Abstract:
A highly efficient analog-to-digital (A/D) converter circuit that converts an external analog signal sequentially generated from an external analog signal source into an n-bit digital data signal (n is an integer equal to or more than two) includes a digital-to-analog (D/A) converter circuit that converts an n-bit digital data signal into an analog signal and outputting the analog signal from a first output terminal, a comparator that compares a signal level of an external analog signal supplied from an external device with a signal level of the analog signal outputted from the first output terminal, and a digital integrator circuit that digitally integrates a 1-bit digital data signal outputted from the comparator and thereby producing an n-bit digital data signal.

Description:
[0001]    This application is based on Japanese Patent Application 2000-160896, filed on May 30, all of the content of which is incorporated in this application by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an analog-to-digital (A/D) converter circuit, and in particular, to a highly efficient A/D converter circuit suitable to process audio signals.  
           [0004]    2. Description of the Related Art  
           [0005]    In the prior art, the highly efficient A/D converter circuits of this kind include an A/D converter of sequential comparison type and over-sampling-type A/D converters such as an A/D converter of Δ modification and an A/D converter of Δ Σ modification. FIG. 4 shows an A/D converter of sequential comparison type. The A/D converter includes a sample-and-hold (S/H) circuit  40 , a digital-to-analog (D/A) converter circuit (DAC)  42 , a sequential-comparison register  44 , and a comparator  46  to compare an output signal from the sample-and-hold circuit  40  and an analog output signal from the D/A converter  42 .  
           [0006]    In the A/D converter circuit of sequential comparison type, an analog signal is inputted via an input terminal  100  to be held by the S/H circuit  40 . On the other hand, an output signal from the sequential-comparison register  44  is fed to the DAC  42  to set its most-significant bit (MSB) to one, i.e., MSB=1. Thereafter, the comparator  46  compares the output signal from the DAC  42  with that from the S/H circuit  40 . If the output signal from the DAC  42  is larger than that from the S/H circuit  40 , the MSB is fixed to one (MSB=1). If the output signal from the DAC  42  is smaller than that from the S/H circuit  40 , the MSB is fixed to zero (MSB=0). Resultantly, the first bit of the output signal from DAC  42  is determined. The comparator  46  continuously and repeatedly conducts the comparing operation for the output from the DAC  42  up to its least-significant bit (LSB). When the output signal from the S/H circuit  40  equals to that from the DAC  42 , the digital output from the sequential-comparison register  44  operating in association with the DAC  42  is determined as the output from the A/D converter of sequential comparison type.  
           [0007]    [0007]FIG. 5 shows, as a highly efficient A/D converter circuit of over-sampling type, constitution of an A/D converter of Δ modification of the prior art. The A/D converter circuit of FIG. 5 includes a comparator  50  which compares a reference voltage (a ground voltage in FIG. 5) with an output voltage from an adder circuit  52  to produce 1-bit digital data according to a result of the comparison, a D/A converter circuit  54  to receive the 1-bit digital data from the comparator  50 , an analog integrator circuit  56  to integrate an analog output from the DAC  54 , an inverter circuit  57  to invert a result of the integration from the analog integrator  56 , and an adder circuit  52  to add an analog voltage signal inputted from an input terminal  101  to an inverted output signal from the inverter  57 .  
           [0008]    The A/D converter of Δ modification outputs 1-bit digital data of “1” or “0” depending on a result of comparison in the comparator  50 , namely, depending on whether or not the reference voltage is higher than the output signal from the adder  52 . The D/A converter  54  converts the 1-bit digital data into an analog signal. The analog integrator  56  integrates the analog signal and sends the integrated signal to the inverter  57 . The inverter  57  inverts the integrated signal to produce an inverted signal. The adder  52  adds the inverted signal to the analog voltage signal inputted from the input terminal  101 . The comparator  50  compares the sum signal resultant from the addition with the reference voltage. Until the analog input voltage from the input terminal  101  matches in a signal level with the output signal from the analog integrator  56 , the comparator  50  repeatedly conducts the comparing operation. The 1-bit digital data sequentially outputted from the comparator  50  constitutes the converted output signal from the A/D converter of Δ modification.  
           [0009]    [0009]FIG. 6 shows, as a highly efficient A/D converter circuit of over-sampling type, constitution of an A/D converter of Δ Σ modification of the prior art. The A/D converter circuit of FIG. 6 includes a comparator  60  which compares a reference voltage (a ground voltage in FIG. 6) with an output voltage from an analog integrator circuit  66  to produce 1-bit digital data according to a result of the comparison, a DAC circuit  62  to receive the 1-bit digital data from the comparator  60  to convert the data into analog data, an inverter circuit  63  to invert an analog output from the DAC  62 , an adder circuit  64  to add an analog voltage inputted from an input terminal  103  to an inverted output from the inverter  63 , and an analog integrator circuit  66  to integrate an output signal from the adder  64 .  
           [0010]    In the A/D converter of Δ Σ modification, the adder  64  adds the analog voltage inputted from the input terminal  103  to the inverted output from the inverter  63 . The analog integrator  66  integrates a sum signal resultant from the addition. The comparator  60  compares the integrated output signal from the analog integrator  66  with the reference voltage signal. According to a result of the comparison, 1-bit digital data is outputted to an output terminal  104  and the DAC  62 .  
           [0011]    The DAC  62  converts the 1-bit digital data into an analog voltage. The inverter  63  inverts the integrated analog voltage to obtain an inverted signal. The adder  64  adds the inverted output signal to the analog voltage signal inputted from the input terminal  103 . A signal resultant from the addition is inputted to the analog integrator  66 .  
           [0012]    Until the reference voltage matches in a signal level with the output signal from the analog integrator  66 , the comparator  60  repeatedly conducts the comparing operation. The 1-bit digital data sequentially outputted from the comparator  60  constitutes the converted output from the A/D converter of Δ Σ modification.  
           [0013]    The A/D converter of sequential comparison required a sample-and-hold circuit. When such an A/D converter operates, for example, with a sampling frequency of 192 KHz to produce a 24-bit digital signal. It is necessary to conduct the sample-and-hold operation within {fraction (1/192)}×10 3 =5.2 microseconds (μs) and 24 sequential comparing operations within the same period of time. That is, there is required an A/D converter which produces a 24-bit output signal and which has a settling time of about 100 nanoseconds (ns). Such a circuit cannot be easily constructed.  
           [0014]    Additionally, in the A/D converter of sequential comparison, the comparator compares the input analog voltage with the voltage about a half of the full-scale voltage of the D/A converter (the voltage value indicated by a bit corresponding to the MSB of the D/A converter) beginning at the MSB. Therefore, when resolution is increased, a monotonous feature is lost in the neighborhood of “full-scale-voltage/2 (center potential)” or a code loss easily occurs. This consequently results in a problem that a highly efficient A/D converter of sequential comparison cannot be easily produced.  
           [0015]    The over-sampling-type A/D converter such as an A/D converter of Δ modification or an A/D converter of Δ Σ modification uses an analog integrator circuit. Therefore, to obtain a high signal-to-noise (S/N) ratio, large capacitor is required in the large-scale integrated (LSI) circuit of the converter. This leads to a problem. That is, to form such large capacitor, the LSI circuit becomes great in size. Furthermore, to improve efficiency of the converter, there arises a problem that an over-sampling clock signal with a high frequency is required.  
           [0016]    Additionally, in an A/D converter of Δ modification or an A/D converter of Δ Σ modification, it is difficult to obtain a dynamic range or a signal-to-noise ratio equal to or more than 120 dB. Since an analog integrator circuit is employed as an integrator circuit in the A/D converter circuit, it is difficult to conduct a multi-channel A/D conversion using time division.  
           [0017]    Although recent high-performance audio apparatuses require 24-bit A/D conversion, the A/D converter circuits of the prior art cannot conduct A/D conversion with a precision of 24 bits.  
         SUMMARY OF THE INVENTION  
         [0018]    It is therefore an object of the present invention to provide a high-performance A/D converter circuit for audio apparatuses to thereby remove the difficulties above. 1.  
           [0019]    To achieve the object according to the present invention, there is provided, an analog-to-digital (A/D) converter circuit that converts an external analog signal sequentially generated from an external analog signal source into an n-bit digital data signal (n is an integer equal to or more than two), comprising, a digital-to-analog (D/A) converter circuit that converts an n-bit digital data signal into an analog signal and outputting the analog signal from a first output terminal, a comparator that compares a signal level of an external analog signal supplied from an external device with a signal level of the analog signal outputted from the first output terminal, and a digital integrator circuit that digitally integrates a 1-bit digital data signal outputted from said comparator and thereby producing an n-bit digital data signal.  
           [0020]    It is therefore possible to conduct an analog-to-digital conversion with a high resolution of 24 bits using a low over-sampling rate (similar to that of the prior art).  
           [0021]    Since the A/D converter circuit according to the present invention is a converter of a feedback type, it is possible to prevent deterioration in the monotonous feature, which is a problem of the A/D converter circuit of sequential comparison.  
           [0022]    According to the present invention, a digital integrator circuit is used as an integrator circuit of the A/D converter circuit and hence an analog integrator circuit is not required. Therefore, the capacity described above is not required and hence there can be obtained an A/D converter circuit suitable for an LSI circuit.  
           [0023]    Since the A/D conversion is carried out using a digital integrator circuit, it is possible to conduct a multichannel A/D conversion using time division.  
           [0024]    Since the A/D converter circuit in an embodiment of the present invention is a converter of a feedback type, it is possible to prevent deterioration in the monotonous feature, which is a problem of the A/D converter circuit of sequential comparison.  
           [0025]    Since a digital integrator circuit is used as an integrator circuit of the A/D converter circuit in an embodiment of the present invention, an analog integrator circuit is not required and the capacity is not required. Consequently, there can be obtained an A/D converter circuit suitable for an LSI circuit.  
           [0026]    Since the A/D conversion is carried out using a digital integrator circuit in an embodiment of the present invention, it is possible to conduct a multi-channel A/D conversion using time division.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    The objects and features of the present invention will become more apparent from the consideration of the following detailed description taken in conjunction with the accompanying drawings in which:  
         [0028]    [0028]FIG. 1 is a diagram schematically showing constitution of an A/D converter circuit in an embodiment of the present invention;  
         [0029]    [0029]FIG. 2 is a diagram showing an example of a specific configuration of a digital integrator circuit in the A/D converter circuit shown in FIG. 1;  
         [0030]    [0030]FIG. 3 is a block diagram showing another example of a specific configuration of a digital integrator circuit in the A/D converter circuit shown in FIG. 1;  
         [0031]    [0031]FIG. 4 is a block diagram showing structure of an S/D converter circuit of sequential comparison in the prior art;  
         [0032]    [0032]FIG. 5 is a block diagram showing structure of an S/D converter circuit of Δ modification in the prior art; and  
         [0033]    [0033]FIG. 6 is a block diagram showing structure of an S/D converter circuit of Δ Σ modification in the prior art. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]    Referring now to the accompanying drawings, description will be given in detail of an embodiment of the present invention. FIG. 1 shows a configuration of an A/D converter circuit in an embodiment of the present invention.  
         [0035]    The A/D converter circuit includes a D/A converter circuit  10 , a comparator  12 , a digital integrator circuit  14 , and a clock signal generator  13 .  
         [0036]    The DAC  10  receives a signal delivered from the digital integrator  14  and can convert an n-bit digital signal into an analog signal, where n is an integer equal to or more than two. In this example, n is 24, that is, a 24-bit digital signal is converted into an analog signal.  
         [0037]    The comparator  12  compares an analog signal received via an input terminal  200  from a signal source  16  with an analog output signal from the DAC  10 . According to a result of the comparison, the comparator  12  outputs 1-bit digital data.  
         [0038]    The digital integrator  14  receives the 1-bit digital data from the comparator  12  and digitally integrates the data according to a synchronizing clock signal from the clock signal generator  13  to resultantly output an n-bit digital signal, where n is 24 in this example.  
         [0039]    Operation of the A/D converter will be described in detail. Having received the analog signal from the signal source  16 , the comparator  12  compares the analog signal with the analog output signal from the D/A converter  10 . If a signal level of the analog input signal from the input terminal  200  is larger than that of the analog signal from the D/A converter  10 , the comparator  12  outputs 1-bit data of “1” to the digital integrator  14 . If the signal level of the analog input signal is larger than that of the analog signal from the D/A converter  10 , the comparator  12  outputs 1-bit data of “0” to the digital integrator  14 . The integrator  14  also receives a clock signal CLK from the clock signal generator  13 .  
         [0040]    The digital integrator  14  digitally integrates the 1-bit digital data, namely, adds the data to each other to produce n-bit digital data (n is 24 in this example). At timing synchronized with a clock signal CLK from the clock signal generator  13 , the 24-bit digital data is fed to the DAC  10  and an external device.  
         [0041]    Since the 1-bit output data can be output at a high frequency, the 1-bit output terminal can be connected to the input terminal of a device operated at high frequency, for example, SACD (Super Audio Compact Disk(CD)) player. It can also be used, for example, for a 1-bit power amplifier.  
         [0042]    The 1-bit output data can be used, for example, for a 1-bit power amplifier.  
         [0043]    The comparator  12  repeatedly conducts the comparison until the analog signal from the input terminal  200  matches the analog output signal from the DAC  10 . When these signals match each other, a 24-bit digital output signal from the digital integrator  14  is obtained as a result of the analog-to-digital conversion of the analog input signal. The 24-bit data is acquired as output data from the A/D converter circuit.  
         [0044]    [0044]FIG. 2 shows an example of a specific configuration of the digital integrator circuit  14  of FIG. 1. The digital integrator  14  of FIG.  2  includes an up-down counter (U/D)  20 , an adder  22 , and a clock signal generator  23 . The counter (U/D)  20  receives via an input terminal  201  (FIG. 1) the 1-bit digital data (up/down (U/D) signal) outputted from the comparator  12  shown in FIG. 1. The adder  22  has a feedback circuit  22   a  to add a count value produced from the counter  20  to a count value accumulated in the adder  22 . In this connection, the clock signal generator  23  inputs a synchronizing signal to the counter  20  and the adder  22 .  
         [0045]    In the configuration, the counter  20  receives via the input terminal  201  the 1-bit digital data representing a result of the comparison between the input analog voltage from the comparator  12  shown in FIG. 1 and the analog output signal from the DAC  10 . If the digital data is “1”, the counter  20  adds one to the accumulated value. If the digital data is “0”, the counter  20  subtracts one from the accumulated value.  
         [0046]    At timing synchronized with the clock signal CLK from the clock signal generator  23 , the counter  20  conducts the addition or the subtraction according to the 1-bit digital data received as above. A result of the count operation (including 24 bits) is outputted to the adder  22 . In FIG. 2, a symbol of 24 with a slash therebelow means that the line represents a 24-bit signal line. This also applies to the other drawings. The output signal from the adder  22  is fed to an output terminal  202  and is also returned via the feedback circuit  22   a  to an input terminal thereof. The feedback circuit  22   a  supplies a result of the preceding addition to the adder  22 . Therefore, in the adder  22 , the count value from the counter  20  is added, in synchronism with the clock signal CLK, to the input representing the preceding addition thus supplied via the feedback circuit  22   a  to resultantly produce a 24-bit signal. The adder  22  outputs the 24-bit signal via the output terminal  202  to the D/A converter  10  of FIG. 1. The digital integration is achieved in this way.  
         [0047]    Since the feedback circuit  22   a  is disposed for the adder  22 , the output signal is obtained as an accumulation of the input signals.  
         [0048]    Next, FIG. 3 shows another circuit configuration of the digital integrator circuit  14 . This differs from FIG. 2 in that the circuit  14  of FIG. 3 includes three adders, namely, a first adder  22 , a second adder  30 , and a third adder  32 . The second adder  30  receives a resultant signal of addition from the first adder  22  to conduct an addition for the signal. The third adder  32  receives output signals respectively from the counter  20 , the first adder  22 , and the second adder  30  with a predetermined ratio set between these signals. Specifically, coefficients K 1  and K 2  are respectively multiplied by the output signals respectively from the first and second adders  22  and  30 .  
         [0049]    Operation of the digital integrator  14  will be described. In the integrator  14  configured as above, the up-down counter  20  receives via an input terminal  203  1-bit digital data indicating “up” or “down” of the count in the counter  20  as a result of the comparison carried out by the comparator  12  of FIG. 1 between the analog input signal from an input terminal  203  and the analog output signal from the DAC  10 .  
         [0050]    The counter (U/D)  20  adds or subtracts one to or from the existing count value thereof according to the 1-bit digital data received from the clock signal generator  23  at timing synchronized with the clock signal CLK to produce a 24-bit resultant count value. The counter  20  outputs the value to the first and third adders  22  and  32 . The first adder  22  adds the value from the counter  20  to the existing value thereof in synchronism with the clock signal CLK from the clock signal generator  23  to produce a value resultant from the addition. The first adder  22  outputs the value to the second and third adders  30  and  32 .  
         [0051]    The second adder  30  adds the value from the first adder  22  to the existing value thereof in synchronism with the clock signal CLK to produce a value resultant from the addition. The second adder  22  outputs the value to the third adder  32 . At timing synchronized with the clock signal CLK, the third adder  32  adds the outputs respectively from the first and second adders  22  and  30  with a predetermined ratio between the coefficients K 1  and K 2  associated respectively with the outputs from the first adder  22  and that from the second adder  30  to produce a value resultant from the addition. The third adder  32  outputs the value to the DAC  10 .  
         [0052]    The first adder  22  outputs a least-significant bit (LSB). The second adder  30  outputs high-order bits. The third adder  32  outputs further high-order bits. When the addition is conducted using only the least-significant bit, there is required a long period of time for the processing. To minimize the processing time, the high-order bits are also used for the addition. The coefficients K 1  and K 2  need only be determined to have a ratio therebetween which can be represented, for example, by eight bits.  
         [0053]    While the digital integrator  14  of FIG. 2 includes one adder to form a one-stage integrator, the digital integrator of FIG. 3 includes a plurality of adders to form a multi-stage integrator. Specifically, the digital integrator of FIG. 3 includes two adders  22  and  30  to digitally conduct a two-stage integration. Specifically, two adders  22  and  30  are used to conduct a digital two-stage integration. Therefore, when compared with the circuit configuration shown in FIG. 2, the circuit constitution shown in FIG. 3 is more advantageous in that the conversion time of the comparator  12  in minimized. That is, in a shorter period of time, the analog input signal from the input terminal  200  becomes substantially equal to the analog output signal from the DAC  10  in the circuit configuration of FIG. 3.  
         [0054]    The DAC  10  receives the output signal from the digital integrator  14 , and the digital output signal is at least either one the output signal from the comparator  12  and that from the digital integrator  14 . Therefore, even if the over-sampling rate is lowered, the ADC can operate with high efficiency.  
         [0055]    As above, the A/D converter of Δ modification (FIG. 5) and the A/D converter of Δ Σ modification (FIG. 5) of the prior art has a resolution of 16 or 20 bits. Therefore, to obtain a resolution of 24 bits in the analog-to-digital conversion using such an A/D converter circuit of the prior art, the over-sampling rate of the prior art (for example, 128 sampling frequencies (fs) or 512 fs) must be increased. To change data of, for example, a 44.1 kHz sampling format to data of 1024 fs or 2048 fs, the clock signal must have a frequency of 45 MHz or 90 MHz. Additionally, for data of the digital video disk (DVD) audio format, the sampling frequency takes a high value such as 192 KHz or 96 KHz. It is necessary to conduct an over-sampling operation for the sampling frequency with the over-sampling rate of 1024 fs or 2048 fs. The clock signal must have a highest frequency of about 400 MHz. This is not practical. In the over-sampling rate of the ADC of the present invention, the comparator compares the input signal with the 24-bit output signal from the DAC  10  to produce a 1-bit output signal. The digital integrator integrates the 1-bit output signal to produce 24-bit data. The output from the digital-to-analog conversion includes 24 bits, not one bit. Therefore, using an over-sampling rate which is lower than the over-sampling rate described above of an ordinary A/D converter of Δ Σ modification and which is similar to that of the prior art, the A/D converter of the present invention can conduct the analog-to-digital conversion with a high resolution of 24 bits.  
         [0056]    Since the A/D converter of the present invention uses a feedback operation, the deterioration of the monotonous feature, which disadvantageously takes place in the A/D converter of the sequential comparison can be presented.  
         [0057]    In the A/D converter of the present invention, the integrator circuit of the A/D converter is a digital integrator circuit. Therefore, the analog integrator circuit used in the prior art is not required, and hence the associated capacity is not required. The A/D converter circuit can be suitably implemented in an LSI chip.  
         [0058]    Since the digital integrator circuit is used to conduct the analog-to-digital conversion, it is possible to achieve a multi-channel analog-to-digital conversion using time division.  
         [0059]    In the A/D converter circuit of the embodiments, the digital integrator circuits respectively have a 1-stage configuration (FIG. 2) and a multi-stage, specifically, two-stage configuration (FIG. 3). The present invention is not restricted by these embodiments. The digital integrator circuit may include three of more stages to achieve the object of the present invention.  
         [0060]    While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by those embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.