Abstract:
A filter circuit includes a plurality of integrator stages, each stage including a voltage-to-current converter to convert an input voltage into a current supplied to an output thereof and a capacitor coupled to the output of the voltage-to-current converter, a voltage charged in the capacitor being supplied to a next stage as an output of each stage, and a capacitor serving as a feed-forward coupling that couples the output of at least one stage of the plurality of integrator stages to a last output node.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to filter circuits, and particularly relates to an active filter circuit with a compensated phase delay and a sigma-delta A/D converter utilizing such a circuit.  
         [0003]     2. Description of the Related Art  
         [0004]     Sigma-delta A/D converters obtain a differential between an analog input signal and a feedback signal fed back after the D/A conversion of the digital output, and performs an A/D conversion with respect to the differential signal after the integration thereof. The circuit portion that performs the A/D conversion of the differential signal after integration may be a low-precision A/D converter having a small number of output bits. For example, a comparator having one bit output may be used for this purpose. In this case, the one-bit digital output of the comparator is D/A-converted and fed back, and a differential between the feedback signal and the analog input signal is obtained and integrated. The integration accumulates differences between the input and the output over time. When the accumulated difference between the input and the output over time exceeds the threshold of the comparator at some point in time, the output of the comparator is inverted.  
         [0005]     The feedback signal made by performing D/A conversion on the output “1” of the comparator is 1 V, and the input signal is 0.75 V, for example. The differential, which is equal to −0.25 V, is accumulated through integration. The output of the comparator that receives the integrated signal as its input then changes from “1” to “0” at some point in time. The feed-back signal made by performing D/A conversion on the output “0” of the comparator is 0 V, for example. Since the input signal is 0.75 V, the difference is +0.75 V. The differential (+0.75V) is accumulated by integration. The output of the comparator that receives the integrated signal as its input then changes from “0” to “1” at some point in time. In this manner, the digital output alternates between “0” and “1”.  
         [0006]     A rate at which the integrated signal changes by integrating +0.75 V is three times greater than the rate at which the integrated signal changes by integrating −0.25V. As a result, the period during which +0.75 V is being accumulated, i.e., the period during which the digital output is “0”, is one third of the period during which −0.25 V is being accumulated, i.e., the period during which the digital output is “1”. The digital output that alternates between “0” and “1” thus has an average that is equal to 0.75, which precisely represents the input analog potential.  
         [0007]     If the digital output is sampled, the sampled digital output in the above example becomes “111011101110 . . . ”, for example. With the over-sampling of the digital output and the averaging by use of a FIR low-pass filter, therefore, the A/D conversion output is obtained with the precision commensurate with the over-sampling rate.  
         [0008]     In this manner, the sigma-delta A/D converter provides high precision by its nature, and has an advantage in that most of the processing is performed digitally, with few analog-based parts.  
         [0009]      FIG. 1  is a diagram showing the construction of a sigma-delta A/D converter. A sigma-delta A/D converter  10  of  FIG. 1  includes an adding/subtracting unit  11 , a loop-filter  12 , an A/D converter  13 , and a DAC (D/A converter)  15 . The A/D converter  13  and the DAC  14  operate based on a clock signal CLK. The DAC  14  performs a D/A conversion with respect to the digital output signal for provision as an analog feedback signal to the adding/subtracting unit  11 . The adding/subtracting unit  11  obtains a differential between the analog input signal and the feedback signal to supply the differential signal to the A/D converter  13 . The loop-filter  12  is a low-pass filter, and has the function to integrate the differential signal supplied from the A/D converter  13 . The integrated signal made by integrating the differential signal is supplied from the loop-filter  12  to the A/D converter  13 . The A/D converter  13  performs an A/D conversion with respect to the integrated signal for provision as a digital signal output.  
         [0010]     The differential signal output from the adding/subtracting unit  11  corresponds to a difference between the input signal and the output signal that is left unrepresented by the limited number of bits of the digital output signal. When an accumulation of this difference grows in the integrated signal, the digital signal output of the A/D converter  13  changes at some point in time. The time period required for the accumulation of the difference and the subsequent change is reciprocal to the difference between the input signal and the output signal that is left unrepresented by the limited number of bits of the digital output signal. Accordingly, a temporal average of the digital output signal of the A/D converter  13  can represent the analog input signal with the precision that exceeds precision achievable by the limited number of bits of the digital output signal. Namely, with the provision of the digital output signal at an over-sampling rate and the averaging of the signal by use of a FIR low-pass filter, it is possible to provide an A/D converter output with the precision commensurate with the over-sampling rate.  
         [0011]     In general, a gm-C-type filter is used as the loop-filter  12  in order to achieve desired filter characteristics. The gm-C-type filter typically has the configuration in which a voltage-to-current converting amplifier (OTA: operational transconductance amplifier) is used in the feed-forward path to compensate for phase delay (Lucien J. Breems, “A 1.8-mW CMOS ΣΔ Modulator with Integrates Mixer for A/D Conversion of IF Signals,” IEEE Journal of Solid-State Circuits, Vol. 35, No. 4, 2000).  
         [0012]     When a filter having a gain is used in a feedback-loop system as shown in  FIG. 1 , a phase delay becomes an issue.  
         [0013]      FIG. 2  is a diagram for explaining the issue of phase delay. The example shown in  FIG. 2  shows a system having two poles. Around each pole, the slope of the gain decreases by 20 dB/dec, and the phase delays by 90 degrees. In a system having two or more poles, the phase ends up delaying 180 degrees on the higher-frequency side. This means that the sign of higher frequency components is reversed between the input and output of the filter with respect to the signals. If the gain is higher than 1 (0 dB) with respect to a frequency having a 180-degree phase delay, the loop undesirably oscillates. A method for compensating for such phase delay includes adding a zero.  
         [0014]      FIG. 3  is a diagram for explaining a case in which a zero is added to compensate for phase delay. Around a zero, the gain increases by 20 dB/dec, and the phase advances by 90 degrees. Accordingly, provision of zeros in the filter as many as the number of poles or one less than the number of poles makes it possible to reduce the phase to less than 180 degrees as shown in  FIG. 3  while the phase had a 180-degree delay in  FIG. 2 .  
         [0015]      FIGS. 4A and 4B  are diagrams showing a construction that compensates for phase by adding zeros in a second-order gm-C-type filter.  FIG. 4A  illustrates a filter for which no phase compensation is provided. This filter includes voltage-to-current converting amplifiers (OTA)  301  and  302 , capacitors  303  and  304 , and a resistor  305 . The voltage-to-current converting amplifiers  301  and  302  each convert an input voltage signal from voltage to current so as to output a current signal. The respective output currents charge the capacitors  303  and  304  connected to the output nodes. As a result, voltages corresponding to the integration of the respective output currents are obtained. In this manner, a single voltage-to-current converting amplifier and a single capacitor together constitute a single-stage integrator (low-pass filter). The filter configuration shown in  FIG. 4A  has a problem in that the phase is delayed by 180 degrees on the higher frequency side.  
         [0016]      FIG. 4B  illustrates a filter for which phase is compensated for by adding zeros. This filter includes voltage-to-current converting amplifiers (OTA)  311  and  312  in addition to the voltage-to-current converting amplifiers (OTA)  301  and  302 , the capacitors  303  and  304 , and the resistor  305  shown in  FIG. 4A . The provision of the feed-forward voltage-to-current converting amplifiers  311  and  312  makes it possible to add zeros to compensate for phase delay.  
         [0017]      FIG. 5  is a diagram showing the construction of a fourth-order filter for which phase is compensated for. The filter shown in  FIG. 5  includes voltage-to-current converting amplifiers (OTA)  401  through  404 , capacitors  405  through  408 , voltage-to-current converting amplifiers (OTA)  409  through  412 , and a resistor  413 . The provision of the feed-forward voltage-to-current converting amplifiers  409  through  412  makes it possible to add zeros to compensate for phase delay. This filter has four poles and three zeros, and is designed to have a 90-degree phase delay on the higher frequency side.  
         [0018]     The second-order filter configuration shown in  FIG. 4B  uses two feed-forward voltage-to-current converting amplifiers for the purpose of phase compensation, and the fourth-order filter configuration shown in  FIG. 5  uses four feed-forward voltage-to-current converting amplifiers for the purpose of phase compensation. In this manner, an n-th order filter configuration generally has n feed-forward voltage-to-current converting amplifiers added thereto. These feed-forward voltage-to-current converting amplifiers for the purpose of phase compensation consume electric power in the same manner as the voltage-to-current converting amplifiers for the purpose of integration provided at the respective stages. Accordingly, power consumption by the amplifiers doubles in the filter having the phase-compensated configuration.  
         [0019]     Accordingly, there is a need for a gm-C-type filter that achieves phase compensation while suppressing power consumption.  
       SUMMARY OF THE INVENTION  
       [0020]     It is a general object of the present invention to provide a filter circuit and a sigma-delta A/D converter that substantially obviates one or more problems caused by the limitations and disadvantages of the related art.  
         [0021]     Features and advantages of the present invention will be presented in the description which follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by a filter circuit and a sigma-delta A/D converter particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention.  
         [0022]     To achieve these and other advantages in accordance with the purpose of the invention, the invention provides a filter circuit, which includes a plurality of integrator stages, each stage including a voltage-to-current converter to convert an input voltage into a current supplied to an output thereof and a capacitor coupled to the output of the voltage-to-current converter, a voltage charged in the capacitor being supplied to a next stage as an output of each stage, and a capacitor serving as a feed-forward coupling that couples the output of at least one stage of the plurality of integrator stages to a last output node.  
         [0023]     According to another aspect of the present invention, a sigma-delta A/D converter includes a filter circuit, which includes a plurality of integrator stages, each stage including a voltage-to-current converter to convert an input voltage into a current supplied to an output thereof and a capacitor coupled to the output of the voltage-to-current converter, a voltage charged in the capacitor being supplied to a next stage as an output of each stage, and a capacitor serving as a feed-forward coupling that couples the output of at least one stage of the plurality of integrator stages to a last output node. The sigma-delta A/D converter further includes an A/D converter having an input thereof coupled to the last output node of the filter circuit to output a digital signal, a D/A converter coupled to the output of the A/D converter to output an analog signal having a signal level responsive to the digital signal, and a differential circuit coupled to an external analog input and the output of the D/A converter to supply a differential between the external analog input and the analog signal to the filter circuit.  
         [0024]     According to at least one embodiment of the present invention, the use of a capacitor element in place of a feed-forward voltage-to-current converting amplifier conventionally used in a feed-forward path achieves phase compensation equivalent to that achieved by the use of the feed-forward voltage-to-current converting amplifier. Since an active device is removed by replacing the feed-forward voltage-to-current converting amplifier with the capacitor element, power consumption is reduced compared with the related-art construction. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings, in which:  
         [0026]      FIG. 1  is a diagram showing the construction of a sigma-delta A/D converter;  
         [0027]      FIG. 2  is a diagram for explaining the issue of phase delay;  
         [0028]      FIG. 3  is a diagram for explaining a case in which a zero is added to compensate for phase delay;  
         [0029]      FIGS. 4A and 4B  are diagrams showing a construction that compensates for phase by adding zeros in a second-order gm-C-type filter;  
         [0030]      FIG. 5  is a diagram showing the construction of a fourth-order filter for which phase is compensated for;  
         [0031]      FIG. 6  is a partial view of a related-art filter showing a single-stage integrator and a corresponding feed-forward section;  
         [0032]      FIG. 7  is a circuit diagram showing the circuit construction of a single-stage integrator and a corresponding feed-forward section according to the present invention;  
         [0033]      FIG. 8  is a circuit diagram showing the construction of a filter according to the present invention that achieves equivalent characteristics to the related-art filter of  FIG. 5 ;  
         [0034]      FIG. 9  is a diagram showing a variation of the filter shown in  FIG. 8 ;  
         [0035]      FIG. 10  is a circuit diagram showing an example of the circuit construction of a voltage-to-current converting amplifier;  
         [0036]      FIG. 11  is a circuit diagram showing a further variation of the filter having the construction shown in  FIG. 9 ; and  
         [0037]      FIG. 12  is a circuit diagram showing a further variation of the filter having the construction shown in  FIG. 11 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]     In the following, embodiments of the present invention will be described with reference to the accompanying drawings.  
         [0039]     The filter of the present invention described in the following is suitable for use as the loop-filter of a sigma-delta A/D converter. The application thereof, however, is not limited to sigma-delta A/D converters, and the present invention is applicable to gm-C-type filters in general.  
         [0040]      FIG. 6  is a partial view of a related-art filter showing a single-stage integrator and a corresponding feed-forward section.  FIG. 7  is a circuit diagram showing the circuit construction of a single-stage integrator and a corresponding feed-forward section according to the present invention.  
         [0041]     The construction shown in  FIG. 6  corresponds to the voltage-to-current converting amplifiers  402  and  410  and the capacitor  406  of the related-art filter shown in  FIG. 5 , for example. The construction of  FIG. 6  includes voltage-to-current converting amplifiers (OTA)  501  and  502  and a capacitor  503 . For the sake of simplicity of explanation, the output of the voltage-to-current converting amplifier  502  is assumed to be coupled directly to the ground without having an intervening output resistor. The impedance of the capacitor  503  is 1/sC1. An output voltage Vout (i.e., an input to the next stage) and a feed-forward current I F  are represented as: 
 
 Vout =( gm 1 /sC 1) Vin   (1)
 
 I   F   =gm 2 ·Vin   (2).
 
 Here, gm1 and gm2 are the mutual conductance of the voltage-to-current converting amplifiers  501  and  502 , respectively. 
 
         [0042]     The configuration of  FIG. 7  according to the present invention includes a voltage-to-current converting amplifier (OTA)  601  and capacitors  602  and  603 . The impedance of the capacitor  603  is 1/sC3, and the impedance of the capacitor  602  is 1/sC4. An output voltage Vout′ (i.e., an input to the next stage) and a feed-forward current I F ′ are represented as: 
 
 Vout ′=( gm 3 /sC 3)·( C 3/( C 3 +C 4))· Vin   (3)
 
 I   F ′=( C 4/( C 3 +C 4))· gm 3 ·Vin   (4).
 
 Here, gm is the mutual conductance of the voltage-to-current converting amplifier  601 . 
 
         [0043]     In the above equations, C3 and C4 are selected such that Vout=Vout′ and I F =I F ′ are satisfied. This allows the construction of  FIG. 7  according to the present invention to achieve the same input/output characteristics as those of the related-art construction shown in  FIG. 6 . Namely, the voltage-to-current converting amplifier  502  of the construction shown in  FIG. 6  outputs the current I F  having the amount corresponding to level of the input voltage Vin. In the construction shown in  FIG. 7  according to the present invention, on the other hand, the output voltage Vout′ made by integrating the input voltage Vin is coupled to the capacitor  602 , thereby letting the capacitor  602  generate a current (=C 4 (dV/dt)) proportional to a differential of the voltage Vout′. Differentiation of the output voltage Vout′ in this manner cancels out the integration of the input voltage Vin, thereby letting the capacitor  602  generate the current I F ′ having the amount corresponding to the level of the input voltage Vin.  
         [0044]     In reality, the output of the filter is not directly connected to the ground, but is connected to the ground through an intervening load impedance. If the load impedance is sufficiently small compared with surrounding impedances, the above equations properly approximate the filter characteristics.  
         [0045]     The present invention as described above employs a capacitor element in place of a feed-forward voltage-to-current converting amplifier, and achieves phase compensation equivalent to that achieved by the use of the feed-forward voltage-to-current converting amplifier. Since an active device is removed by replacing the feed-forward voltage-to-current converting amplifier with the capacitor element, the present invention can reduce power consumption by a corresponding amount.  
         [0046]      FIG. 8  is a circuit diagram showing the construction of a filter according to the present invention that achieves equivalent characteristics to the related-art filter of  FIG. 5 . The filter of  FIG. 8  includes voltage-to-current converting amplifiers  701  through  704 , integration capacitors  705  through  708 , feed-forward capacitors  709  through  711 , and a feed-forward voltage-to-current converting amplifier  712 , and a load resistor  713 .  
         [0047]     The feed-forward capacitor  709  generates a feed-forward current equivalent to the output current of the feed-forward voltage-to-current converting amplifier  409  shown in  FIG. 5 . The feed-forward capacitor  710  generates a feed-forward current equivalent to the output current of the feed-forward voltage-to-current converting amplifier  410  shown in  FIG. 5 . The feed-forward capacitor  711  generates a feed-forward current equivalent to the output current of the feed-forward voltage-to-current converting amplifier  411  shown in  FIG. 5 . Further, the feed-forward voltage-to-current converting amplifier  712  corresponds to the feed-forward voltage-to-current converting amplifier  412  shown in  FIG. 5 .  
         [0048]     In the construction shown in  FIG. 8 , the feed-forward voltage-to-current converting amplifier  712  cannot be replaced by a capacitor because it is located at the last stage. Since a voltage-to-current converting amplifier is an amplifier that converts a voltage to a current, it can easily be replaced by a resistor element.  
         [0049]      FIG. 9  is a diagram showing a variation of the filter shown in  FIG. 8 . In  FIG. 9 , the same elements as those of  FIG. 8  are referred to by the same numerals, and a description thereof will be omitted. In  FIG. 9 , a resistor element  812  is used in place of the feed-forward voltage-to-current converting amplifier  712 . The resistor element  812  lets a current flow in the amount proportional to a voltage drop across the resistor in response to the voltage at the input side (where the feed-forward voltage-to-current converting amplifier  704  is situated). The replacement of the voltage-to-current converting function by the resistor element  812  makes it possible to further reduce the number of active devices, thereby further suppressing the power consumption.  
         [0050]      FIG. 10  is a circuit diagram showing an example of the circuit construction of a voltage-to-current converting amplifier. The voltage-to-current converting amplifier shown in  FIG. 10  demonstrates a configuration in which differential inputs and differential outputs are used.  
         [0051]     The voltage-to-current converting amplifier of  FIG. 10  includes NMOS transistors  901  and  902  forming an input differential pair, PMOS transistors  903  and  904  serving as a current source pair, NMOS transistors  905  and  906  serving as a current source pair, and voltage-to-current converting resistors  907  and  908 . The NMOS transistors  901  and  902  forming an input differential pair function as a source follower. Accordingly, a voltage difference proportional to the input voltage difference is applied to the opposite ends of the series of the resistors  907  and  908 , resulting in a current responsive to this voltage difference flowing through the resistors  907  and  908 .  
         [0052]     The voltage-to-current converting amplifier of  FIG. 10  performs a amplification operation, with biases being applied by the PMOS transistors  903  and  904  and the NMOS transistors  905  and  906 . The gates of the NMOS transistors  905  and  906  receive common feedback (CMFB). The currents running through the PMOS transistors  903  and  904  are the same amount, and the currents running through the NMOS transistors  905  and  906  are also the same amount. As a result, a current equal to the current flowing though the voltage-to-current converting resistors  907  and  908  is output as a differential current output. This achieves a voltage-to-current conversion.  
         [0053]      FIG. 11  is a circuit diagram showing a further variation of the filter having the construction shown in  FIG. 9 . In  FIG. 11 , the same elements as those of  FIG. 9  are referred to by the same numerals, and a description thereof will be omitted.  
         [0054]     In the construction shown in  FIG. 11 , an operational amplifier  1001 , a resistor  1002 , and a capacitor  1003  are provided in place of the voltage-to-current converting amplifier  701  and the capacitor  705  constituting the first stage integrator in  FIG. 9 . The operational amplifier  1001 , resistor  1002 , and capacitor  1003  together form a RC active filter. The resistor  1002  generates a current corresponding to the input voltage, and this current is supplied to the capacitor  1003  to generate a voltage that is an integration of the input voltage. The use of the RC active filter at the first stage of the filter improves the distortion characteristics of the first stage of the filter.  
         [0055]      FIG. 12  is a circuit diagram showing a further variation of the filter having the construction shown in  FIG. 11 . In  FIG. 12 , the same elements as those of  FIG. 11  are referred to by the same numerals, and a description thereof will be omitted.  
         [0056]     In the construction shown in  FIG. 12 , a feed-forward path is additionally provided that extends from the output of the active filter at the first stage of the filter. The provision of the additional feed-forward path and a capacitor for compensating for phase makes it possible to set the phase delay to 0 degree rather than to 90 degrees on the higher frequency side. This further improves the filter characteristics.  
         [0057]     The filter illustrated in  FIG. 11  has fourth-order characteristics for lower frequencies and first-order characteristics for higher frequencies. The filter illustrated in  FIG. 12  has fourth-order characteristics for lower frequencies and 0th-order characteristics for higher frequencies. These filters are applicable to higher-order sigma-delta A/D converters.  
         [0058]     Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention.  
         [0059]     The present application is based on Japanese priority application No. 2004-330895 filed on Nov. 15, 2004, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.