schematic diagram of a delay cell using double- sampling switched capacitor analog circuit technique and timing diagram of the required clock signals for the delay cell, respectively; FIGS. 7A-7B are schematic diagrams of a resonator circuit within the bandpass MASH ΣΔ ADC and the feed-forward gain circuit, respectively, both of which are implemented using double-sampling switched capacitor analog circuit technique; FIG. 8 is a block diagram of an exemplary eighth order bandpass MASH 4-4 ΣΔ ADC of the present invention; FIG. 9 is a plot of the simulated signal-to-noise ratio (SNR) performance of the bandpass MASH 4-4 ΣΔ ADC of the present invention; FIGS. 10A-10B are exemplary schematic diagrams of a delay cell based resonator and a Forward-Euler resonator, respectively, both implemented using single-sampling switched capacitor analog circuit technique; FIGS. 10C-10D are exemplary schematic diagrams of a two-path interleaved resonator implemented using pseudo two-path single-sampling switched capacitor analog circuit and the timing diagram of the required clock signals for the pseudo two-path circuit, respectively; FIGS. 10E-10F are exemplary schematic diagrams of a two-path interleaved resonator implemented using two independent path double- sampling switched capacitor analog circuits; FIGS. 10G-10H are exemplary schematic diagrams of a double-sampled delay cell based resonator circuit and timing diagram of the required clock signals for the resonator circuit, respectively; FIGS. 11 through 13 show simplified block diagrams of three specific embodiments of a control circuit for controlling a multi-stage circuit, which can be a ΣΔ ADC as shown in Fig. 11; FIG. 14 shows a simplified block diagram of a specific embodiment of a control circuit that can be used as the control circuits in FIGS. 11 through 13; FIG. 15A shows a graph of the standard deviation of the detected signal from a detector stage versus input signal amplitude; FIG. 15B shows a graph of the distribution densities of the signal levels of the detected signal; and FIG. 15C shows a graph of the count value from a signal processor versus detector input signal amplitude. DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS A high-speed sigma-delta analog-to-digital converter (ΣΔ ADC) performs analog-to-digital conversion of the input signal by making successive one-bit approximations of the change in amplitude of the input signal, since the previous sample has already been approximated, at a sampling rate which is many times higher than the bandwidth of the input signal. The output samples comprise the input signal and the quantization noise. The ΣΔ ADC can be designed such that the quantization noise is pushed (or noise shaped) to an out- of-band frequency where filtering is more easily performed. A ΣΔ ADC can provide high signal-to-noise ratio (SNR), good linearity, and low DC offset because of the inherent structure of ΣΔ ADC. For example, high SNR can be obtained by selecting a sufficient oversampling ratio (OSR) and the appropriate noise shaping filters. Additionally, good linearity and low DC offset are obtained because of the simple one-bit quantizer within the ΣΔ ADC. High-speed bandpass ΣΔ ADC can be used to perform the required analog-to-digital conversion of narrowband signals at an intermediate frequency (IF). Exemplary applications include CDMA communication system and HDTV. In a bandpass ΣΔ ADC, the input signal is at an IF frequency instead of baseband. Sampling at IF allows for the elimination of a downconversion stage in the communication system, thus reducing circuit complexity, decreasing cost, and improving reliability. Furthermore, the noise shaping filters within the bandpass ΣΔ ADC can be designed such that the quantization noise around the band of interest is pushed to out-of-band frequencies where filtering is more easily performed. ΣΔ ADC samples an analog waveform in continuous time to provide discrete samples at evenly spaced time intervals. ΣΔ ADC has the following transfer function : F(z) = G(z) - X(z) + H(z)- E(z)| , (1) where Y(ζ) is the output from the ΣΔ ADC in the z-transform domain, X(z) is the input to the ADC, Ε(z) is the quantization noise, G(z) is the transfer function from the input to the output, and Η(z) is the noise transfer function from the quantizer to the output. Thus, the ADC output Y(z) comprises the input signal X(z) that is shaped by the transfer function G(z) plus the quantization noise E(z) which is shaped by the noise transfer function H(z). To avoid distortion of the input signal X(z), the transfer function G(z) is typically designed such that it is frequency independent. For example, G(z) can be an allpass function 1 m comprising a fixed gain (Aj) and delay elements (z " ), such as A # z " . The quantization noise E(z) can be shaped by the noise transfer function H(z) such that the quantization noise in the band of interest (e.g. the band where the input signal is present) is pushed out-of-band where filtering is more easily performed. The characteristics of the noise transfer function H(z) is selected based on the application for which the ΣΔ ADC is used and is designed to provide the required performance. I. ΣΔ ADC Architectures ΣΔ ADC can be designed using one of many architectures. A block diagram of an exemplary single-loop ΣΔ ADC 10 is shown in FIG. 1. Single- loop ΣΔ ADC 10 comprises input summer 22 that subtracts the quantized ADC output from the ADC input. The error signal from summer 22 is provided to first filter 24 that filters the error signal in accordance with the transfer function of first filter 24. The output of first filter 24 is provided to summer 26 that subtracts the quantized ADC output from the output of first filter 24. The error signal from summer 26 is provided to second filter 28 that filters the error signal in accordance with the transfer function of second filter 28. The output of second filter 28 is quantized, typically to one bit although more bits can also be used, and provided as the quantized ADC output. FIG. 1 illustrates a single loop ΣΔ ADC with two filter sections. The filter sections determine the noise shaping characteristics of the ΣΔ ADC and is designed based on the application for which the ΣΔ ADC is used. More filter sections can be interposed between second filter 28 and quantizer 30. However, single-loop ΣΔ ADCs are typically designed with two or less filter sections because of concerns with instability of higher order single-loop ΣΔ ADCs. A block diagram of an exemplary MASH ΣΔ ADC architecture is shown in FIG. 2. A MASH ADC can be designed with two or more loops, depending on the desired noise transfer function H(z). However, MASH ADCs with more than three loops are not typically utilized because small incremental improvement is achieved for additional loops greater than three. The MASH architecture is preferred for a higher order ΣΔ ADC since the MASH architecture is inherently stable. As shown in FIG. 2, a MASH ADC 12 comprises three loops 40a, 40b, and 40c. Loop 40a quantizes the ADC input and provides the output Yl to each loop. After noise cancellation, the dynamic range, and thus the output of MASH ADC 12, can be three bits or more. Within each loop 40, summer 42 receives the input signal and the quantizer output from quantizer 46. Summer 42 subtracts the quantizer output from the input signal and provides the error signal to loop filter 44. Loop filter 44 filters the error signal and provides the filtered output to quantizer 46 where it is quantized to one-bit values. Loop filter 44 is designed to produce the desired noise transfer function H(z) that is based on the application for which the ΣΔ ADC is used. The filtered output from loop filter 44 in all but the last loop 40c is provided to gain element 52 and scaled with a first gain. The output from quantizer 46 in all but the last loop 40c is provided to gain element 54 and scaled with a second gain. The scaled signal from gain element 54 is subtracted from the scaled signal from gain element 52 by summer 56 and the error signal is provided to gain element 56. Gain element 56 scales the error signal with a third gain and provides the scaled error signal to the subsequent loop 40. The gains of gain elements 52, 54, and 58 affect the noise transfer function H(z) of MASH ADC 12. Each loop filter 44 can comprise one or more filter sections, depending on the desired noise transfer function. More filter sections allow for implementation of higher order ΣΔ ADC to produce the desired performance, such as high SNR. A loop filter design is described in detail below. In this specification, a MASH ΣΔ ADC is designated in accordance with the following nomenclature. MASH A-B-C denotes three loops (A, B, and C) which are of the order designated by the values of A, B, and C. For example, MASH 4-2-2 denotes a three loop architecture, with the first loop having a fourth order filter, the second loop having a second order filter, and the third loop also having a second order filter. Overall, MASH 4-2-2 is an eighth order ΣΔ ADC. The present invention can be directed toward a MASH 2-2, MASH 4- 2, MASH 4-4, MASH 4-2-2, MASH 4-4-2, MASH 4-4-4 and other orders of MASH ΣΔ ADC. The selection of the desired ΣΔ ADC architecture, single-loop or MASH, depends on a number of factors. One important factor is the required signal-to- noise ratio (SNR). SNR is defined as the ratio of the power of the largest input signal to the power of the quantization noise. For a full-scale sinewave input, the SNR for a ΣΔ ADC can be calculated according to the following equation : where L is the order of the loop filter used for noise shaping and OSR is the oversampling ratio. OSR is defined as the ratio of the sampling rate over the OSR = ^- two-sided signal bandwidth, *BW . Equation (2) is based on the simple theory using only white quantization noise and a unity gain quantizer. Using equation (2) the SNR is calculated for an exemplary CDMA application wherein the 2-sided signal bandwidth 2 ^ = 2.4576 MHz and the sampling rate is approximately 78.64 MHz. These frequencies produce an OSR of 32. The SNR is calculated for various loop filter order L and the results are tabulated in Table 1. The loop filter order is the summation of the order of all filters within the ΣΔ ADC. For a bandpass ΣΔ ADC, the loop filter order L is half of the overall order of the bandpass filters. An n-th order bandpass filter has an effective order of L = n/2 because the poles in the bandpass transfer function are divided evenly between the upper half of the z-plane and the lower half of the z-plane. The calculated SNR values in Table 1 represent the upper bound that can be achieved. The actual SNR may be 10 dB to 15 dB less than the calculated values due to non-ideal circuit components and limitation of the input signal to less than the full-scale value. For the CDMA application described above, the SNR is simulated to be 70 dB for a bandpass MASH 4-2 ADC, 85 dB for a MASH 4-4, 60 dB for a sixth order single-loop bandpass ΣΔ ADC, and 62 dB for an eighth order single-loop ADC. The simulated results are also tabulated in Table 1. The simulated results assume an input signal at -10 dB below full-scale and coincident zeros (e.g. all zeros are placed at z = ±j) in the noise transfer function H(z). The simulated SNR value for the sixth order single-loop ΣΔ ADC can be improved to 70 dB if the zeros are spread across the signal bandwidth. Similarly, the simulated SNR value for the eighth order single-loop ΣΔ ADC can be improved to 80 dB with zero spreading. However, zero spreading can be difficult to achieve due to non-ideal circuit components. The simulated SNR of a MASH architecture is better than that of the single-loop architecture. Table 1 - Calculated and Simulated SNR for ΣΔ ADC of Various Order In actual implementation, the MASH architecture has the additional advantages of inherent stability and ease of loop filter design. However, circuit matching is more critical for the MASH architecture because of the need to cancel the quantization noise between multiple loops. The single-loop architecture is stable for second order or lower order loops but may be unstable for higher order loops. Higher order single-loop designs can be made stable by careful circuit design and thorough simulation. However, the single-loop architecture is more tolerant to circuit mismatch. The selection of the single- loop or MASH architecture is dependent on the requirements of the application. For most applications, the MASH architecture is preferred over the single-loop architecture. The noise transfer function H(z) of MASH ADC 12 is determined by the design of loop filters 44. For example, a baseband MASH ΣΔ ADC can be designed by implementing loop filters 44 with lowpass filters. Similarly, a bandpass ΣΔ ADC can be designed by implementing loop filters 44 with bandpass filters. The noise transfer function H(z) of the quantization noise is the inverse of the filter characteristics because the loop filters reside within the loops. An exemplary baseband MASH ΣΔ ADC can be designed by implementing loop filters 44 with one or more integrators having the lowpass transfer function 1 _ £ . The number of integrators within loop filters 44 depends on the desired noise transfer function H(z). As shown in FIG. 3 A, the lowpass transfer function has a pole at z = +1 and a zero at the origin z = 0. The frequency response of the lowpass transfer function is shown in FIG. 3B by the solid line. The lowpass filter has the highest gain at DC because of the pole at z = +1, a gain of 1.0 at fs/6, and a gain of 0.5 at fs/2 where fs is the sampling frequency. The frequency response of the noise transfer function is shown in FIG. 3B by the dashed line. The quantization noise around DC is pushed toward higher frequency. An exemplary bandpass MASH ΣΔ ADC can be designed by implementing loop filters 44 with one or more resonators having the bandpass -z 1 -2 transfer function 1 + 2 . The number of resonators within loop filters 44 depends on the desired noise transfer function H(z). For example, a fourth order loop requires two resonator sections each having the bandpass transfer function as described above. A bandpass transfer function can be obtained from a lowpass transfer function by substituting z in the lowpass transfer -1 function with -z . As shown in FIG. 3C, the bandpass transfer function has a pair of poles at z = ±j and two zeros at the origin z = 0. The frequency response of the bandpass transfer function is shown in FIG. 3D by the solid line. The bandpass filter has the highest gain at fs/4, because of the poles at z = ±j, and a gain of 0.5 at DC and at fs/2. The frequency response of the noise transfer function is shown in FIG. 3D by the dashed line. The quantization noise around fs/4 is pushed away from fs/4, the frequency band of interest, toward DC and fs/2. II. Bandpass MASH ΣΔ ADC Architecture A block diagram of an exemplary two loop bandpass MASH ΣΔ ADC is illustrated in FIG. 4. MASH ADC 100 comprises two loops 110a and 110b, feed forward element 150, and noise cancellation logic 160. In the exemplary embodiment, MASH ADC 100 receives an analog ADC input and produces a digital ADC output having at least two bits, at least one bit for each loop 110. The ADC input is provided to loop 110a that produces a 1-bit output Yl in response thereto. A fraction of the ADC input and quantization noise (X2) from loop 110a is provided to loop 110b where additional noise shaping is performed. The outputs Yl and Y2 from loops 110a and 110b, respectively, are provided to noise cancellation logic 160. Noise cancellation logic 160 combines the outputs Yl and Y2 and produces the ADC output. Within loop 110a, summer 128a receives the ADC input and the 1-bit output Yl from quantizer 140a. Summer 128a subtracts Yl from the ADC input and provides the error signal to resonator 130a. Resonator 130a filters the error signal and provides the filtered output (VI) to summer 128b. Summer 128b also receives Yl from quantizer 140a and subtracts Yl from VI. The error signal from summer 128b is provided to resonator 130b that further filters the error signal. The filtered output (N2) from resonator 130b is provided to quantizer 140a that produces the 1-bit output Yl in response thereto. Loop 110b is connected in similar manner as loop 110a. The filtered output N2 from resonator 130b is also provided to gain element 142 which scales N2 by the scaling factor l/k^. The output Yl from quantizer 140a is provided to gain element 144 that scales Yl by the scaling factor h. The outputs from gain elements 142 and 144 are provided to summer 146 which subtracts the output from gain element 144 from the output from gain element 142. The error signal from summer 146 is provided to gain element 148 that scales the error signal by the scaling factor 1/G. The output from gain element 148 comprises X2 that is provided to loop 110b. Within noise cancellation logic 160, the output Yl is provided to delay element 172 which delays Yl by a time interval equal to the processing delay of loop 110b. The delayed Yl from delay element 172 is time aligned with Y2. The output Y2 is provided to gain element 162 which scales Y2 by the scaling factor G. The delayed output Yl is provided to gain element 166 that scales the delayed Yl by the scaling factor h-1. The outputs from gain elements 162 and 166 are provided to summer 164 that sums the two scaled outputs. The combined signal from summer 164 is provided to element 168 that filters the combined signal with the transfer function Ν(z). The output from element 168 and the delayed Yl are provided to summer 170 which sums the two signals to produce the ADC output. For a bandpass ΣΔ ADC, each resonator 130 in MASH ADC 100 is implemented with a bandpass transfer function 1 + z . where k n is the gain of the n-th resonator 130 within loop 110 and m = 1 or 2. Each resonator 130 comprises a pair of poles and is second order. Since each loop 110 contains two resonators 130, the order of each loop is four. Overall, MASH ADC 100 is an eighth order MASH 4-4 ADC. The transfer function N(z) within element 168 is selected based on the characteristics of the ΣΔ ADC. For a bandpass ΣΔ ADC, (ϊ i _ 2 \ -9m _. . v „, • . Delay element 172 has a transfer function of z " . The gains kj, k2, h, and G which are reflected in the scaling factors of gain elements 142, 144, 148, 162, and 166 determine the location of the zeros of the noise transfer function H(z). The poles in resonators 130 are transformed into zeros in the noise transfer function H(z) because the resonators reside within a feedback loop. Initially, the zeros in H(z) are selected to be at z = ±j for the bandpass ΣΔ ADC. MASH ADC 100, as illustrated in FIG. 4, is a bandpass ΣΔ ADC. The same topology can be used to implement a baseband ΣΔ ADC. This can be easily achieved by substituting each resonator 130 with an integrator having the z ~λ lowpass transfer function 1 ~~ Z . implementing element 168 with the transfer tϊ → f function N(z) = , and providing delay element 172 with the transfer -2 function z . With these substitutions, MASH ADC 100 is transformed into a fourth order baseband MASH 2-2 ADC. In the exemplary embodiment, all elements of MASH ADC 100, except for noise cancellation logic 160, are implemented as analog circuits. However, the optimal implementation of an element in either analog or digital circuit may depend on the IC process used in implementing the ΣΔ ADC. Therefore, the various combinations of analog and digital circuits to synthesize the required elements within MASH ADC 100 are within the scope of the present invention. III. Bandpass Resonator Design A bandpass MASH 4-4 ADC can be designed by implementing z '2 resonators 130 in MASH ADC 100 with a bandpass transfer function 1 + Z ~ as described above. Thus, resonators 130a, 130b, 130c, and 130d have the same structure. However, the gain of resonators 130a and 130c is kj whereas the gain of resonators 130b and 130d is k^ Resonator 130 is illustrated in FIG. 5A. Resonator 130 can be implemented by many resonator structures, four of which are illustrated in FIGS. 5B-5E. Resonators 131, 132, 133 and 134 receive the input signal Rin and produce the output signal Rout. A block diagram of an exemplary delay cell resonator 131 is shown in FIG. 5B. The input signal Rin is provided to gain element 192 that scales the input signal with the gain k n . The scaled Rin is provided to summer 194 that also receives the output signal Rout and subtracts Rout from the scaled Rin. The error signal from summer 194 is provided to delay element 200a that delays the signal by one clock cycle of the sampling clock. The delayed error signal from delay element 200a is provided to delay element 200b which further delays the signal by one sampling clock cycle. The signal from delay element 200b comprises the output signal Rout. A block diagram of an exemplary lossless discrete integrator (LDI) resonator 132 is shown in FIG. 5C. The input signal Rin is provided to gain element 260 that scales the input signal with the gain k n . The scaled Rin is provided to summer 262 that also receives the scaled output signal Rout and subtracts the scaled Rout from the scaled Rin. The error signal from summer 262 is provided to filter 264 which filters the signal with the transfer function z- 1 l -£ . The filtered error signal from filter 264 is provided to filter 266 that 1 further filters the signal with the transfer function 1 ~~ z . The signal from filter 266 comprises the output signal Rout. Rout is provided to gain element 268 that scales Rout with the gain β. In the exemplary embodiment, β = 2 and the k ' z '1 -2 overall transfer function of resonator 132 is 1 +-Z . By proper selection of β, the zeros of the noise transfer function can be spread in the signal band. A block diagram of an exemplary Forward-Euler (FE) resonator 133 is shown in FIG. 5D. The input signal Rin is provided to gain element 270 that scales the input signal with the gain k n . The scaled Rin is provided to summer 272a that also receives the scaled output signal Rout and subtracts the scaled output signal Rout from the scaled input signal Rin. The error signal from summer 272a is provided to filter 274a that filters the signal with the transfer K 'z '2 function + z . The filtered error signal from filter 274a is provided to summer 272b that also receives the scaled Rout and subtracts the scaled Rout from the filtered error signal. The error signal from summer 272b is provided to filter K ' z- 2 —1 274b that filters the signal with the transfer function 1+ z . The signal from filter 274b comprises the output signal Rout. Rout is provided to gain element 276 that scales the output signal Rout with the gain β. In the exemplary k * z -2 embodiment, β = 2 and the overall transfer function of resonator 133 is 1 + z . A block diagram of an exemplary two-path interleaved resonator 134 is shown in FIG. 5E. The input signal Rin is provided to gain element 280 that scales the input signal with the gain k n . The scaled Rin is provided to switches 282a and 282b which connects the scaled Rin to summers 284a and 284b, respectively. Summer 284 also receives the delayed error signal from delay elements 286 and subtracts the delayed error signal from the scaled Rin. The error signal from summer 284 is provided to delay element 286 which delays the error signal by one sampling clock cycle. The delayed error signal from delay elements 286a and 286b are provided to switches 288a and 288b, respectively. Switches 288a and 288b connect together and comprise the output of resonator 134. Switches 282a and 288a are clocked by one phase of a switching clock and switches 282b and 288b are clocked by a second phase of the switching clock. The clock signals are described in detail below. The overall transfer function of resonator 134 is + z . Resonators 131, 132, 133 and 134 can be implemented by numerous analog circuit techniques. For example, resonators 131, 132, 133 and 134 can be implemented with continuous time analog circuit techniques such as active-RC, gm-C, and MOSFET-C. Resonators 131, 132, 133 and 134 can also be implemented with sampled-data analog circuit techniques such as switched capacitor and switched current. The selection of the analog circuit technique depends on the requirement of the application for which the ΣΔ ADC is used. For an exemplary CDMA application wherein a 12-bit ΣΔ ADC operating at a sampling rate of 80 MHz, the performance of the various circuit techniques is tabulated in Table 2. Table 2 - Performance of Various Analog Circuit Techniques The implementation of the functions as described herein using any one of the circuit techniques listed in Table 2, or its equivalents, are within the scope of the present invention. In the preferred embodiment, resonators 131, 132, 133 and 134 are implemented with a switched capacitor circuit technique because of superior performance in SNR, accuracy, speed, and cost. The design of resonator 131 using the switched capacitor circuit technique is described in detail below. Within resonator 131, each delay element 200 can be implemented by one of many analog circuit techniques. In the preferred embodiment, delay element 200 is implemented with a double- sampling switched capacitor delay circuit 210 as illustrated in FIG. 6A. For optimal linearity and noise performance, delay circuit 210 is implemented as a fully differential circuit, where the input comprises Rin+ and Rin- and the output comprises Rout÷ and Rout-. Within delay circuit 210, the input signal Rin+ is provided to two signal paths through switches 220a and 224a. Switch 220a connects to one end of capacitor 228a and switch 236a. The other end of capacitor 228a connects to switches 222a and 232a. Switch 222a also connects to AC ground 202. Switch 232a also connects to the inverting input of amplifier 250 and switch 236a also connects to the non-inverting output of amplifier 250. In similar manner, switch 224a connects to one end of capacitor 230a and switch 238a. The other end of capacitor 230a connects to switches 226a and 234a. Switch 226a also connects to AC ground 202. Switch 234a also connects to the inverting input of amplifier 250 and switch 238a also connects to the non-inverting output of amplifier 250. Delay circuit 210 is a fully differential circuit. The lower half of delay circuit 210 is a mirror image of the upper half. AC ground 202 is implemented as a DC bias voltage with a capacitor bypass to ground. The DC bias voltage determines the mid-scale voltage of the differential signal at that node. For best linearity, the signals Rin+ and Rin- are normally biased near the operating center of amplifier 250. In some circuit design, the differential output Rout+ and Rout- can have a different optimal DC bias voltage than that of the input Rin. Delay circuit 210 samples the input signal Rin on two phases of the switching clock. Referring to FIG. 6B, the sampling clock fs is divided by two to obtain the switching clock. In the exemplary embodiment, the clock signal CLK1 having the first clock phase øl is provided to the switches which are shown without the bubble (e.g. switch 224a). The clock signal CLK2 having the second clock phase ø2 is provided to the switches which are shown with the bubble (e.g. switch 220a). Each clock signal should have a duty cycle that is less than 50 percent. The minimum width of the clock signals is determined by the charging time of the capacitors which, in turn, is determined by the size of the capacitor and the ON resistance of the switches. Referring to FIG. 6A, during the first clock phase øl, switches 224a and 226a are switched ON and capacitor 230a is charged with the input signal Rin+. During the second clock phase ø2, switches 224a and 226a are switched OFF, switches 234a and 238a are switched ON, and the voltage across capacitor 230a is provided to the output Rout+. Capacitor 230a is charged during the first clock phase øl and provided to the output Rout+ during the second clock phase ø2. Therefore, the delay provided by delay circuit 210 is a half switching clock cycle, or one sampling clock cycle. Similarly, capacitor 228a is charged during the second clock phase ø2 and provided to the output Rout+ during the first clock phase øl. The two signal paths, one through capacitor 230a and the second through capacitor 228a, operate on different clock phases and only share amplifier 250. Using double-sampling switched capacitor circuit, the input signal Rin is provided to the output Rout on both phases of the switching clock, through two signal paths, thereby resulting in the sampling of Rin at the sampling clock frequency fs even though the switches are switched ON and OFF at half the sampling clock (fs/2). A double-sampling switched capacitor circuit allows the switches to be clocked at half the sampling frequency, thus allowing the capacitors and amplifier more time to settle to the final value. Since the operating speed of a switched capacitor circuit is determined by the settling time of the amplifier used in the circuit, using the same amplifier during both phases of the switching clock increases the sampling rate by a factor of two without requiring faster settling amplifier. However, double-sampling switched capacitor circuits are sensitive to path mismatch. Mismatch in the first sampling stage of the ΣΔ ADC can cause degradation in the output samples. Mismatch in the subsequent stages is noise shaped and does not result in noticeable degradation. Any mismatch between the two signal paths, such as mismatch in the capacitors or mismatch due to uneven clock phases, in the first stage produces an image of the input signal to appear at the output samples. By using good circuit design rules, the capacitor mismatch can be reduced to one percent or less, thereby minimizing the amplitude of the image to -40 dB or more below the amplitude of the input signal. The switching clocks can be designed to minimize uneven clock phases. Alternatively, the first sampling stage can be clocked with a master clock, before the divide-by-two operation to obtain the switching clocks. Clock jitter can be reduced by using a clean external clock source. This topology also have faster settling time than the single-sampled topology because of less amplifier loading. IV. Bandpass MASH 4-4 ADC Design Referring to FIG. 4, each loop 110 comprises two resonator sections 120. Each resonator section 120 comprises summer 128 and resonator 130. Resonator 130 can be implemented as delay cell resonator 131 as shown in FIG. 5B. Each delay cell resonator 131 comprises two delay elements 200. Each delay element 200 can be implemented with double-sampling switched capacitor delay circuit 210 as shown in FIG. 6A. A schematic diagram of double-sampling switched capacitor resonator circuit 121, which implements resonator section 120, is shown in FIG. 7A. Resonator circuit 121 comprises delay element and summer circuit 300 and delay circuit 310. Circuit 300 incorporates summer 128, gain element 192, summer 194, and delay element 200a (see FIGS. 4 and 5B). Delay circuit 310 implements delay element 200b. Referring to FIG. 7A, within circuit 300, the input signal Nip is provided to two signal paths, the first signal path through switch 324a. Switch 324a connects to one end of capacitor 330a and switch 314a. The other end of capacitor 330a connects to switches 326a and 334a. Switch 326a also connects to AC ground 202 and switch 334a also connects to the inverting input of amplifier 350a. Switch 314a connects to the quantizer output Ypøl that is described below. Switches 326a and 334a connect to one end of capacitor 318a. The other end of capacitor 318a connects to switches 344a and 338a. Switch 338a also connects to the non-inverting output of amplifier 350a. Switch 344a also connects to the inverting output of amplifier 350b within delay circuit 310. The operation of the first signal path in circuit 300 can be described as follows. During the first clock phase øl, switches 324a and 326a are switched ON and capacitor 330a is charged with the input signal Nip. During the second clock phase ø2, switches 324a and 326a are switched OFF and switches 314a, 334a, and 338a are switched ON. The input signal Yxpøl and the voltage across capacitor 330a are scaled by the ratio of capacitors 330a and 318a (Cs/Cf) and provided to the non-inverting output of amplifier 350a. Also during the first clock phase øl, switch 344a is switched ON and the signal from the inverting output of amplifier 350b is fed back, charging capacitor 318a. The voltage across capacitor 318a is reflected at the non-inverting output of amplifier 350a during the second clock phase ø2. The above discussion describes the circuit connection and operation of the first signal path within circuit 300. An identical circuit is provided for the second signal path which operates in the same manner as that of the first signal path, except the switches are clocked at the alternative phase of the switching clock. Thus, the input signal Nip is provided to the output of amplifier 350a on both phases of the switching clock and results in the sampling of the input signal at the sampling rate. Circuit 300 is a fully differential circuit. An identical circuit is provided for the inverting input signal Nin. The lower half of circuit 300 is the mirror image of the upper half. Circuit 300 includes the functions of summer 128, gain element 192, and summer 194 (see FIGS. 4 and 5B). The function of summer 194 is provided by switches 342 and 344 which connects the output of the second delay element to capacitors 316 and 318, respectively. The voltage Non is stored in capacitor 318a during the first clock phase øl and subtracted from the voltage at Nb during the second clock phase ø2. The function of summer 128 is provided by switches 312 and 314 that connect the quantizer output to capacitors 328 and 330, respectively. The quantizer output voltage Ypøl is provided to capacitor 330a during the second clock phase ø2 and is added to the voltage at Nb. Delay circuit 310 is identical to delay circuit 210 in FIG. 6A and operates in the same manner as that described above for delay circuit 210. Delay circuit 310 delays the output from circuit 300 by a half switching clock cycle, or one sampling clock cycle. The output from amplifier 350b comprises the output of resonator circuit 121. Resonator circuit 121 has the following transfer function from Nip to Nop The transfer function from Yxpøl to Nop is -HR(Z). In this nomenclature, Yx denotes the quantizer output from the first (x = 1) or second (x = 2) loop, p or n denotes a (+) or (-) signal, and øl or ø2 denotes the clock phase of the quantizer output. The voltage gain from Yxpøl to Nop is -Cs/Cf, the ratio of capacitor 330a to capacitor 318a. Thus, the gain of gain element 192 can be set as k n = Cs/Cf. Having implemented each resonator section 120 with resonator circuit 121, MASH ADC 100 in FIG. 4 can be implemented as an eighth order bandpass MASH 4-4 ADC 101 as shown in FIG. 8. Each resonator section 120 in FIG. 4 is replaced with double-sampling switched capacitor resonator circuit 121 in FIG. 8. Within resonator circuit 121, the feedback from delay circuit 310 to circuit 300 is not shown for simplicity. Also, note that noise cancellation logic 160 in FIG. 4 is not shown in FIG. 8 for simplicity. Quantizer 140a in FIG. 4 is implemented with quantizer 141a which comprises two synchronous comparators 390a and 390b. Comparator 390a is clocked by CLK1 having the first phase øl and comparator 390b is clocked by CLK2 having the second phase ø2 (see FIG. 6B). The differential input signals to comparators 390a and 390b are provided by the output of circuit 300b. This is because quantizer 141a has a delay of a half switching clock cycle. The input to quantizer 141a is taken before delay circuit 310b which also provides a delay of a half switching clock cycle. Connected in this manner, quantizer 141a is properly aligned in time. Each comparator 390 provides a differential output. Comparator 390a provides the differential output signals Ylpøl and Ylnøl and comparator 390b provides the differential output signals Ylpø2 and Ylnø2. The four quantizer outputs, collectively referred to as Yl, are provided to circuits 300a, 300b, and 151 as shown by FIGS. 4 and 8. Referring to FIGS. 4 and 8, feed forward gain element 150 can be incorporated into circuit 300c to simplify the circuit design. Referring to FIG. 4, the gain from the output (N2) of resonator 130b to X2 is l/k^G and the gain from the output (Yl) of quantizer 140a to X2 is -h/G. The overall transfer function of feed-forward gain element 150 can be calculated as X2 = Avl»V2- Av2»Yl, where Avl = l/kføG and Av2 = h/G. The gains k^, k.2, h, and G of bandpass MASH 4-4 ADC 101 are selected for optimal SΝR and ease of circuit implementation. Using mixed analog and digital design simulation tools, the following exemplary values are selected for the gains : \ = 0.5, k 2 = 0.5, ■ 2, G = 4\ (4) Other values for gains k^, k2, h, and G can also be utilized and are within the scope of the present invention. Using the gain values as shown in equation (4), and an oversampling ratio of 32, the SΝR versus input signal level is plotted in FIG. 9. The peak SΝR surpasses 90 dB. A schematic diagram of an exemplary feed-forward gain circuit 151, which in conjunction with circuit 300c implements feed-forward gain element 150, is illustrated in FIG. 7B. The quantizer outputs Ylpø2, Ylpøl, Ylnøl, and Ylnø2 from quantizer 141a (see FIG. 8) are provided to switches 372a, 376a, 376b, and 372b, respectively. Switches 372a, 376a, 376b, and 372b connect to one end of capacitors 380a, 382a, 382b, and 380b which are connected to node Na, Nb, Nc, and Nd within resonator circuit 121 in FIG. 7A, respectively. The other end of capacitors 380a, 382a, 382b, and 380b, connect to switches 374a and 384a, 378a and 386a, 378b and 386b, and 374b and 384b, respectively. Switches 374a, 378a, 378b, 374b, 384a, 386a, 386b, and 384b also connect to AC ground 202. The gain values Avl and Av2 can be calculated and incorporated into feed forward gain circuit 151. Using the values of k^ = 0.5, k2 = 0.5, h = 2, and G=4 from equation (4), the gain values become Avl = 1.0 and Av2 = 0.5. Referring to FIGS. 7A and 7B, the gain from the output Ylpøl of quantizer 141a to the non-inverting output of amplifier 350a is determined by the ratio of capacitors 382a and 318a, or Av2 = Cq/Cf = 0.5. Therefore, the capacitance of capacitor 382a is synthesized to be half of the value of capacitor 318a. Referring to FIG. 8, the gain from the output N2p of resonator circuit 121b (corresponding to Nip in FIG. 7A) to the non-inverting output of amplifier 350a is determined by the ratio of capacitors 330a and 318a, or Avl = Cs/Cf = 1.0. Thus, the value of capacitor 330a is synthesized to be the same capacitance as capacitor 318a. However, capacitors 330a and 318a also implement gain element 192 (see FIG. 5B). In the exemplary embodiment, the gains kn = k^ = k2 = 0.5 as shown in equation (4). Therefore, capacitor 330a is selected to be half the capacitance of capacitor 318a. In the exemplary embodiment, noise cancellation circuit 160 in FIG. 4 is implemented in digital logic. For an eighth order bandpass MASH ΣΔ ADC, delay element 172 has a transfer function of z " and can be implemented with four D flip-flops connected in cascade, the design and implementation of which are known in the art. The transfer function of element 168 is Ν(z) = v z ) which can be implemented with two summers and four sets of D flip-flops, the implementation of which is also known in the art. As stated above, double-sampling switched capacitor circuits are sensitive to path mismatch. However, path mismatch in the stages subsequent to the first sampling stage is noise shaped and does not cause a noticeable image. Referring to FIG. 8, within delay element and summer circuit 300a, which is illustrated in FIG. 7A, only the input sampling capacitors 328 and 330 are sensitive to mismatch in capacitor values and only input sampling switches 320, 322, 324, and 326 are sensitive to uneven clock phases of the switching clocks. Path mismatch can be minimized by the use of circuit design techniques described below. Referring to FIG. 3D, bandpass MASH 4-4 ADC 101 provides noise shaping of the quantization noise such that the spectral components around fs/4 is pushed toward DC and fs/2. For optimal performance, the input signal being sampled should be placed close to fs/4. For undersampling applications, wherein the input signal is centered at an IF which is higher than the sampling frequency and the aliasing property of sampling is used to downconvert the input signal from IF to a lower frequency, the input signal should be placed close to IF = (2n+l)»fs/4, where n is an integer greater than or equal to zero. V. Alternative Bandpass Resonator Design A bandpass resonator can be synthesized by various other structures, three of which are illustrated in FIGS. 5C-5E. In the exemplary embodiment, lossless discrete integrator (LDI) resonator 132 in FIG. 5C is implemented with single-sampling switched capacitor LDI resonator circuit 402 in FIG. 10A, Forward-Euler (FE) resonator 133 in FIG. 5D is implemented with single- sampling switched capacitor FE resonator circuit 403 in FIG. 10B, and two-path interleaved resonator 134 in FIG. 5E is implemented with pseudo two-path switched capacitor resonator circuit 502 in FIG. 10C and two independent path resonator circuit 503 in FIGS. 10E-10F. These are exemplary implementations of resonators 132, 133 and 134. Other implementations utilizing the circuit techniques listed in Table 2 are within the scope of the present invention. An implementation of delay cell based resonator 132 using single- sampling switched capacitor circuit is shown in FIG. 10A. Within the first section of LDI resonator circuit 402, the input signal Nip is provided to switch 414a. Switch 414a connects to one end of capacitor 422a and switch 418a. The other end of capacitor 422a connects to switches 424a and 426a. Switches 418a and 424a also connect to AC ground 202. Switches 426a and 430a and one end of capacitor 436a connect to the inverting input of amplifier 450a. The other end of capacitor 436a connects to switches 440a and 444a. Switch 440a also connects to AC ground 202 and switch 444a also connects to the non-inverting output of amplifier 450a. Switch 430a also connects to switch 432a and one end of capacitor 434a. The other end of capacitor 434a connects to switches 438a and 442a. Switches 432a and 438a also connect to AC ground 202 and switch 442a also connects to the non-inverting output of amplifier 450a. A second section identical to the first second is connected in cascade with the first section. The output of the second section is fed back to the first section. The inverting output of amplifier 450b connects to switch 412a. Switch 412a also connects to switch 416a and one end of capacitor 420a. Switch 416a also connects to AC ground 202. The other end of capacitor 420a connects to switches 424a and 426a. LDI resonator circuit 402 is a fully differential circuit. The lower half of LDI resonator circuit 402 is a mirror image of the upper half. The output of amplifier 450b comprises the output of resonator circuit 402. LDI resonator circuit 402 is clocked at the sampling frequency. LDI resonator circuit 402 has a resonant frequency that is a function of the sampling frequency and the capacitor ratios. The transfer function of LDI resonator circuit 402 is : where Cs = Ch = Ci and β = Cf/Cs. By changing β, the zeros of the noise transfer function H(z) for a ΣΔ ADC utilizing LDI resonators can be spread about fs/4. LDI resonator circuit 402 is not effective for oversampling ratio of greater than 16 because of sensitivity to capacitor mismatch. An implementation of FE resonator 133 using single-sampling switched capacitor circuit is shown in FIG. 10B. Within the first section of FE resonator circuit 403, the input signal Nip is provided to switch 472a. Switch 472a connects to one end of capacitor 476a and switch 474a. The other end of capacitor 476a connects to switches 478a and 482a and one end of capacitor 480a. Switches 474a and 478a also connect to AC ground 202. Switch 482a also connects to the inverting input of amplifier 490a. Capacitor 484a connects to the inverting input and the non-inverting output of amplifier 490a. A second section identical to the first section is connected in cascade with the first section. The output of the second section is fed back to the first section. The inverting output of amplifier 490b connects to switch 488c. Switch 488c connects to switch 486c and the other end of capacitors 480a and 480c. Switch 486c also connects to AC ground 202. FE resonator circuit 403 is a fully differential circuit. The lower half of FE resonator circuit 403 is a mirror image of the upper half. The output of amplifier 490b comprises the output of resonator circuit 403. FE resonator circuit 403 is clocked at the sampling frequency. FE resonator circuit 403 has a resonant frequency that is a function of the sampling frequency and the capacitor ratios. The transfer function of FE resonator circuit 403 is : where Cfl = Cf2 = Cf, Csl = Cs2 = Cil = Ci2 = Ci, and β = Cf/Ci. By changing β, the zeros of the noise transfer function H(z) for a ΣΔ ADC utilizing FE resonators can be spread about fs/4. FE resonator circuit 403 has a faster settling time than LDI resonator circuit 402. An implementation of two-path interleaved resonator 134 using pseudo two-path single-sampling switched capacitor circuit is shown in FIG. IOC. Within resonator circuit 502, the input signal Nip is provided to switch 512a. Switch 512a connects to one end of capacitor 516a and switch 514a. The other end of capacitor 516a connects to switches 518a and 520a. Switches 514a and 518a also connect to AC ground 202. Switches 520a and 524a and one end of capacitor 534a connect to the inverting input of amplifier 550. The other end of capacitor 534a connects to switches 540a and 546a. Switch 540a also connects to AC ground 202 and switch 546a also connects to the non-inverting output of amplifier 550. Switch 524a also connects to switches 522a, 526a, and 528a. Switch 522a also connects to the non-inverting input of amplifier 550. Switches 526a and 528a also connect to one end of capacitors 530a and 532a, respectively. The other end of capacitor 530a connects to switches 536a and 542a. The other end of capacitor 532a connects to switches 538a and 544a. Switches 536a and 538a also connect to AC ground 202 and switches 542a and 544a also connect to the non-inverting output of amplifier 550. Resonator circuit 502 is a fully differential circuit. The lower half of resonator circuit 502 is a mirror image of the upper half. The output of amplifier 550 comprises the output of resonator circuit 502. Resonator circuit 502 is clocked at the sampling frequency. Resonator circuit 502 has a resonant frequency that is a function of the sampling frequency and the capacitor ratios. The advantage of resonator circuit 502 is that only one amplifier 550 is required for two delays. The disadvantages are the needs for eight clock phases and the need to operate resonator circuit 502 at the sampling frequency. The required clock signals for resonator circuit 502 are shown in FIG. 10D. The transfer function of resonator circuit 502 is : H FrP ω . (?) Two-path interleaved resonator 134 can also be implemented using two independent path single-sampling switched capacitor circuit as shown in FIGS. 10E-10F. Within resonator circuit 503a, the input signal Nip is provided to switch 562a. Switch 562a connects to one end of capacitor 566a and switch 564a. The other end of capacitor 566a connects to switches 568a and 570a. Switches 564a and 568a also connect to AC ground 202. Switch 570a and one end of capacitor 578a connect to the inverting input of amplifier 590a. The other end of capacitor 578a connects to the non-inverting output of amplifier 590a. Switch 574a connects to the non-inverting input of amplifier 590a. Switch 574a also connects to switch 572 and one end of capacitor 576a. The other end of capacitor 576a connects to switches 580a and 582a. Switches 572a and 580a also connect to AC ground 202. Switch 582a also connects to the non-inverting output of amplifier 590a. The non-inverting output of amplifier 590a connects to switch 584a. The other end of switch 584a comprises the output signal Nop. Resonator circuit 503a is a fully differential circuit. The lower half of resonator circuit 503a is a mirror image of the upper half. Resonator circuit 503a comprises one signal path of the input signal. An identical resonator circuit 503b comprises the second signal path. Resonator circuit 503b is connected in the same manner as resonator circuit 503a but the switches operate on the alternative clock phases. Resonator circuit 503 is clocked at half the sampling frequency. Resonator circuit 503 has a resonant frequency that is a function of the sampling frequency and the capacitor ratios. Resonator circuit 503 has a fast settling time. However, because of the two independent paths, path matching is more difficult to maintain. The transfer function of resonator circuit 503 is : z H TIP (z) =- -2 (8) 1 + z " VI. Multi-Sampling Bandpass Resonator Design The double-sampling switched capacitor bandpass resonator circuit of the present invention can be further expanded to multi-sampling resonator circuits. A schematic diagram of an exemplary quadruple-sampling switched capacitor resonator circuit 802 is illustrated in FIG. 10G. FIG. 10G only illustrates the upper half of resonator circuit 802. The lower half, to which the Nin of the differential input is applied, is identical to the upper half and not shown for simplicity. Within resonator circuit 802, the input signal Nip is provided to four signal paths, the first signal path through switch 820a. Switch 820a connects to one end of capacitor 824a and switch 826a. Switch 826a connects to the quantizer output Yxpøl. The other end of capacitor 824a connects to switches 822a and 830a and one end of capacitor 828a. Switch 822a also connects to AC ground 202 and switch 830a also connects to the inverting input of amplifier 850a. The other end of capacitor 828a connects to switches 832a and 834a. Switch 832a also connects to the non-inverting output of amplifier 850a and switch 834a also connects to the inverting output of amplifier 850a. The operation of the first signal path in resonator circuit 802 can be described as follows. During the first clock phase øl, switches 820a and 822a are switched ON and capacitor 824a is charged with the input signal Nip. During the third clock phase ø3, switches 820a and 822a are switched OFF and switches 826a, 830a, and 834a are switched ON. The signal Yxpøl and the voltage across capacitor 824a are scaled by the ratio of capacitors 824a and 828a (Cs/Cf) and provided to the non-inverting output of amplifier 850a. Also during the first clock phase øl, switch 832a is switched ON and the signal from the inverting output of amplifier 850a is fed back, charging capacitor 828a. The voltage across capacitor 828a is reflected at the non-inverting output of amplifier 850a during the third clock phase ø3. The non-inverting output from amplifiers 850a and 850b comprises the outputs Vopl3 and Nop23, respectively, which are provided to the next resonator section. The other three signal paths are connected in similar manner as shown in FIG. 10G. The other three signal paths also operate in similar manner as the first signal path. However, the switches in the other three signal paths are switched with switching clocks having different phases as shown in FIG. 10H. Thus, each switch in resonator circuit 802 is switched ON and OFF every four sampling clock cycles. This allows amplifiers 850 more time to settle to the final value. Viewed in another way, an amplifier having a specified performance can be used to implement a ΣΔ ADC which is effectively sampled at four time the switching frequency. However, path mismatch due to mismatch in capacitor values, uneven clock phases of the switching clocks, and amplifier mismatch can cause images to appear the ADC output. VII. Other Considerations Double-sampling switched capacitor circuits for the ΣΔ ADC of the present invention are sensitive to path mismatch which can result from mismatch in capacitor values and/or uneven phases of the switching clocks. Capacitor mismatch can be reduced to less than one percent by utilizing circuit design techniques which are known in the art, such as the common centroid layout technique. Double-sampling switched capacitor circuits sample the signal on two phases of the switching clock. The switching clock is a divide-by-two of the sampling clock (see FIG. 6B). If the divide-by-two causes any phase asymmetry, the phase mismatch causes an image of the input signal to appear at the output. Using the master clock, e.g. the sampling clock, before the divide-by-two operation, to clock the first sampling stage (switches 320, 322, 324, and 326 in FIG. 7A) will resolve this problem. Clock jitter in the first sampling stage is also critical. Clock jitter translates to quantization noise. Clock jitter can be reduced by clocking the first sampling stage with a clean external clock source. For undersampling application wherein the ADC is used to downconverts a signal at IF down to a lower frequency, the jitter spectral density is increased by the square of the undersampling ratio. For example, for an IF of 220 MHz and a sampling rate of 80 MHz, the phase noise is increased by 8.8 dB [201og(220 MHz/80 MHz)]. For undersampling applications, the clock jitter requirement is more stringent. The ΣΔ ADC of the present invention has been described in detail for a bandpass MASH 4-4 ΣΔ ADC which is implemented with double-sampling switched capacitor circuits. The circuit design techniques described above can also be applied to a single-loop ΣΔ ADC architecture which is shown in FIG. 1. Thus single-loop ΣΔ ADCs are within the scope of the present invention. A baseband ΣΔ ADC can be designed by implementing the filters in FIGS. 1-2 with a lowpass filter. For example, a baseband MASH 2-2 ADC can be designed by substituting resonators 130 in FIG. 4 with integrators having the z- 1 lowpass transfer function 1 -^ . Thus, baseband single-loop and MASH ΣΔ ADCs are within the scope of the present invention. The filters in the ΣΔ ADCs of the present invention can be implemented with various analog circuit design techniques, including active RC, gm-C, MOSFET-C, switched capacitor, and switched current. Furthermore, the switched capacitor and switched current circuits can be single-sampling, double-sampling, or multi-sampling designs. Therefore, the various combinations and permutations of bandpass and baseband ΣΔ ADC implemented with single-loop and MASH architectures which are synthesized with active RC, gm-C, MOSFET-C, switched capacitor, or switched current utilizing single-sampling, double-sampling, or multi-sampling designs are within the scope of the present invention. Some embodiments of the invention have been described with circuitry implemented using MOSFETs. The invention can also be implemented with other circuits including BJTs, FETs, , MESFETs, HBTs, P-HEMTs, and others. Also, P-MOS and N-MOS can be used to implement the invention. As used herein, "transistor" generically refers to any active circuit, and is not limited to a BJT. VIII. Minimizing Power Consumption In many applications, such as CDMA communication system, power consumption is an important design consideration because of the portable nature of the telephone wherein the ΣΔ ADC of the present invention reside. The ΣΔ ADC can be designed to minimize power consumption by allowing for selective sections of the ΣΔ ADC to be disabled when high dynamic range is not required. In addition, the ΣΔ ADC can be designed to allow for adjustment of the bias current based on the signal level of the ADC input and the required performance. In the exemplary embodiment, the ΣΔ ADC provides 12-bits of resolution. This design anticipates the worse case signal level into the ΣΔ ADC. For CDMA applications, approximately 4-bits of resolution is needed for the desired signal (e.g. the CDMA signal) and the remaining 8-bits of resolution are reserved for spurious signals of large amplitude (or jammers) and for AGC control. In the exemplary embodiment, the 12-bits of resolution is provided by a two-loop MASH 4-4 architecture. Referring to FIG. 4, loop 110a provides high dynamic range and a low noise floor. Loop 110b provides additional dynamic range but has a slightly higher noise floor than loop 110a. The lower noise floor of loop 110a is the result of having larger capacitors and biasing the amplifiers within loop 110a with higher bias current. In the present invention, each loop can be selectively disabled, based on the signal level of the ADC input and the required performance, to minimize power consumption. Furthermore, the bias current of the amplifier within each resonator 130 can be adjusted based on the signal level of the ADC input and the required performance. When high dynamic range is required, the ADC input is provided to loop 110a, the bias current of all amplifiers is set high, and MASH ADC 100 operates in the manner described above. This situation may result from an ADC input comprising the CDMA signal and two large jammers at +58 dBc or an ADC input comprising the CDMA signal and one large jammer at +72 dBc. These requirements are specified by the "TIA/EIA/IS-98-A Intermodulation Spurious Response Attenuation", hereinafter the IS-98-A standard. In practice, this situation occurs infrequently. As the jammer amplitude decreases, high dynamic range is not required. When this occurs, loop 110b can be disabled and the output Yl from loop 110a comprises the ΣΔ ADC output. Alternatively, loop 110a can be disabled, the ADC input can be provided to loop 110b, and the output Y2 from loop 110b comprises the ΣΔ ADC output. Thus, one to two loops can be enabled to provide the required dynamic range. The bias current of the amplifier in each resonator 130 can be adjusted to minimize power consumption while providing the required performance. In the exemplary embodiment, loop 110a is designed to consume a maximum of 10 mA of bias current and second loop 110b is designed to consume a maximum of 8 mA of bias current. In the exemplary embodiment, within loop 110a, the amplifier within resonator 130a is designed to consume 6 mA and the amplifier within resonator 130b is designed to consume 4 mA. When high dynamic range is required, the bias current for each amplifier is set high. When high dynamic range is not required, the bias current can be decreased. For example, the bias current of the amplifier within resonator 130a can be decreased from 6 mA down to 2 mA and the bias current of the amplifier within resonator 130b can be decreased from 4 mA down to 2 mA. Similarly, the bias current for the amplifiers within loop 110b and be decreased accordingly when high dynamic range is not required. Adjustment of the amplifier bias current can be performed independently of the disablement of the loops, or can be performed in conjunction with the disablement of the loops. In fact, analysis and measurements can be performed to ascertain the dynamic range of various configurations of the ΣΔ ADC. Then, based on the required dynamic range, the ΣΔ ADC can be configured accordingly. The various methods used to configure the ΣΔ ADC to minimize power consumption are within the scope of the present invention. In the exemplary embodiment, the required dynamic range can be estimated by measuring the power level of the desire signal (e.g. the CDMA signal) and the power level of the ADC input. The power level of the ADC input can be measured by a power detector. The power detector can be implemented in the manner known in the art. The power level of the desired signal can be measured by computing the RSSI of the desired signal, after the digital signal processing to remove undesirable images and spurious signals. The RSSI measurement is described in detail in U.S. Patent No. 5,107,225, entitled "HIGH DYNAMIC RANGE CLOSED LOOP AUTOMATIC GAIN CONTROL CIRCUIT", issued April 21, 1992, assigned to the assignee of the present invention and incorporated by reference herein. Alternatively, the required dynamic range can be determined based on the operating mode of the receiver wherein the ΣΔ ADC resides. IX. Control Circuit As noted above, to reduce power consumption while still provide the required data conversion performance, a control mechanism is used to selectively enable one or more loops of the ΣΔ ADC and to disable the remaining loops. The control mechanism measures one or more characteristics (e.g., signal level) of the ADC input signal, compares the measured characteristic(s) to particular threshold level(s), and controls the loops such that the desired or required performance is achieved. Many challenges arise in designing such control mechanism. First, for a ΣΔ ADC that is used in a receiver of a communications device, the amplitude of the input signal is typically very small, even after the signal conditioning (e.g., low noise amplification, and so on). In fact, for a cellular application, the input signal amplitude can be as small as 30 mV peak-to-peak or less. Thus, a detector within the control mechanism should be able to accurately measure a small amplitude input signal. Second, for a ΣΔ ADC that is used as a bandpass sampling converter, the input signal is centered at IF and can have high frequency components. For a specific CDMA application, the input signal can have frequency components as high as 240 MHz or more. To avoid attenuating high frequency components, the detector can be designed with components (e.g., switches, transistors, and so on) having large dimensions, which can provide low loss at high frequency. However, large-sized components increase die area and cost. Higher operating frequency also typically requires larger amounts of bias current, which is undesirable in portable applications such as cellular telephone. Third, the ADC input signal is typically buffered by an amplifier or a buffer that can introduce a DC offset to the signal. The DC offset can be a large percentage of the input signal amplitude. For example, an input signal having an amplitude of 30 mNpp may also have a DC offset of 10 mN, or possibly more. Thus, the detector should, to an extent, be insensitive to the DC offset in the input signal. Moreover, the detector should also be insensitive to its internally generate DC offset. As can be seen, a control mechanism that addresses the above challenges is highly desirable. FIG. 11 shows a simplified block diagram of a specific embodiment of a control circuit 1110 for controlling a ΣΔ ADC 1120. As shown in FIG. 11, ΣΔ ADC 1120 includes two cascaded ΣΔ stages 1122a and 1122b driven by a buffer (BUF) 1124. Each ΣΔ stage 1122 can represent a loop of a multi-loop ΣΔ ADC, such as loop 110 and it associated feed forward element 150 shown in FIG. 4. Each ΣΔ stage 1122 can also represent a filter section of a multi-section ΣΔ ADC, such as filter section 24 or 28 shown in FIG. 1. Generally, each ΣΔ stage 1122 can represent any portion of a circuit that can be selectively enabled and disabled. When a stage is disabled, internal circuitry within the stage (not shown in FIG. 11) provides a bypass path such that the signal at the input of the stage is provide to the output of the stage. As shown in FIG. 11, within ΣΔ ADC 1120, the input signal is provided to buffer 1124 that buffers the signal. The buffered signal comprises the ΣΔ modulator input signal that is provided to the first ΣΔ stage 1122a. ΣΔ stage 1122a noise shapes and quantizes the signal in the manner described above and provides the processed signal to the second ΣΔ stage 1122b. ΣΔ stage 1122b further noise shapes and quantizes the signal and generates the output data samples. The outputs from ΣΔ stages 1122a and 1122b may be combined by a noise cancellation circuit (not shown in FIG. 11) for a multi-loop ΣΔ ADC. Within control circuit 1110, the modulator input signal is also provided to a detector ΣΔ stage 1112 that also noise shapes and quantizes the signal to generate a detected signal. The detected signal is then provided to a conditioning circuit 1114 that conditions and quantizes the signal to generate digital samples. The signal conditioning may include, for example, signal amplification, filtering, comparison, and so on. The samples are provided to a signal processor 1116 that further processes the samples to generate a control signal. The control signal is used to selectively enable and disable ΣΔ stage 1122a, and may also be used to selectively enable and disable ΣΔ stage 1122b (as shown by the dashed line). A reference generator 1118 can be included within control circuit 1110, and is used to provide one or more reference voltages to ΣΔ stages 1122, detector ΣΔ stage 1112, and conditioning circuit 1114. The elements of control circuit 1110 are further described below. Generally, one or more ΣΔ stages 1122 in the signal path are enabled to provide the required data conversion performance, e.g., the required signal-to- noise ratio (SNR). For a cellular application, the modulator input signal includes the desired signal (e.g., the CDMA signal) and possible undesired jammers. The jammers can be much larger than the desired signal. Since a gain control mechanism is typically used to maintain the modulator input signal at a particular signal level, to avoid clipping by the ΣΔ ADC, the desired signal can be very small relative to the ΣΔ ADC input range when large amplitude jammers are present. In this situation, higher dynamic range is required to allow for quantization of the small amplitude desired signal with the required SNR. In accordance with an aspect of the invention, jammers are detected by J measuring the amplitude of the modulator input signal. Detector ΣΔ stage 1112 processes the modulator input signal in similar manner as one of ΣΔ stages 1122 in ΣΔ ADC 1120 and provides the detected signal that is indicative of the amplitude of the modulator input signal. In an embodiment, for a cellular application, if the amplitude of the modulator input signal is determined to be less than a particular signal level, one of ΣΔ stages 1122 can be disabled since jammers are not present (or are at low signal levels) and high dynamic range is not required. Alternatively, if the amplitude of the modulator input signal is determined to be greater than the particular signal level, one or more large amplitude jammers are presumed to be present in the input signal. Both ΣΔ stages 1122 are then enabled to provide high dynamic range such that the required SNR is maintained. Specifically, the high dynamic range allows ΣΔ ADC 1120 to quantize the desired signal with the required SNR even in the presence of large amplitude jammers. As noted above, each ΣΔ stage 1122 can be a loop of a multi-loop ΣΔ ADC or a filter section of a multi-section ΣΔ ADC. The ΣΔ stages can be implemented with different orders (e.g., a fourth order in cascade with a second order). In a specific embodiment, each ΣΔ stage 1122 is a second order lowpass modulator for a baseband ΣΔ ADC and a fourth order bandpass modulator for a bandpass ΣΔ ADC. When the ΣΔ stages are the same order, the second ΣΔ stage 1122b can be implemented as a "shrunken" replica of the first ΣΔ stage 1122a, as described above. The first ΣΔ stage 1122a can be designed with larger- sized components (e.g., switches, capacitors, and so on) and biased with greater current to provide enhanced noise performance, when enabled. The second ΣΔ stage 1122b can be designed with smaller-sized components and biased with less current since high dynamic range is not required when the input signal amplitude is larger. Detector ΣΔ stage 1112 can be implemented as a "shrunken" replica of the second ΣΔ stage 1122b, and can be designed with even smaller-sized components and biased with even less current. Detector ΣΔ stage 1112 is used to measure signal amplitude, and high dynamic range or high SNR is typically not required. FIG. 12 shows a simplified block diagram of a specific embodiment of a control circuit 1210 for controlling a multi-stage circuit 1220. Multi-stage circuit 1220 can be a multi-loop ΣΔ ADC, a multi-section ΣΔ ADC, or other circuits having multiple stages that can be selectively enabled and disabled (and possibly bypassed). An example of such multi-stage circuit is an amplifier having a set of cascaded amplification stages. As shown in FIG. 12, within multi-stage circuit 1220, the input signal is provided to a buffer (BUF) 1224 that buffers the signal. The buffered signal is provided to a high performance stage 1222a that is enabled when high performance (e.g., high dynamic range) is required. The output from stage 1222a is provided to a medium performance stage 1222b that is enabled when medium performance (e.g., medium dynamic range) is required. The signal from stage 1222b is provided to a low performance stage 1222c that provides a low level of performance (e.g., low dynamic range). In an embodiment, only stage 1222c is enabled when low dynamic range is required, stages 1222b and 1222c are enabled when medium dynamic range is required, and all three stages 1222a through 1222c are enabled when high dynamic range is required. Stages 1222a and 1222b include MUXes 1226a and 12226b, respectively. Each MUX 1226 selects either the processed signal or the bypassed signal and provides the selected signal to the output of the stage. The bypassed signal is selected when the stage is disabled. Each stage 1222 can be implemented independently from other stages. For example, for a ΣΔ ADC, each stage 1222 can have a different order. In a specific embodiment, each stage 1222 is a second order lowpass modulator for a baseband ΣΔ ADC and a fourth order bandpass modulator for a bandpass ΣΔ ADC. In this embodiment, stage 1222b can be implemented as a shrunken replica of stage 1222a, and stage 1222c can be implemented as a shrunken replica of stage 1222b. As shown in FIG. 12, within control circuit 1210, the buffered signal is provided to two detection paths. In the first detection path, a low performance detector stage 1212a processes the buffered signal and provides a first detected signal to a conditioning circuit/ signal processor 1214a. Circuit/processor 1214a conditions, quantizes, and further processes the detected signal to generate a first control signal that is used to selectively enable and disable high performance stage 1222a and medium performance stage 1222b. In the second detection path, a medium performance detector stage 1212b processes the buffered signal and provides the processed signal to a low performance detector stage 1212c. Detector stage 1212c further processes the signal and provides a second detected signal to a conditioning circuit/ signal processor 1214b. Circuit/processor 1214b conditions, quantizes, and further processes the detected signal to generate a second control signal that is used to selectively enable and disable high performance stage 1222a. In an embodiment, each of low performance detector stages 1212a and 1212c is implemented as a replica of low performance stage 1222c, and medium performance detector stages 1212b is implemented as a replica of medium performance stage 1222b. The replicas can be implemented using smaller-sized components and can also be operated using less bias current. In the embodiment shown in FIG. 12, the stage(s) to be disabled are controlled by a control signal from a detection path that includes the replica(s) of the stage(s) that are enabled. For example, high performance stage 1222a and medium performance stage 1222b are controlled by the first control signal from the detection path that includes a replica of low performance stage 1222c. Similarly, high performance stage 1222a is controlled by the second control signal from the detection path that includes the replicas of the low and medium performance stages 1222a and 1222b. In an embodiment, the detector stage(s) in each detection path measure the amplitude of the buffered signal. In an embodiment, the stages in the signal path are enabled based on the detected signal amplitude (e.g., to provide higher dynamic range when the signal amplitude is large). For example, stages 1222a through 1222c can be enabled if the input signal amplitude is greater than a first signal level, stages 1222b and 1222c can be enabled if the input signal amplitude is between the first signal level and a second signal level, and stage 1222c can be enabled if the input signal amplitude is less than the second signal level. The stages can also be enabled based on other detected signal characteristics, and can also be enabled in different order and configurations. FIG. 13 shows a simplified block diagram of a specific embodiment of a control circuit 1310 for controlling a multi-stage circuit 1320. Similar to multi- stage circuit 1220, multi-stage circuit 1320 can be a multi-loop ΣΔ ADC, a multi- section ΣΔ ADC, or other circuits having multiple stages that can be selectively enabled and disabled (and possibly bypassed). Each stage (possibly with the exception of the last stage 1322n) includes a MUX 1326 that selects either the processed signal or the bypassed signal and provides the selected signal to the output of the stage. The bypassed signal is selected when the stage is disabled. As shown in FIG. 13, multi-stage circuit 1320 includes a number of stages 1322a through 1322n and a buffer (BUF) 1324. The input signal is provided to buffer 1324 that buffers the signal and provides the buffered signal to the first stage 1322a. Each stage 1322 processes the signal and provides the processed signal to a subsequent stage. The output from the n-th stage 1322n comprises the output from circuit 1320. In an embodiment, each stage (again, possibly with the exception of the last stage 1322n) can be selectively enabled and disabled. A sufficient number of stages is enabled to provide the required performance (e.g., the required dynamic range or the required SNR), and remaining stages are disabled to conserve power. In a specific embodiment, the highest dynamic range is provided when all stages are enabled, the next highest dynamic range is provided when all but one stage (e.g., first stage 1322a) are enabled, and the lowest dynamic range is provided when only one stage (e.g., n-th stage 1332n) is enabled. In a specific embodiment, the stages are disabled in accordance with their relative location in the circuit. As an example, the first stage 1322a is disabled first, the second stage 1322b is disabled next, and the (n-l)-th stage is disabled last. In an embodiment, the n-th stage 1322n is enabled at all times, or whenever circuit 1320 is turned on. In other embodiments, the stages can be enabled in different configurations and disabled in different orders, and this is within the scope of the invention. For example, the first stage (instead of the last stage) can be enabled at all times. Within control circuit 1310, the buffered signal is provided to a set of one or more detector stages 1312. Detector stage(s) 1312 process the buffered signal and provide a detected signal to a conditioning circuit 1314 that conditions and quantizes the signal to generate digital samples. The samples are provided to a signal processor 1316 that processes the samples and generates a set of control signals. The control signals are used to selectively enable and disable the stages of multi-stage circuit 1320. A reference generator 1318 can also be included within control circuit 1310 to provide one or more reference voltages to stages 1322, detector stage(s) 1312, and conditioning circuit 1314. In an embodiment, each of detector stage(s) 1312 in the detector path is implemented as a replica of a stage 1322 in the signal path. Again, the replica(s) can be shrunken to reduce die area and can also be operated with less bias current to conserve power. FIG. 14 shows a simplified block diagram of a specific embodiment of a control circuit 1410 that can be used as the control circuits in FIGS. 11 through 13. Control circuit 1410 includes detector stage(s) 1412, a conditioning circuit 1414, and a signal processor 1416 coupled in series. Detector stage(s) 1412 receive and process the input signal (e.g., the modulator input signal from buffer 1124 in FIG. 1). Signal processor 1416 provides the control signal that is used to enable /disable one or more stages of a multi-stage circuit. A reference generator 1418 couples to detector stage(s) 1412 and conditioning circuit 1414 and provides the necessary reference signals to these circuit elements. For clarity, control circuit 1410 will now be described for used in conjunction with the specific two-stage ΣΔ ADC design shown in FIG. 11. In a specific embodiment, ΣΔ ADC 1120 is an eight order bandpass MASH 4-4 ADC, and each ΣΔ stage 1122 comprises a fourth order bandpass modulator. As noted above, ΣΔ stage 1122b provides a particular dynamic range and noise performance, and ΣΔ stage 1122a provides additional dynamic range and enhanced noise performance when enabled. The enhanced noise performance of ΣΔ stage 1122a can be provided with larger-sized components and greater bias current. In an embodiment, detector stage 1412 is a "shrunken" replica of one of the ΣΔ stages (i.e., ΣΔ stage 1122a or 1122b), and is also a fourth order bandpass modulator. For example, the components of detector stage 1412 can be implemented at one tenth of the size of the components in ΣΔ stage 1122a. In another embodiment, detector stage 1412 is a lower order (e.g., second order) modulator, which may be adequate for detecting the input signal amplitude while utilizing less complex circuitry. Detector stage 1412 noise shapes and quantizes the input signal in similar manner as the ΣΔ stage it replicates. The differential output, Op and On, from detector stage 1412 is provided to conditioning circuit 1414. Generally, a ΣΔ modulator's analog output amplitudes are indicative of its input signal level. For some designs, the ΣΔ modulator has tendency to become unstable as the input signal amplitude exceeds the reference voltage. In fact, it can be shown that the ΣΔ modulator's analog output amplitudes grow significantly as the input signal amplitude exceeds the reference voltage (i.e., the peak-to-peak signal amplitude exceeds the difference between the high and low reference voltages). When the ΣΔ modulator becomes unstable, the standard deviation of its analog output signal is much larger than that of the ΣΔ modulator when it is stable. In accordance with an aspect of the invention, these characteristics are used detect the amplitude of the modulator input signal. FIG. 15A shows a graph of the standard deviation of the detected signal from detector stage 1412 versus input signal amplitude. FIG. 15A includes a set of plots for various input signal frequencies. In FIG. 15A, the standard deviation on the vertical axis and the input signal amplitude on the horizontal axis are normalized to the detector reference voltage (i.e., N DEC p and N DEC Ν in FIG. 14). When the frequency of the input signal (e.g., 0.58 MHz) is within the bandwidth of the ΣΔ modulator (e.g., 0.70 MHz), as exemplified by a plot 1510a, the standard deviation of the detected signal increases sharply as the input signal amplitude approaches a normalized value of 1.0. A normalized value of 1.0 corresponds to the point where the peak-to-peak amplitude of the input signal equals the difference between N DECJ , and N DEC _ Ν . It can be seen that the standard deviation increases from less than 1.0 to greater than 20 (a twenty-plus fold increase) when the input signal amplitude increases from 0.9 to 1.0 (an eleven-plus percent increase). However, when the frequency of the input signal (e.g., 15.4 MHz) greatly exceeds the bandwidth of the ΣΔ modulator (e.g., 0.70 MHz), as exemplified by a plot 1510k, the standard deviation of the detected signal increases slowly as the amplitude of the input signal increases past a normalized value of 1.5. The plots 15a through 15k in FIG. 15A indicate that higher frequency components are filtered by the ΣΔ modulator. The lowpass characteristic of the ΣΔ modulator reduces the impact due to high frequency components. In the embodiment shown in FIG. 14, conditioning circuit 1414 includes a comparator 1438 that compares the detected signal from the detector stage against a comparison signal (or a voltage) and provides the comparison result to signal processor 1416. Within conditioning circuit 1414, the outputs, Op and On, from detector stage(s) 1412 are provided to one end of switches 1432b and 1432c, respectively. Comparison voltages, N COMPj > and N COMPJ ^, are provided to one end of switches 1432a and 1432d, respectively. The other ends of switches 1432a and 1432b couple together and to one end of a capacitor 1434a, and the other ends of switches 1432c and 1432d couple together and to one end of a capacitor 1434b. The other end of capacitor 1434a couples to one end of a switch 1436a and to a non-inverting input of comparator 1438. Similarly, the other end of capacitor 1434b couples to one end of a switch 1436b and to an inverting input of comparator 1438. The other ends of switches 1436a and 1436b couple to an input common-mode voltage N ICM , which is the mid-scale or common-mode voltage of the detector outputs, Op and On. Switches 1432b, 1432c, 1436a, and 1436b are controlled by a clock signal having a first clock phase φl, and switches 1432a and 1432d are controlled by a clock signal having a second clock phase φ2. During the first clock phase φl, switches 1432b, 1432c, 1436a, and 1436b are closed and the outputs Op and On from detector stage 1412 charge capacitors 1434a and 1434b, respectively. During the second clock phase φ2, switches 1432a and 1432d are closed and the voltage captured on capacitors 1434a and 1434b are respectively compared against the high and low comparison voltages, N C0MP P and V COMPj3 ' by comparator 1438. Capacitors 1434a and 1434b thus respectively sample the detector outputs, Op and On, during the first clock phase φl, and respectively sample the comparison voltages, V- o P > an d N coMP - Ν ' during the second clock phase φ2. Comparator 1438 generates a one (i.e., logic high) when the detector output exceeds the comparison voltage and a zero (i.e., logic low) otherwise. FIG. 14 shows a specific embodiment of conditioning circuit 1414. Other conditioning circuits can be designed and are within the scope of the invention. FIG. 15B shows a graph of the distribution densities of the signal levels of the detected signal. FIG. 15B includes plots 1520a and 1520b for two input signal amplitudes, Al and A2, respectively. The detected signal has a density shown by plot 1520a when the input signal has an amplitude of Al, and a density shown by plot 1520b when the input signal has an amplitude of A2, where A2 is greater than Al. As shown in FIG. 15B, the density distributions are approximately Gaussian, and the standard deviation of the Gaussian distribution increases with larger input signal amplitude. When the detected signal exceeds the comparison voltage N C0MP , as indicated by shaded areas 1524a and 1524b, the conditioning circuit outputs a one. The comparison voltage thus affects the percentage of ones and zeros from conditioning circuit 1414. By decreasing the comparison voltage, the percentage of ones increases, and a faster control mechanism may be obtained. Alternatively, by increasing the comparison voltage, a more accurate detection may be obtained, which may reduce the likelihood of false detection. The distribution bi-nodal when the ΣΔ modulator oscillates. In an embodiment, signal processor 1416 receives the digital samples from comparator 1438 and counts the number of ones within a particular time period. As shown in FIG. 15A, the comparison voltage N C0MP can be expressed on the vertical axis. The number of ones (i.e., the number of times a particular plot exceeds N COMP ) is small when the standard deviation of the detected signal is below the comparison voltage N COMP and is large when the standard deviation of the detected signal exceeds the comparison voltage N COMP . FIG. 15C shows a graph of the count value from signal processor 1416 versus detector input signal amplitude. FIG. 15C includes a set of plots 1530a through 1530k for various comparison voltages N C0MP . As noted above, the digital samples from conditioning circuit 1414 are provided to signal processor 1416 that counts the number of ones within a particular time period. The number of ones increases noticeably as the input signal amplitude increases from a normalized value of 0.9 to 1.0. The comparison voltage N C0MP effects the count value when the input signal amplitude is between the normalized values of 0.9 and 1.0 but has less impact when the input signal amplitude exceeds the normalized value 1.0. When the comparison voltage is low, as indicated by plot 1530a, the count value saturates when the input signal amplitude reaches the normalized value of 0.95. However, when the comparison voltage is higher, as indicated by plot 1530k, the count value does not saturate until the input signal amplitude reaches the normalized value of 1.0. When the input signal amplitude exceeds a normalized value of approximately 1.0, the count value saturates at a particular count value. In an embodiment, signal processor 1416 compares the count value with a count threshold. If the count value exceeds the count threshold (e.g., 6000 in a specific implementation), the input signal amplitude is deemed to be greater than a particular percentage (e.g., 95%) of the detector reference voltage N DEC , and the control signal is configured to enable one or more additional ΣΔ stages in the ΣΔ ADC. Signal processor 1416 can be implemented with an accumulator that counts the number of ones from comparator 1438 and is reset at the start of each counting interval. The accumulator value at the end of the counting interval is compared against the count threshold. If the accumulator value exceeds the count threshold, the detector stage is presumed to be in oscillation and the amplitude A Vin of the detector input signal is deemed to have exceeded a particular percentage of the detector reference voltage N DEC (e.g., A Vjn > 0.95 N V DEC )' The counting interval can be adjusted based on system requirements. Generally, a longer counter interval provides increased accuracy. However, a shorter counting interval can provide a faster response time. In an embodiment, the ΣΔ stages are enabled and disabled based on the detected input signal amplitude. If the detected input signal amplitude exceeds an particular signal level, high dynamic range is required and additional ΣΔ stage(s) are enabled. Alternatively, if the detected input signal amplitude is below the particular signal level, high dynamic range is not required and zero or more ΣΔ stages can be disabled. In an embodiment, the input signal amplitude is determined by adjusting the detector reference voltage N DEC and monitoring the count value. As the detector reference voltage is adjusted, if the count value noticeably changes in value, the input signal amplitude is determined as a percentage (e.g., 95%) of the detector reference voltage N DEC . Alternatively, as the detector reference voltage is adjusted, if the count value exceeds a particular amount (e.g., 6000), the input signal amplitude is determined to be greater than a particular percentage (e.g., 95%) of the detector reference voltage N DEC . In some applications, it is not necessary to determine the input signal amplitude with a great deal of precision. Rather, it is only necessary to determine whether the input signal amplitude exceeds particular signal levels. The signal levels can correspond to, for example, the levels at which the stages in the multi-stage circuit are to be enabled /disabled. For example, for a three- stage circuit, the detector stage can be designed with two detector reference voltages (e.g., a high and a low detector reference voltage). If the count value is high for the high detector reference voltage, the input signal has a large amplitude and all three stages may be enabled to provide the required performance. If the count value is low for the low detector reference voltage, the input signal has a small amplitude and one stage may be adequate to provide the required performance. The remaining stages may then be disabled to conserve power. And if the count value is low for the high detector reference voltage but high for the low detector reference voltage, the input signal has a medium amplitude and two stages may be enabled. The high and low detector reference voltages can be selected to correspond to signal levels at which the stages are enabled/disabled. In another embodiment, the input signal amplitude can be determined by adjusting the gain of the detector stage. The detector stage can be designed with multiple gain settings. The gain settings can correspond to, for example, the levels at which the stages in the multi-stage circuit are to be enabled/disabled. For example, for a three-stage circuit, the detector stage can be designed with two gain settings (e.g., a high and a low gain setting). If the count value is high at the low gain setting, the input signal has a large amplitude and all three stages may be enabled to provide the required performance. If the count value is low at the high gain setting, the input signal has a small amplitude and one stage may be adequate to provide the required performance. And if the count value is low at the low gain setting but high at the high gain setting, the input signal has a medium amplitude and two stages may be required. The high and low gain settings can thus be selected to correspond to signal levels at which the stages are enabled/ disabled. For clarity, the invention has been described for a particular (e.g., cellular) application in which the input signal may include large amplitude jammers. The control mechanism described above presumes that large amplitude jammers are present when the input signal amplitude is high, and additional ΣΔ stage(s) are enabled to provide increased dynamic range. For other applications, large spurious signals may not be present in the input signal and the reverse could be true. That is, the ΣΔ stages can be disabled as the input signal amplitude increases. The large signal amplitude could indicate a desired signal having a higher SNR and requiring less dynamic range. Thus, the control mechanism is designed based, to an extent, on the characterization of the input signal. Also for clarity, the invention has been mainly described for a specific application to ΣΔ ADC. However, the invention can be adopted for use with many other multi-stage circuits. Generally, the multi-stage circuit includes a number of signal stages, some of which can be selectively enabled and disabled. In an embodiment, the control circuit includes one or more detector stages, with each detector stage being a replica of one of the signal stages. The detector stage(s) are used to measure a particular signal characteristic (e.g., signal amplitude). The measured signal characteristic is then used to control the signal stages. The multi-stage circuit can be, for example: (1) an amplifier having multiple gain stages, (2) a power amplifier having multiple output drivers (e.g., coupled in parallel), (3) an active filter having multiple filter sections, and other circuits. The signal stages can be coupled in series or in parallel, or a combination thereof. The detector stages are typically coupled in a similar configuration as the signal stages. The control mechanism described above provides many advantages. The detector stage receives the same input signal that is provided to the multistage circuit. Extra circuitry is not required to generate a signal especially for the control mechanism. In addition, the control mechanism described above is particularly advantageous when used for controlling a ΣΔ ADC. These additional benefits are described below. First, the detector stage models the signal stage that is enabled and provides a measurement that more accurately indicates the actual amplitude of the input signal. As note above, the ΣΔ modulator filters the input signal such that higher frequency components (i.e., relative to the center frequency) are attenuated more than lower frequency components. Since the detector stage is implemented as a replica of one of the stages, the input signal is attenuated by the detector stage in a similar manner (i.e., with a similar frequency response). The detector stage thus provides a detected signal having spectral components approximately matching that of the stage in the signal path. The detector stage is particularly accurate in measuring jammers in a cellular application. High frequency jammers are attenuated by the detector stage in similar manner as the enabled stage(s) in the signal path. Thus, even though the amplitude of the jammer may be high, the detected signal could be small if the jammer frequency is sufficiently high. In this case, a low dynamic range setting may be adequate since the jammers are similarly filtered by the stage in the signal path. In contrast, a detector employing a flat frequency response may falsely declare a high input signal amplitude for large out-of- band jammers and erroneously turn on additional signal stages that may not be required. Second, in some embodiment, the detector stage is a "shrunken" replica of the stage in the signal path. Thus, the detector stage can be implemented with components (e.g., switches and capacitor) having dimensions that are a fraction of those of the stage in the signal path. For example, the scaling for the detector stage can be a tenth of the size of the stage being replicated. Third, the detector stage can be operated at a fraction of the bias current of the stage its replicates. The detector stage is generally used to detect signal amplitude, and high dynamic range or high SNR is typically not required. The bias current for the detector stage can thus be substantially reduced. Fourth, the detector reference voltage V DEC can be adjusted (e.g., in small and accurate increments) to allow for accurate determination of the input signal amplitude, if necessary or desired. Accurate detector reference voltages can be readily generated using, for example, a bandgap reference and a DAC in a manner known in the art. Fifth, the comparator in the conditioning circuit can be designed to detect large signal amplitudes from the detector stage rather than the small amplitude of the input signal. The detected signal from the detector stage typically has several hundreds of millivolts of peak-to-peak swing when the detector is driven unstable. The comparator reference voltage N COMP can thus be set at a much higher level (e.g., one half volt differential) rather than the small (e.g., 30 mN) differential level of the input signal being measured. The large comparator reference voltage enables a control circuit design that is more tolerant to offset in the detector stage and the reference generator. Sixth, for a bandpass ΣΔ ADC, the detector stage also downconverts the IF input signal to baseband or other low output frequencies. Thus, the subsequent circuitry (e.g., the switches within the conditioning circuit) can be implemented with smaller sizes. The foregoing description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. Narious modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. WE CLAIM:

Claims: CLAIMS 1. A data conversion circuit comprising: a sigma-delta analog-to-digital converter (ΣΔ ADC) configured to receive an input signal and provide data samples, the ΣΔ ADC including a plurality of ΣΔ stages coupled in cascade; and a control circuit coupled to the ΣΔ ADC and configured to provide a control signal that selectively disables zero or more of the plurality of ΣΔ stages, the control circuit including one or more detector stages configured to receive the input signal and provide a detected signal, a conditioning circuit coupled to the one or more detector stages, the conditioning circuit configured to receive the detected signal and provide conditioned samples, and a signal processor coupled to the conditioning circuit, the signal processor configured to receive the conditioned samples and provide the control signal. 2. The circuit of claim 1, wherein the control circuit includes one detector stage implemented as a replica of one of the ΣΔ stages. 3. The circuit of claim 2, wherein the detector stage is implemented as a fourth order bandpass ΣΔ modulator. 4. The circuit of claim 2, wherein the detector stage is implemented as a second order lowpass ΣΔ modulator. 5. The circuit of claim 1, wherein at least one detector stage is implemented as a replica of one of the ΣΔ stages. 6. The circuit of claim 5, wherein the at least one detector stage is implemented with components having dimensions that are a fraction of the ΣΔ stage being replicated. 7. The circuit of claim 5, wherein the at least one detector stage is biased with a fraction of a bias current for the ΣΔ stage being replicated. 8. The circuit of claim 1, wherein the detected signal is indicative of an amplitude of the input signal. 9. The circuit of claim 1, wherein the one or more detector stages are further configured to receive a detector reference voltage, and wherein the detected signal is indicative of an amplitude of the input signal relative to the detector reference voltage. 10. The circuit of claim 1, wherein zero or more ΣΔ stages are disabled based, in part, on a detected amplitude of the input signal. 11. The circuit of claim 10, wherein a first ΣΔ stage is disabled if the detected amplitude falls below a first signal level. 12. The circuit of claim 11, wherein a second ΣΔ stage is disabled if the detected amplitude falls below a second signal level. 13. The circuit of claim 10, wherein all ∑Δ stages are enabled if the detected amplitude exceeds a third signal level. 14. The circuit of claim 10, wherein the ΣΔ stages are disabled based, in part, on relative locations of the ΣΔ stages within the ΣΔ ADC. 15. The circuit of claim 1, wherein the conditioning circuit includes a comparison circuit configured to receive the detected signal and a comparison signal, compare the detected and comparison signals, and provide the conditioned samples based on the results of the comparison. 16. The circuit of claim 15, wherein the comparison circuit is implemented using a switched capacitor circuit. 17. The circuit of claim 1, wherein the ΣΔ ADC includes two ΣΔ stages, each ΣΔ stage comprising a fourth order bandpass ΣΔ modulator. 18. The circuit of claim 1, wherein the ΣΔ ADC includes two ΣΔ stages, each ΣΔ stage comprising a second order lowpass ΣΔ modulator. 19. The circuit of claim 1, wherein the ΣΔ ADC is a double-sampling ΣΔ ADC. 20. The circuit of claim 1, wherein the ΣΔ ADC is a quadruple- sampling ΣΔ ADC. 21. The circuit of claim 1, wherein the control circuit further includes a reference generator configured to provide at least one reference signals. 22. A CDMA receiver comprising the data conversion circuit of claim 1. 23. A method for controlling ΣΔ stages in a sigma-delta analog-to- digital converter (ΣΔ ADC) comprising: detecting a characteristic of an input signal provided to the ΣΔ ADC with one or more detector stages, wherein at least one detector stage is implemented as a replica of one of the ΣΔ stages; comparing the detected characteristic against a comparison level; generating a control signal based, in part, on the comparing; and selectively disabling zero or more ΣΔ stages in accordance with the control signal. 24. The method of claim 23, wherein the characteristic being detected is a signal amplitude. 25. The method of claim 24, wherein the selectively disabling includes disabling a first ΣΔ stage if the detected signal amplitude falls below a first signal level. 26. The method of claim 25, wherein the selectively disabling further includes disabling a second ΣΔ stage if the detected signal amplitude falls below a second signal level. 27. The method of claim 24, wherein the selectively disabling includes enabling all ΣΔ stages if the detected signal amplitude exceeds a third signal level. 28. The method of claim 23, wherein the detecting includes receiving a detector reference level; and generating a detected signal based, in part, on the input signal and the detector signal level, wherein the detected signal is indicative of an amplitude of the input signal. 29. An electronic circuit comprising: a multi-stage circuit configured to receive an input signal and provide an output signal, the multi-stage circuit including a plurality of N signal stages coupled in a particular configuration; and a control circuit coupled to the multi-stage circuit and configured to provide a control signal that selectively disables zero or more of the N signal stages, the control circuit including one or more detector stages configured to receive the input signal and provide a detected signal, a conditioning circuit coupled to the one or more detector stages, the conditioning circuit configured to receive the detected signal and provide a conditioned signal, and a signal processor coupled to the conditioning circuit, the signal processor configured to receive the conditioned signal and provide the control signal. 30. The circuit of claim 29, wherein the detected signal is indicative of an amplitude of the input signal. 31. The circuit of claim 29, wherein zero or more signal stages are disabled based, in part, on a detected amplitude of the input signal. 32. The circuit of claim 29, wherein at least one detector stage is implemented as a replica of one of the signal stages. 33. The circuit of claim 32, wherein the at least one detector stage is implemented with components having dimensions that are a fraction of the signal stage being replicated. 34. The circuit of claim 32, wherein the at least one detector stage is biased with a fraction of a bias current for the signal stage being replicated. 35. The circuit of claim 32, wherein the at least one detector stage is configured to have a frequency response resembling that of the signal stage being replicated. 36. The circuit of claim 29, wherein the signal stages are coupled in cascade. 37. The circuit of claim 29, wherein the signal stages are coupled in parallel. 38. A control circuit for controlling a multi-stage circuit that includes a plurality of signal stages, the control circuit comprising: one or more detector stages configured to receive an input signal and provide a detected signal, wherein at least one detector stage is implemented as a replica of one of the signal stages; a conditioning circuit coupled to the one or more detector stages, the conditioning circuit configured to receive the detected signal and provide a conditioned signal; and a signal processor coupled to the conditioning circuit, the signal processor configured to receive the conditioned signal and provide the control signal, wherein the control signal selectively disables zero or more signal stages in the multi-stage circuit. 39. The circuit of claim 38, wherein the multi-stage circuit is a ΣΔ ADC.

Inventors: BAZARJANI SEYFOLLAH, PELUSO VINCENZO, WANG SEAN, BAZARJANI, SEYFOLLAH, PELUSO, VINCENZO, WANG, SEAN
Applicants: QUALCOMM INC, QUALCOMM INCORPORATED
Application year: 2001
Main classification: H03M3
IPC codes: H03M3