US20080143568A1 - Methods and apparatus for a multi-mode analog-to-digital converter - Google Patents
Methods and apparatus for a multi-mode analog-to-digital converter Download PDFInfo
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- 238000006243 chemical reaction Methods 0.000 claims description 7
- 239000004065 semiconductor Substances 0.000 claims description 4
- 239000000872 buffer Substances 0.000 description 7
- 238000007493 shaping process Methods 0.000 description 5
- 230000010354 integration Effects 0.000 description 4
- 238000012545 processing Methods 0.000 description 4
- 238000005070 sampling Methods 0.000 description 2
- 238000012546 transfer Methods 0.000 description 2
- 210000001520 comb Anatomy 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 230000007423 decrease Effects 0.000 description 1
- 230000003111 delayed effect Effects 0.000 description 1
- 238000013461 design Methods 0.000 description 1
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- 230000009977 dual effect Effects 0.000 description 1
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M3/00—Conversion of analogue values to or from differential modulation
- H03M3/30—Delta-sigma modulation
- H03M3/39—Structural details of delta-sigma modulators, e.g. incremental delta-sigma modulators
- H03M3/392—Arrangements for selecting among plural operation modes, e.g. for multi-standard operation
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M3/00—Conversion of analogue values to or from differential modulation
- H03M3/30—Delta-sigma modulation
- H03M3/39—Structural details of delta-sigma modulators, e.g. incremental delta-sigma modulators
- H03M3/412—Structural details of delta-sigma modulators, e.g. incremental delta-sigma modulators characterised by the number of quantisers and their type and resolution
- H03M3/422—Structural details of delta-sigma modulators, e.g. incremental delta-sigma modulators characterised by the number of quantisers and their type and resolution having one quantiser only
- H03M3/43—Structural details of delta-sigma modulators, e.g. incremental delta-sigma modulators characterised by the number of quantisers and their type and resolution having one quantiser only the quantiser being a single bit one
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M3/00—Conversion of analogue values to or from differential modulation
- H03M3/30—Delta-sigma modulation
- H03M3/39—Structural details of delta-sigma modulators, e.g. incremental delta-sigma modulators
- H03M3/436—Structural details of delta-sigma modulators, e.g. incremental delta-sigma modulators characterised by the order of the loop filter, e.g. error feedback type
- H03M3/438—Structural details of delta-sigma modulators, e.g. incremental delta-sigma modulators characterised by the order of the loop filter, e.g. error feedback type the modulator having a higher order loop filter in the feedforward path
- H03M3/452—Structural details of delta-sigma modulators, e.g. incremental delta-sigma modulators characterised by the order of the loop filter, e.g. error feedback type the modulator having a higher order loop filter in the feedforward path with weighted feedforward summation, i.e. with feedforward paths from more than one filter stage to the quantiser input
Definitions
- the subject matter described herein generally relates to analog-to-digital converters, and more particularly relates to analog-to-digital converters used for conversion of both AC and DC analog signals.
- both AC and DC signals are converted to digital form using dedicated analog-to-digital converters (ADC)—i.e., one ADC for AC signals, and another ADC for DC signals.
- ADC analog-to-digital converters
- the use of dedicated ADCs is necessitated by the very different nature of the signals themselves.
- AC signals such as audio signals or RF signals vary significantly over a short period of time, but are generally contained within a specified frequency range.
- the required performance of the ADC relates to signal-to-noise ratio, dynamic range, and the like, while offset and gain errors are tolerable.
- Over-sampled sigma-delta noise shaping ADCs are often used in this type of application.
- DC signals in contrast, do not vary significantly over a short period of time, and include such signals as battery voltage, charging current, digital-to-analog converter (DAC) output voltage under calibration, and the like.
- DAC digital-to-analog converter
- Low speed Nyquist rate ADCs SAR, dual slope, etc.
- over-sampled incremental ADCs are often used.
- Incremental ADCs provide precision DC measurement (i.e., greater than 16 bit), but the standard sigma-delta noise-shaping ADC can not be directly used as an incremental ADC.
- FIG. 1 is an analog-to-digital converter (ADC) circuit in accordance with one embodiment
- FIG. 2 is an exemplary filter circuit used in conjunction with the circuit shown in FIG. 1 ;
- FIG. 3 is waveform depicting operation of the combined circuits of FIGS. 1 and 2 in a incremental mode.
- a multi-mode analog-to-digital converter in accordance with various embodiments is configured to be selectively used as a standard noise-shaping ADC for AC signals, or as an incremental ADC for DC signals, thus combining both functions in a single circuit and saving power and semiconductor component space.
- a multi-mode ADC in accordance with one embodiment includes two coupled circuits: the ADC circuit 100 of FIG. 1 , and the filter circuit 200 of FIG. 2 , both of which will now be described.
- FIG. 1 presents an exemplary ADC circuit (or simply “circuit”) 100 in accordance with one embodiment.
- circuit 100 receives an analog input signal V in 102 , and produces a digital output d k 140 responsive to V in , which may be an AC or DC signal.
- V in analog input signal
- d k 140 digital output responsive to V in
- clock signals, control lines, and reference signals are not illustrated in this figure.
- Circuit 100 falls within the general class of sigma-delta modulators in that it evaluates the input signal 102 , measures the error (via summer 106 ), integrates the error (via integrators 108 , 114 , and 124 ), then compensates for that error to produce an output ( 140 ).
- the output d k 140 is a bit stream (i.e., a one-bit serial signal) whose mean output converges to the value of input 102 .
- the input 102 is oversampled to reduce the noise in the band of interest, while noise at high frequencies (e.g., quantization noise) is filtered as described in further detail below.
- noise shaping modulators.
- Circuit 100 includes three integrators—integrators 108 , 114 , and 124 (annotated in Z-transform notation)—configured in a feed-forward manner and leading to summer 136 .
- Each integrator 108 , 114 , and 124 has an associated reset signal 110 , 116 , and 126 . These reset signals are applied (e.g., by external control circuitry, not illustrated) to force the integrator to an initial state (e.g., a zero state). While a third-order CIFF circuit is illustrated, other configurations may also be used.
- Input voltage 102 is coupled to a buffer 104 (e.g., buffered op-amp, or the like), having a buffer coefficient b, which is coupled to summing node (or “summer”) 106 .
- the output of summer 106 is applied to integrator 108 .
- the output of integrator 108 is coupled to buffer 112 (having a coefficient c 1 ), which is coupled to integrator 114 .
- the output of integrator 114 is coupled to buffer 122 (having a coefficient c 2 ), which in turn is coupled to the final integrator 124 .
- the outputs of integrators 108 , 114 , and 124 are coupled to buffers 134 , 132 , and 130 (having coefficients a 1 , a 2 , and a 3 ), the outputs of which are summed in summer 136 along with input voltage 102 .
- the use of weighted feedforward paths in this way helps to provide system stability. Values for b, c 1 , c 2 , a 1 , a 2 , a 3 may be selected to achieve the desired stability and system performance, given a particular clock frequency.
- These coefficient values are typically derived based on the targeted noise transfer and signal transfer functions, and are a well known aspect of delta-sigma modulator design.
- the output of summer 136 is coupled to one input of comparator 138 , which produces a bit stream at output 140 (d k ).
- a feedback loop including digital-to-analog converter (DAC) 142 and buffer 144 (having coefficient that is opposite in sign but equal in magnitude of that of buffer 104 ) is coupled between output 140 and summer 106 .
- DAC digital-to-analog converter
- buffer 144 having coefficient that is opposite in sign but equal in magnitude of that of buffer 104
- the sum of this bit stream (where a digital “1” corresponds to a positive reference voltage, and a digital “0” corresponds to a negative reference voltage) converges to a mean value corresponding to the voltage input 102 .
- the relationship between input 102 and output 140 (dk) is given by:
- n is the number of clock cycles
- Vref is a suitable reference voltage
- circuit (or “filter”) 200 is a 4 th -order cascaded integrator-comb (CIC) filter configured to generate the desired digital value (output 202 ) corresponding to analog input voltage 102 .
- CIC filter is an implementation of a narrowband low-pass filter and is often used for decimation, anti-aliasing, and/or interpolation. It generally operates by adding a delayed version of the signal to itself, causing a form of constructive and destructive interference. The resulting frequency response exhibits a comb-like series of regularly-spaced spikes.
- Such filters are often used in applications having a large excess sample rate—i.e., applications where the sample rate is much larger than the bandwidth occupied by the signal.
- Circuit 200 illustrated in FIG. 2 implements a decimation filter, and generally includes a series of summers 204 , a series of ideal integrators (or “filter integrators”) 210 , a series of ideal differentiators (or “combs”) 211 , and a decimation operator 213 positioned therebetween.
- Each integration stage (i.e., combination of summer 204 and integrator 210 ) of circuit 200 is a one-pole filter with a unity feedback coefficient, and operates at a predetermined clock frequency.
- Each bit of bitstream d k 140 progress through integrators 210 and summers 204 in the conventional manner, while decimation operator 213 functions to discard all by the Rth sample received from output 140 , resulting in a reduced sampling rate.
- the value of R used in a particular application will depend on a number of factors, including sampling rate, etc., as is known in the art.
- Differentiators 211 function in a manner that is opposite that of integrators 210 , but operate on the reduced bit stream produced by decimation operator 213 .
- circuit 200 may be used in one of two modes: as a standard noise-shaping ADC for processing AC signals (“Standard” or “AC” mode), and as an incremental ADC for processing DC signals (“Incremental” or “DC” mode).
- circuit 200 has two outputs: output 202 , which is used in the AC mode, and output 212 , which is used in the DC mode as described in further detail below.
- each integrator 108 is reset via application of reset signals 110 , 116 , and 126 .
- reset signals may be received, for example, from a controller or other microprocessor (now shown).
- a fixed number of integration steps (clock cycles) n are then performed, depending upon the desired accuracy. This number, which may be configurable, varies depending upon the application and desired accuracy, and may range from two to any suitably large number of clock cycles.
- the analog DC input signal 102 is incremented through integrators 108 , 114 , and 124 , and a bit stream is produced at output 140 as described above.
- the average value of this bit stream converges to the magnitude of input signal 102 .
- the output of the final integrator 124 is frozen after the desired number of cycles (e.g., by stopping the clock), and the output 212 of filter 200 is used to determine the correct digital representation of input signal 102 .
- output 212 is positioned after a subset of integrators 210 —in this case, three such integrators. Because the circuit shown in FIG. 1 is 3 rd order, it desirable to use only the first three integrators 210 of CIC filter 200 to properly filter the signal. Furthermore, by doing so, the conversion time in incremental mode is reduced. Thus, with reference to the equation for V in shown above, the triple integration in this equation is performed by the first three integrators 210 in circuit 200 . Since the number of clock cycles, n, is known, the digital representation of V in is also known. Because signal-to-noise ratio and other AC-related metrics are of little importance to DC signals, there is little disadvantage in using only three of the integrators 210 to derive the digital signal. At the same time, offset and gain errors are reduced.
- output 202 of filter 200 is used to determine the correct digital representation of input signal 102 .
- the resulting output 202 has a higher signal-to-noise ratio and better dynamic range than that of output 212 .
- Output 202 is more susceptible to gain and offset errors, which are not as important in an AC context as they are in the DC context.
- the combined ADC is used in normal mode to measure a 1 KHz-500 KHz GSM/WCDMA AC signal, and is used in incremental mode to measure a calibration signal for a 14 bit DAC.
- the range of embodiments is not so limited, however, and may be used in a variety of contexts.
- FIG. 3 plots example operation of circuits 100 and 200 in incremental ADC mode.
- the output 212 , an ADC RESET signal 302 , and an ADC end-of-convert (EOC) signal 304 are simultaneously plotted.
- ADC RESET signal 302 is characterized by a leading edge 308 and trailing edge 310 , and corresponds to the application of reset signals 110 , 116 , and 126 in FIG. 1 .
- ADC EOC signal 304 is characterized by a leading edge 312 , and trailing edge 314 , where trailing edge 314 generally corresponds to trailing edge 310 .
- the digital filter and ADC circuitry are combined in a single semiconductor device, thus reducing the chip area normally required for dedicated ADCs. At the same time, power consumption is reduced.
- a multi-mode analog-to-digital converter configured to receive an analog input and produce a digital output corresponding to the analog input, comprises: a delta-sigma analog-to-digital converter circuit configured to receive the analog input and produce a digital bit-stream associated therewith, the delta-sigma analog-to-digital converter circuit including at least one integrator configured to reset to an initial state in response to a reset signal; and a digital filter circuit configured to receive the digital bit-stream, the digital filter circuit configured to produce a first filtered output and a second filtered output derived from the digital bit-stream; wherein, during a first mode, the delta-sigma analog-to-digital converter circuit is configured to receive the reset signal and produce the digital bit-stream for a predetermined number of clock cycles, and wherein the digital output corresponds to the first filtered output; and wherein, during a second mode, the delta-sigma analog-to-digital converter circuit is configured to continuously produce the digital bit-stream, and wherein the digital output corresponds to the second filtered
- the delta-sigma analog-to-digital converter circuit comprises a cascade-of-integrators feed-forward (CIFF) analog-to-digital converter.
- the CIFF is a third-order CIFF.
- the digital filter circuit may comprise a cascaded-integrator-comb (CIC) filter.
- the CIC filter may include a series of integrators, wherein the first filtered output corresponds to the end of a subset of the series of integrators.
- the analog-to-digital converter is an n-th order converter, and the subset of the series of integrators includes n integrators.
- a method of converting an analog input to a digital output includes: providing a delta-sigma analog-to-digital converter circuit configured to receive the analog input and produce a digital bit-stream associated therewith, the delta-sigma analog-to-digital converter circuit including at least one integrator configured to reset to an initial state in response to a reset signal; providing a digital filter circuit configured to receive the digital bit-stream, the digital filter circuit configured to produce a first filtered output and a second filtered output derived from the digital bit-stream; selecting a mode of operation, wherein the mode of operation is selected from the group consisting of an AC mode and a DC mode; if the DC mode is selected: applying the reset signal, instructing the delta-sigma converter circuit to produce the digital bit-stream for a predetermined number of clock cycles, and deriving the digital output from the first filtered output; if the AC mode is selected: producing the digital bit-stream continuously and deriving the digital output from the second filtered output.
- providing the delta-sigma analog-to-digital converter circuit includes providing a cascade-of-integrators feed-forward (CIFF) analog-to-digital converter, and wherein instructing the delta-sigma converter circuit includes instructing the CIFF analog-to-digital converter to produce the digital bit-stream for a predetermined number of clock cycles.
- providing the delta-sigma analog-to-digital converter circuit includes providing a third-order CIFF.
- the second filtered output corresponds to the end of a series of integrators, and wherein first filtered output corresponds to the end of a subset of the series of integrators.
- the delta-sigma analog-to-digital converter circuit is a third-order converter, and wherein the subset of the series of integrators includes three integrators in the series of integrators.
- a multi-mode analog-to-digital converter configured to receive an analog input and produce a digital output corresponding to the analog input includes: a cascade-of-integrators feed-forward (CIFF) analog-to-digital converter circuit configured to receive the analog input and produce a digital bit-stream associated therewith, the CIFF analog-to-digital converter circuit including a plurality of integrators, each configured to reset to an initial state in response to a corresponding reset signal; and a cascaded-integrator comb (CIC) filter circuit configured to receive the digital bit-stream, produce a first filtered output after a first number of filter integrators, and produce a second filtered output after a second number of filter integrators; wherein the CIFF analog-to-digital converter circuit and the CIC filter circuit are configured to selectively operate in an incremental mode for conversion of DC signals, and a normal mode for conversion of AC signals; wherein, during the incremental mode, the CIFF analog-to-digital converter circuit is configured to receive the corresponding reset signals and produce the digital bit
- the CIFF analog-to-digital converter circuit includes a third-order CIFF.
- the first number of filter integrators comprises three filter integrators, and the second number of filter integrators comprises eight filter integrators.
- the predetermined number of clock cycles may be configurable by the controller.
- the CIFF analog-to-digital converter and the CIC filter circuit may be integrated into a single semiconductor device.
- the clock may terminate after the predetermined number of clock cycles in response to an end-of-convert signal.
Abstract
Description
- The subject matter described herein generally relates to analog-to-digital converters, and more particularly relates to analog-to-digital converters used for conversion of both AC and DC analog signals.
- In many integrated circuits applications, both AC and DC signals are converted to digital form using dedicated analog-to-digital converters (ADC)—i.e., one ADC for AC signals, and another ADC for DC signals. The use of dedicated ADCs is necessitated by the very different nature of the signals themselves. AC signals such as audio signals or RF signals vary significantly over a short period of time, but are generally contained within a specified frequency range. In such applications, the required performance of the ADC relates to signal-to-noise ratio, dynamic range, and the like, while offset and gain errors are tolerable. Over-sampled sigma-delta noise shaping ADCs are often used in this type of application.
- DC signals, in contrast, do not vary significantly over a short period of time, and include such signals as battery voltage, charging current, digital-to-analog converter (DAC) output voltage under calibration, and the like. In such applications, where low offset and gain error are desired, low speed Nyquist rate ADCs (SAR, dual slope, etc.) or over-sampled incremental ADCs are often used. Incremental ADCs provide precision DC measurement (i.e., greater than 16 bit), but the standard sigma-delta noise-shaping ADC can not be directly used as an incremental ADC.
- Accordingly, it is desirable to provide an ADC circuit that can be used for processing both AC and DC signals. Other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.
- A more complete understanding of the various embodiments may be derived by referring to the detailed description and claims when considered in conjunction with the following figures, wherein like reference numbers refer to similar elements throughout the figures.
-
FIG. 1 is an analog-to-digital converter (ADC) circuit in accordance with one embodiment; -
FIG. 2 is an exemplary filter circuit used in conjunction with the circuit shown inFIG. 1 ; and -
FIG. 3 is waveform depicting operation of the combined circuits ofFIGS. 1 and 2 in a incremental mode. - The following detailed description is merely illustrative in nature and is not intended to limit the scope or application of possible embodiments. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.
- Various embodiments may be described herein in terms of functional and/or logical block components and various processing steps. It should be appreciated that such block components may be realized by any number of hardware, software, and/or firmware components configured to perform the specified functions. For the sake of brevity, conventional techniques and components related to analog and digital circuits, filters, and the like are not described in detail.
- The terms “first,” “second,” “third,” “fourth” and the like in the description and the claims, if any, may be used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. These terms, so used, are interchangeable under appropriate circumstances. The embodiments of the invention described herein are, for example, capable of use in sequences other than those illustrated or otherwise described herein.
- The terms “comprise,” “include,” “have” and any variations thereof are used synonymously to denote non-exclusive inclusion. The terms “left,” right,” “in,” “out,” “front,” “back,” “up,” “down,” and other such directional terms are used to describe relative positions, not necessarily absolute positions in space. The term “exemplary” is used in the sense of “example,” rather than “ideal.”
- In general, a multi-mode analog-to-digital converter (ADC) in accordance with various embodiments is configured to be selectively used as a standard noise-shaping ADC for AC signals, or as an incremental ADC for DC signals, thus combining both functions in a single circuit and saving power and semiconductor component space. A multi-mode ADC in accordance with one embodiment includes two coupled circuits: the
ADC circuit 100 ofFIG. 1 , and thefilter circuit 200 ofFIG. 2 , both of which will now be described. -
FIG. 1 presents an exemplary ADC circuit (or simply “circuit”) 100 in accordance with one embodiment. In general,circuit 100 receives an analoginput signal V in 102, and produces adigital output d k 140 responsive to Vin, which may be an AC or DC signal. For the purposes of clarity, clock signals, control lines, and reference signals (e.g., for DAC 142) are not illustrated in this figure. -
Circuit 100 falls within the general class of sigma-delta modulators in that it evaluates theinput signal 102, measures the error (via summer 106), integrates the error (viaintegrators output d k 140 is a bit stream (i.e., a one-bit serial signal) whose mean output converges to the value ofinput 102. Theinput 102 is oversampled to reduce the noise in the band of interest, while noise at high frequencies (e.g., quantization noise) is filtered as described in further detail below. Thus, such sigma-delta modulators are referred to as “noise shaping” modulators. - Unlike conventional Nyquist rate ADCs, sigma-delta converters generate a stream of digital samples whose pulse density is a measure of the analog input. For example, in an application with a reference voltage of 2.5 V, where a digital “1” represents 2.5V and a “0” represents −2.5V, a bit stream dk of 10111 represents an average voltage of (2.5−2.5+2.5+2.5+2.5)/5=1.5V.
- The specific topology shown in
FIG. 1 is a third-order cascade-of-integrators feed-forward (CIFF) ADC, also referred to as an “incremental” ADC.Circuit 100 includes three integrators—integrators summer 136. Eachintegrator reset signal -
Input voltage 102 is coupled to a buffer 104 (e.g., buffered op-amp, or the like), having a buffer coefficient b, which is coupled to summing node (or “summer”) 106. The output ofsummer 106 is applied tointegrator 108. The output ofintegrator 108 is coupled to buffer 112 (having a coefficient c1), which is coupled tointegrator 114. The output ofintegrator 114 is coupled to buffer 122 (having a coefficient c2), which in turn is coupled to thefinal integrator 124. - The outputs of
integrators buffers summer 136 along withinput voltage 102. The use of weighted feedforward paths in this way helps to provide system stability. Values for b, c1, c2, a1, a2, a3 may be selected to achieve the desired stability and system performance, given a particular clock frequency. These coefficient values are typically derived based on the targeted noise transfer and signal transfer functions, and are a well known aspect of delta-sigma modulator design. - The output of
summer 136 is coupled to one input ofcomparator 138, which produces a bit stream at output 140 (dk). A feedback loop including digital-to-analog converter (DAC) 142 and buffer 144 (having coefficient that is opposite in sign but equal in magnitude of that of buffer 104) is coupled betweenoutput 140 andsummer 106. As with other forms of delta-sigma modulators, the sum of this bit stream (where a digital “1” corresponds to a positive reference voltage, and a digital “0” corresponds to a negative reference voltage) converges to a mean value corresponding to thevoltage input 102. The relationship betweeninput 102 and output 140 (dk) is given by: -
- where n is the number of clock cycles, and Vref is a suitable reference voltage.
- The output of
comparator 138 is applied to a digital filter such as that shown inFIG. 2 . The purpose of this filter is to downsample the bitstream received from comparator while filtering out various forms of noise. In this embodiment, circuit (or “filter”) 200 is a 4th-order cascaded integrator-comb (CIC) filter configured to generate the desired digital value (output 202) corresponding toanalog input voltage 102. As is known in the art, a CIC filter is an implementation of a narrowband low-pass filter and is often used for decimation, anti-aliasing, and/or interpolation. It generally operates by adding a delayed version of the signal to itself, causing a form of constructive and destructive interference. The resulting frequency response exhibits a comb-like series of regularly-spaced spikes. Such filters are often used in applications having a large excess sample rate—i.e., applications where the sample rate is much larger than the bandwidth occupied by the signal. -
Circuit 200 illustrated inFIG. 2 implements a decimation filter, and generally includes a series ofsummers 204, a series of ideal integrators (or “filter integrators”) 210, a series of ideal differentiators (or “combs”) 211, and adecimation operator 213 positioned therebetween. Each integration stage (i.e., combination ofsummer 204 and integrator 210) ofcircuit 200 is a one-pole filter with a unity feedback coefficient, and operates at a predetermined clock frequency. - Each bit of
bitstream d k 140 progress throughintegrators 210 andsummers 204 in the conventional manner, whiledecimation operator 213 functions to discard all by the Rth sample received fromoutput 140, resulting in a reduced sampling rate. The value of R used in a particular application will depend on a number of factors, including sampling rate, etc., as is known in the art. Differentiators 211 function in a manner that is opposite that ofintegrators 210, but operate on the reduced bit stream produced bydecimation operator 213. - As mentioned above, the illustrated embodiment may be used in one of two modes: as a standard noise-shaping ADC for processing AC signals (“Standard” or “AC” mode), and as an incremental ADC for processing DC signals (“Incremental” or “DC” mode). Thus, unlike conventional CIC filters, which are used to convert AC signals,
circuit 200 has two outputs:output 202, which is used in the AC mode, andoutput 212, which is used in the DC mode as described in further detail below. - Referring to
FIGS. 1 and 2 , in order to utilizecircuits integrator 108 is reset via application of reset signals 110, 116, and 126. These reset signals may be received, for example, from a controller or other microprocessor (now shown). A fixed number of integration steps (clock cycles) n are then performed, depending upon the desired accuracy. This number, which may be configurable, varies depending upon the application and desired accuracy, and may range from two to any suitably large number of clock cycles. - As the integration steps proceed, the analog
DC input signal 102 is incremented throughintegrators output 140 as described above. The average value of this bit stream converges to the magnitude ofinput signal 102. The output of thefinal integrator 124 is frozen after the desired number of cycles (e.g., by stopping the clock), and theoutput 212 offilter 200 is used to determine the correct digital representation ofinput signal 102. - As shown in
FIG. 2 ,output 212 is positioned after a subset ofintegrators 210—in this case, three such integrators. Because the circuit shown inFIG. 1 is 3rd order, it desirable to use only the first threeintegrators 210 ofCIC filter 200 to properly filter the signal. Furthermore, by doing so, the conversion time in incremental mode is reduced. Thus, with reference to the equation for Vin shown above, the triple integration in this equation is performed by the first threeintegrators 210 incircuit 200. Since the number of clock cycles, n, is known, the digital representation of Vin is also known. Because signal-to-noise ratio and other AC-related metrics are of little importance to DC signals, there is little disadvantage in using only three of theintegrators 210 to derive the digital signal. At the same time, offset and gain errors are reduced. - In order to utilize
circuits integrators circuit 100 are allowed to run continuously over time. At the same time,output 202 offilter 200 is used to determine the correct digital representation ofinput signal 102. As is desirable when converting AC signals, the resultingoutput 202 has a higher signal-to-noise ratio and better dynamic range than that ofoutput 212.Output 202 is more susceptible to gain and offset errors, which are not as important in an AC context as they are in the DC context. - During normal operation, it will often be desirable to use the combined ADC in standard mode to measure AC signals for a period of time, then interrupt its operation to use it in incremental mode to process high-precision DC samples. In one embodiment, for example, the combined ADC is used in normal mode to measure a 1 KHz-500 KHz GSM/WCDMA AC signal, and is used in incremental mode to measure a calibration signal for a 14 bit DAC. The range of embodiments is not so limited, however, and may be used in a variety of contexts.
-
FIG. 3 plots example operation ofcircuits output 212, anADC RESET signal 302, and an ADC end-of-convert (EOC) signal 304 are simultaneously plotted.ADC RESET signal 302 is characterized by aleading edge 308 and trailingedge 310, and corresponds to the application of reset signals 110, 116, and 126 inFIG. 1 .ADC EOC signal 304 is characterized by aleading edge 312, and trailingedge 314, where trailingedge 314 generally corresponds to trailingedge 310. - In this example plot, it is assumed that an ADC RESET has occurred, and that a positive DC voltage has been applied to
input 102. As shown, theoutput 212 gradually rises from an initial value (in this illustration, zero) to a positive value inregion 316 as the number of clock cycles increase. This positive value converges to theanalog input 102 ofFIG. 1 . An ADC EOC signal pulse is then applied, at which time the clock is slopped, and the value ofoutput 212 is read within region 318 (where it remains constant). Next, an ADC RESET pulse is applied, and, in this example, a negative DC voltage is applied toinput 102. Theoutput 212 resets (region 320), then decreases inregion 322 until it is frozen to a final value inregion 324 upon application of a subsequent ADC EOC pulse. - Thus, what has been disclosed is a multi-mode analog-to-digital converter that can efficiently process both AC and DC input signals. In accordance with one embodiment, the digital filter and ADC circuitry are combined in a single semiconductor device, thus reducing the chip area normally required for dedicated ADCs. At the same time, power consumption is reduced.
- In one embodiment, a multi-mode analog-to-digital converter configured to receive an analog input and produce a digital output corresponding to the analog input, comprises: a delta-sigma analog-to-digital converter circuit configured to receive the analog input and produce a digital bit-stream associated therewith, the delta-sigma analog-to-digital converter circuit including at least one integrator configured to reset to an initial state in response to a reset signal; and a digital filter circuit configured to receive the digital bit-stream, the digital filter circuit configured to produce a first filtered output and a second filtered output derived from the digital bit-stream; wherein, during a first mode, the delta-sigma analog-to-digital converter circuit is configured to receive the reset signal and produce the digital bit-stream for a predetermined number of clock cycles, and wherein the digital output corresponds to the first filtered output; and wherein, during a second mode, the delta-sigma analog-to-digital converter circuit is configured to continuously produce the digital bit-stream, and wherein the digital output corresponds to the second filtered output. In one embodiment, the first mode is configured to convert a DC signal, and the second mode is configured to convert an AC signal.
- In one embodiment, the delta-sigma analog-to-digital converter circuit comprises a cascade-of-integrators feed-forward (CIFF) analog-to-digital converter. In a particular embodiment, the CIFF is a third-order CIFF. The digital filter circuit may comprise a cascaded-integrator-comb (CIC) filter. The CIC filter may include a series of integrators, wherein the first filtered output corresponds to the end of a subset of the series of integrators. In one embodiment, the analog-to-digital converter is an n-th order converter, and the subset of the series of integrators includes n integrators.
- A method of converting an analog input to a digital output includes: providing a delta-sigma analog-to-digital converter circuit configured to receive the analog input and produce a digital bit-stream associated therewith, the delta-sigma analog-to-digital converter circuit including at least one integrator configured to reset to an initial state in response to a reset signal; providing a digital filter circuit configured to receive the digital bit-stream, the digital filter circuit configured to produce a first filtered output and a second filtered output derived from the digital bit-stream; selecting a mode of operation, wherein the mode of operation is selected from the group consisting of an AC mode and a DC mode; if the DC mode is selected: applying the reset signal, instructing the delta-sigma converter circuit to produce the digital bit-stream for a predetermined number of clock cycles, and deriving the digital output from the first filtered output; if the AC mode is selected: producing the digital bit-stream continuously and deriving the digital output from the second filtered output.
- In one embodiment, providing the delta-sigma analog-to-digital converter circuit includes providing a cascade-of-integrators feed-forward (CIFF) analog-to-digital converter, and wherein instructing the delta-sigma converter circuit includes instructing the CIFF analog-to-digital converter to produce the digital bit-stream for a predetermined number of clock cycles. In another, providing the delta-sigma analog-to-digital converter circuit includes providing a third-order CIFF. In one embodiment, the second filtered output corresponds to the end of a series of integrators, and wherein first filtered output corresponds to the end of a subset of the series of integrators. In another embodiment, the delta-sigma analog-to-digital converter circuit is a third-order converter, and wherein the subset of the series of integrators includes three integrators in the series of integrators.
- A multi-mode analog-to-digital converter configured to receive an analog input and produce a digital output corresponding to the analog input includes: a cascade-of-integrators feed-forward (CIFF) analog-to-digital converter circuit configured to receive the analog input and produce a digital bit-stream associated therewith, the CIFF analog-to-digital converter circuit including a plurality of integrators, each configured to reset to an initial state in response to a corresponding reset signal; and a cascaded-integrator comb (CIC) filter circuit configured to receive the digital bit-stream, produce a first filtered output after a first number of filter integrators, and produce a second filtered output after a second number of filter integrators; wherein the CIFF analog-to-digital converter circuit and the CIC filter circuit are configured to selectively operate in an incremental mode for conversion of DC signals, and a normal mode for conversion of AC signals; wherein, during the incremental mode, the CIFF analog-to-digital converter circuit is configured to receive the corresponding reset signals and produce the digital bit-stream for a predetermined number of clock cycles, and wherein the digital output corresponds to the first filtered output of the CIC filter circuit; and wherein, during the normal mode, the CIFF analog-to-digital converter circuit is configured to continuously produce the digital bit-stream, and wherein the digital output corresponds to the second filtered output of the CIC filter circuit. In one embodiment, the CIFF analog-to-digital converter circuit includes a third-order CIFF. In a particular embodiment, the first number of filter integrators comprises three filter integrators, and the second number of filter integrators comprises eight filter integrators. The predetermined number of clock cycles may be configurable by the controller. The CIFF analog-to-digital converter and the CIC filter circuit may be integrated into a single semiconductor device. The clock may terminate after the predetermined number of clock cycles in response to an end-of-convert signal.
- While at least one example embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the example embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the various embodiments in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope set forth in the appended claims and the legal equivalents thereof.
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US11/639,676 US7379002B1 (en) | 2006-12-15 | 2006-12-15 | Methods and apparatus for a multi-mode analog-to-digital converter |
PCT/US2007/082585 WO2008076519A2 (en) | 2006-12-15 | 2007-10-26 | Methods and apparatus for a multi-mode analog-to-digital converter |
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