Abstract:
An analogue-to-digital sigma-delta modulator for converting analogue input signals to digital output signals comprises a feedback path ( 1, 101, 201 ) for producing analogue feedback signals that are a function of the digital output signals (y, Y), an ‘N’-stage (‘N’≧2) integrator path ( 9  to  14, 109  to  114 ) for integrating analogue difference signals that are a difference function of the input signal and the analogue feedback signals, and a quantizer ( 3, 103 ) responsive to the signals integrated by the integrator means ( 9  to  14, 109  to  114 ) for producing the digital output signals (y, Y) at clock intervals. The feedback path includes ‘N’ feedback stages ( 15  to  17, 115  to  117 ) for respective integrator stages ( 9  to  14, 109  to  114 ).  
     Each of the ‘N’ feedback stages ( 15  to  17, 115  to  117 ) comprises finite impulse response (‘FIR’) filters ( 15  to  19, 115  to  117 ), each of the FIR filters being of the same order ‘M’, where ‘M’ is at least two; at least the filter ( 15, 115 ) of the feedback stage that feeds back to the first integrator stage is a low pass filter.  
     The integrator stages may be discrete-time integrators; the FIR filters reduce their sensitivity to feedback voltage step changes that would cause non-linearities due to slew-rate limitations. Alternatively, the integrator stages may be continuous-time integrators; the FIR filters reduce their sensitivity to clock pulse jitters. In the embodiment shown in  FIG. 11 , the first integrator stage ( 109, 110 ) is a continuous-time integrator stage, and the remainder of the integrator stages ( 11  to  14 ) are discrete-time integrator stages.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to analogue-to-digital sigma-delta modulators.  
       BACKGROUND OF THE INVENTION  
       [0002]     Sigma-delta modulators are now widely used for conversion between analogue and digital signals, especially with advances in very large-scale integrated circuit technology (VLSI).  
         [0003]     An article by P. M. Aziz, H. V. Sorensen and J. van der Spiegel in IEEE Signal Processing magazine, January 1996 gives an overview of analogue-to-digital sigma-delta modulators as used in analogue-to-digital converters for example. An article by P. F. Ferguson, Jr. A. Ganesan and R. W. Adams, “One Bit Higher-Order Sigma-Delta A/D Converters”, IEEE International Symposium on Circuits and Systems, pp. 890-893, 1990 gives a presentation of the general form of higher-order signal-delta modulators including filters in both feed-forward and feedback paths.  
         [0004]     Two basic kinds of sigma-delta modulators exist: discrete-time and continuous-time.  FIG. 1  of the accompanying drawings shows a typical basic discrete-time sigma-delta modulator of second order (that is to say comprising two integrator stages) and  FIG. 2  shows the circuit configuration of a typical switched-capacitor integrator stage in the modulator of  FIG. 1 .  FIG. 3  shows the transfer functions of a typical basic second order continuous-time sigma-delta modulator.  
         [0005]     In general terms, an analogue-to-digital sigma-delta modulator receives analogue input signals X (that is to say whose amplitude represents data) and converts them at clock intervals to encoded digital output signals Y (that is to say pulses whose amplitude is constant and whose repetition rate represents the data). The modulator comprises a feedback path  1  for producing analogue feedback signals that are a function of the digital output signals, an integrator  2  for integrating analogue difference signals that are a difference function of the analogue input signal and the analogue feedback signals, and a quantizer  3  responsive to the signals integrated by the integrator  2  for producing the digital output signals at clock intervals defined by a clock signal CK.  
         [0006]     An article by D. K. Su and B. A. Wooley, “A CMOS Oversampling D/A Converter with a Current-Mode Semidigital Reconstruction Fllter”, IEEE Journal of Solid-State Circuits, Vol. 28, No. 12, December 1993, pp. 1224-1233 gives a description of a digital-to-analogue modulator including a finite impulse response (‘FIR’) filter, in particular a semi-digital FIR filter in the output path. The technique proposed is not applicable to an analogue-to-digital modulator.  
         [0007]     In U.S. Pat. No. 5,357,252, assigned to the assignee of the present invention, first-order FIR filtering in the feedback path of the first stage of an analogue-to-digital modulator is proposed to combat the pattern noise. This method changes the noise transfer function of the modulator and its extension to higher-order filtering is not practicable.  
         [0008]     An article by T. Okamoto, Y. Maruyama and A. Yukawa, “A Stable High-Order-Delta-Sigma Modulator with an FIR Spectrum Distributor”, IEEE Journal of Solid-State Circuits, Vol. 28, No. 7, pp. 730-735, July 1993, describes a noise-shaper circuit including an FIR spectrum distributor, used to improve the stability of higher-order modulators. The order of FIR filtering is limited to twice the modulator order. The orders of the successive FIR filters are not the same but are stepped from one up to the modulator order along the feedback path. No improvement in terms of power or distortion is apparent in this architecture. It is noted that the article describes a digital-to-analogue modulator whose feedback path includes a plurality of feedback stages including finite impulse response filters of differing orders.  
         [0009]     Concerns that arise in the design of analogue-to-digital sigma-delta modulators include their sensitivity to the effects of feedback voltage step changes and clock pulse instabilities. The present invention provides novel analogue-to-digital sigma-delta modulators that address these concerns, among others.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention provides an analogue-to-digital sigma-delta modulator as described in the accompanying claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a block schematic diagram of a typical basic second order discrete-time sigma-delta modulator,  
         [0012]      FIG. 2  is a schematic diagram of a typical switched-capacitor integrator stage in the modulator of  FIG. 1 ,  
         [0013]      FIG. 3  is a schematic diagram showing the operation of a typical basic second-order continuous-time sigma-delta modulator,  
         [0014]      FIG. 4  is a schematic diagram showing transfer functions of a basic Nth order discrete-time sigma-delta modulator in accordance with one embodiment of the invention, given by way of example,  
         [0015]      FIG. 5  is a block schematic diagram of a basic second order discrete-time sigma-delta modulator of the kind shown in  FIG. 4 ,  
         [0016]      FIG. 6  is a schematic diagram showing transfer functions of a semi-digital finite impulse response filter in the sigma-delta modulator of  FIG. 5 ,  
         [0017]      FIG. 7  is a circuit diagram of the discrete-time sigma-delta modulator of  FIG. 5 ,  
         [0018]      FIG. 8  is a schematic diagram showing the operation of a basic Nth order continuous-time sigma-delta modulator in accordance with another embodiment of the invention,  
         [0019]      FIG. 9  is a block schematic diagram of a basic second order continuous-time sigma-delta modulator of the kind shown in  FIG. 8 , given by way of example,  
         [0020]      FIG. 10  is a circuit diagram of a current switch and proportioning circuit in he sigma-delta modulator of  FIG. 9 ,  
         [0021]      FIG. 11  is a schematic diagram showing transfer functions of a basic Nth order mixed-time sigma-delta modulator in accordance with yet another embodiment of the invention, given by way of example,  
         [0022]      FIG. 12  is a graph showing an improvement factor of a modulator of the kinds shown in  FIG. 8  and  FIG. 11  relative to a modulator of the kind shown in  FIG. 3  as a function of input signal amplitude, and  
         [0023]      FIG. 13  is a graph showing a signal-to-noise ratio of a modulator of the kinds shown in  FIG. 8  and  FIG. 11  as a function of input signal amplitude. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]     The design of a sigma-delta modulator must take account of many different criteria and achieve a suitable compromise between sometimes conflicting requirements. Thus, power consumption in a sigma-delta modulator is a direct function of its sampling frequency and dynamic range. Sampling capacitors such as  4  in the discrete-time switched-capacitor (SC) implementation shown in  FIG. 2  of an stage of the integrator  2  shown in  FIG. 1  are chosen with regard to the thermal noise, KT/C. On the other hand, capacitors determine the circuit speed. Transient response requirements for an SC-integrator in a sigma-delta modulator depend on: the total load capacitance, the amplifier architecture and the input signal activity.  
         [0025]     During the integration phase, the output of an SC-integrator  2  changes from an initial level to a new level. For large input voltage steps, the integrator amplifier  5  first goes through a large-signal response and when the output signal approaches the final level, the integrator settles according to its small-signal performance. The large-signal transient response of an operational amplifier is dominated by its slew-rate while its small-signal performance depends on its gain-bandwidth and phase margin. For a given clock frequency, reducing the voltage steps allows for an integrator amplifier  5  with a smaller slew-rate.  
         [0026]     In the prior art discrete-time sigma-delta modulator structure shown in  FIG. 1 , the feedback path  1  includes a digital-to-analogue converter  6  that converts the output signal Y(z) to an analogue signal that is fed back at each clock pulse to each of two feedback amplifiers  7  and  8  having respective gains a 2  and a 1 . The feedback signal from the first stage amplifier  7  is applied to the subtraction input of an adder  9 , whose addition input receives the analogue input signal X(z), the output of the adder  9  being applied to the input of a first integrator stage  10 . Likewise, the feedback signal from the second stage amplifier  8  is applied to the subtraction input of an adder  11 , whose addition input receives the first integrator stage  10 , the output of the adder  11  being applied to the input of a second integrator stage  12 . The gains a 2  and a 1  of the feedback amplifiers  7  and  8  determine the respective feedback coefficients.  
         [0027]     In such a prior art sigma-delta modulator, the feedback path is responsible for the abruptness of the voltage variations. The embodiments of the present invention shown in FIGS.  4  to  11  of the drawings enable improvements in this and other respects; they use finite impulse response (‘FIR’) filters in the feedback path of the modulator. At least the FIR filter in the feedback path of the first integrator stage is a low pass filter. The FIR filters feed back an analogue signal at each clock pulse whose magnitude is a function not only of the output signal Y(z) at the current (immediately preceding) clock pulse but also of its value at previous clock pulses, the order of the FIR filter being defined as the number of clock pulses that it can take into account. The orders of the FIR filters are the same for each integrator stage. One consequence is to reduce the abruptness of the voltage variations, reducing the slew rate and hence the harmonic distortion of the integrator amplifiers  5 , especially at the first stage  10 . Moreover, if this can be achieved without increasing the capacitive load of the integrator amplifier  5  of the first stage  10 , a substantial saving in power consumption can be obtained.  
         [0028]     Referring now to  FIG. 4 , a discrete-time sigma-delta modulator of nth order in accordance with an embodiment of the present invention is shown including an integration path  2 , a quantizer  3  and a feedback path  1 . The integration path includes N integrator stages of which the first and second stages  10  and  12  and the Nth stage  14  are shown, together with associated feedback adders  9 ,  11  and  13  respectively. The output signal y[n] is fed back to respective FIR filters of which filters  15  and  16  are shown for the first and second stages and  16  for the Nth stage. In this embodiment, all the FIR filters produce an analogue feedback signal that is a function of the preceding M clock intervals of the output signal y[n]. The first stage FIR filter  15  is a low pass filter. The analogue feedback signals from the FIR filters  15  to  17  are applied to the negative inputs of the adders  9  to  13  respectively. The analogue feedback signal from an FIR filter i is a function F i (z) of the output signal.  
         [0029]     In calculating the coefficients for the FIR filters, it is convenient to calculate the function F N (z) of the first filter  15  initially and then calculate the coefficients of the subsequent FIR filters to achieve the target noise transfer function. Such calculations will be described with reference to  FIG. 5 , which is a block schematic diagram of a second order sigma-delta modulator of the kind shown in  FIG. 4 . The FIR filters  15  and  16  are analogue filters, both of the same order M. The output of the modulator can be written in the z-domain as:  
               Y   ⁡     (   z   )       =           z     -   2       ⁢     X   ⁡     (   z   )         +         (     1   -     z     -   1         )     2     ⁢     Q   ⁡     (   z   )             D   ⁡     (   z   )                 Equation   ⁢           ⁢   1             
 
 where (Qz) is the noise introduced by the quantizer  3 , 
 
         [0031]     k 2 =g 2 , k 1 =g 1 g 2 , and 
 
 D ( z )=(1 −z   −1 ) −2   +k   2   B ( z ) z   −1 (1 −z   −1 )+ k   i   A ( z ) z   −2 
 
         [0032]     The transfer function A(z) and B(z) of the FIR filters  15  and  16  can be written as:  
               A   ⁡     (   z   )       =         a   0     +       a   1     ⁢     z     -   1         +       a   2     ⁢     z     -   2         +   …     =       ∑     m   =   1     M     ⁢           ⁢       a   m     ⁢     z     -   m                     Equation   ⁢           ⁢   2                 B   ⁡     (   z   )       =         b   0     +       b   1     ⁢     z     -   1         +       b   2     ⁢     z     -   2         +   …     =       ∑     m   =   1     M     ⁢           ⁢       b   m     ⁢     z     -   m                     Equation   ⁢           ⁢   3             
 
         [0033]     If, for the sake of simplicity, D(z) is required to remain a second order polynomial, it follows that: 
 
 D ( z )=1 +αz   −1   +βz   −2   Equation 4
 
         [0034]     The coefficients of A(z) can be obtained as:  
                 a   0     +     a   1     +   …   +     a   M       =       α   +   β   +   1       k   1               Equation   ⁢           ⁢   5             
 
         [0035]     and the coefficients of B(z) can then be calculated.  
         [0036]     In a practical implementation, calculation of an ideal filter from quadratic programming offers the optimum noise suppression for a given filter length. In practical integrated circuit design, it is convenient to implement all coefficients using capacitors that are integer multiples of a unit capacitor and, in a particular embodiment of the present invention, a comb filter is used as the FIR filter  15  for the first integrator stage, that is to say in which all the coefficients of the filter are equal. The optimum coefficients for this sub-ideal filter are:  
               a   0     =       a   1     =     …   =       a   M     =       α   +   β   +   1         k   1     ⁡     (     M   +   1     )                       Equation   ⁢           ⁢   6             
 
         [0037]     The corresponding optimum values for the second integrator stage FIR filter can be shown to be:  
                       b   0     =       α   +   2       k   2                       b   m     =             (     M   -   m   +   1     )     ⁢     (     α   +   β   +   1     )           k   2     ⁡     (     M   +   1     )         ⁢           ⁢   m     =   1       ,   …   ⁢           ,     M   -   1             }           Equation   ⁢           ⁢   7             
 
         [0038]     Examples of the FIR filter coefficients for M=0 (corresponding to a modulator as shown in  FIG. 1 ) and FIR filters of orders  1  to  5  (corresponding to the modulator of  FIG. 5  in accordance with this embodiment of the invention) are summarised in the following table:  
                                                                                   M   a 0     a 1     a 2     a 3     a 4     a 5     b 0     b 1     b 2     b 3     b 4     b 5     Σb m                     0   1                       1                       1       1   ½   ½                   1   ¼                   1.25       2   ⅓   ⅓   ⅓               1   ⅓   ⅙               1.5       3   ¼   ¼   ¼   ¼           1   ⅜   ¼   ⅛           1.75       4   ⅕   ⅕   ⅕   ⅕   ⅕       1   {fraction (4/10)}   {fraction (3/10)}   {fraction (2/10)}   {fraction (1/10)}       2.00       5   ⅙   ⅙   ⅙   ⅙   ⅙   ⅙   1   {fraction (5/12)}   ⅓   ¼   ⅙   {fraction (1/12)}   2.25                  
 
         [0039]     The last column of the table shows the sum of coefficients of b(z), which represents the factor by which the feedback capacitor of the second integrator increases.  
         [0040]     The FIR filters used in the embodiments of the present invention shown in FIGS.  4  to  11  of the accompanying drawings may be purely digital filters, which require multi-bit DACs in addition. However, in the preferred embodiments of the present invention, semi-digital FIR filters are used having transfer functions as illustrated in  FIG. 6  of the drawings. For a filter of order M, M successive flip-flop circuits form an M-bit shift register  18 . The digital signals at successive stages of the shift register  18  control respective amplifier stages  19  whose gains a 1 , a 2  to a M  define the amplitudes of corresponding analogue signals that define the coefficients of the filter and are summed at  20 .  
         [0041]      FIG. 7  shows a practical implementation of a preferred embodiment of the discrete-time modulator of  FIG. 5  with semi-digital FIR filters as shown in  FIG. 6 . This embodiment of the invention is implemented in switched-capacitor technology in the feedback path  1 , the amplification and addition transfer functions indicated by elements  19  and  20  in  FIG. 6  being achieved by the connections of capacitors  23  in the FIR filters alternately with reference voltages and with the amplifiers  10  and  12  of the integrator stages; the switching of the capacitors  23  at controlled phases of the clock cycles, the sizes chosen for the capacitors and the magnitudes of the reference voltages define the coefficients of the FIR filter transfer functions.  
         [0042]     Each stage of each FIR filter comprises a voltage reference switch  22  that switches between voltage references +V ref  and −V ref  under the control of the corresponding stage of the shift register  18 . The voltage at the pole of the switch  22  is sampled during one phase of the clock cycle by the closure of a switch φ 1  connected to the left plate of capacitor  23  whose right plate is connected to ground through another switch closed at phase φ 1 . The left plate of the capacitor  23  is connected to ground through a switch that is closed at phase φ 2  of the clock cycle and the right plate is connected through another switch that is closed at phase φ 2  to the input  24  of the integrator amplifier of the corresponding stage of the integrator  2 . Respective integrator capacitors  21  are connected between the outputs and the inputs of each of the integrator amplifiers  10  and  12 . The input voltage for each stage of the integrator  2 , the input voltage V in  of the integrator  2  or the ouput of the first stage amplifier  10  as the case may be, is supplied through a phase φ 1  switch to the left hand plate of a sampling capacitor  25  whose right plate is connected to ground through another phase φ 1  switch and the left hand plate of the capacitor  25  is connected to ground through a phase φ 2  switch, the right hand plate of the capacitor  25  being connected to the integrator amplifier input  24  through a phase φ 2  switch.  
         [0043]     If the size of the integrator capacitor  21  is C, the size of the capacitor  23  of the mth stage of the FIR filter  15  of the first integrator stage is chosen to be a m C/2 and the size of the sampling capacitor  25  is chosen to be C/2 for g 1 =½ and g 2 =½. Similarly, for the mth stage of the FIR filter  16  of the second integrator stage, the size of the filter capacitor  23  is chosen to be b m C/2. The values a m  and b m  correspond to the values of the coefficients of the FIR filter transfer function.  
         [0044]     It will be appreciated that the description given above with reference to FIGS.  4  to  7  of the accompanying drawings relates to the basic sigma-delta modulator functions and that additional functions may be utilised. In particular, double sampling may be used, which consists of applying the feedback signal on both clock phases so as to balance the capacitive load seen by the integrator amplifiers during each clock phase. Since the architecture using FIR filters comprises capacitors split into several capacitor elements, this need not involve the addition of supplementary capacitors, it being sufficient to divide the capacities of each FIR filter into two bunches of capacitor elements controlled by the different clock phases.  
         [0045]     In addition, feed forward, interpolation and resonance paths may be added.  
         [0046]     The choice of the order of the FIR filters, which are identical for each integrator stage, may be a compromise. A higher order decreases the voltage steps applied to the integrator first stage, enables the speed of the first stage to be reduced and hence enables a reduction in the parasitic capacitance and power consumption of the first integrator stage. On the other hand, a lower order FIR filter reduces the complexity of the clock routing connections. A suitable compromise for a two-stage discrete-time integrator may be a fourth or fifth order for the FIR filters.  
         [0047]     The discrete-time sigma-delta modulator embodiments of the present invention described above with reference to FIGS.  4  to  7  are of substantial interest because of the inherent properties that switched capacitor technology offers. However, even with the benefit of the present invention, they present disadvantages in certain circumstances. For example, the full bandwidth of the amplifiers is not exploited, due to the fact that the amplifier&#39;s bandwidth must be several times larger than the sampling frequency so that the integrators settle completely within each clock phase. Also, due to the sampling operation, broadband thermal noise is folded back into the band-of-interest (‘aliasing’) and has to be countered.  
         [0048]     The present invention is also applicable to continuous-time (‘CT’) sigma-delta modulators. In continuous-time modulators the amplifier bandwidth can be quite close to the sampling frequency and thermal noise is not aliased. In addition, the structure inherently presents an anti-aliasing pre-filtering with regard to the input signal. However, a prior art CT-modulator such as shown in  FIG. 3  is sensitive to clock jitters, which are random variations of the clock edge. These corrupt the feedback signal and give rise to in-band noise. The use of FIR filters in the feedback path enables improvements in these respects also due to the averaging effect of the filters.  
         [0049]     The operation of a typical basic second-order continuous-time sigma-delta modulator is illustrated schematically in  FIG. 3 . The analogue input signal X, in the time domain, passes through analogue integrator stages  110  and  112  that have a time constant T S  and is sampled by a sampling switch  126  at the end of the integrator path  102  before passing to the quantizer  103 , the feedback path  101  supplying analogue signals to the negative inputs of the adders  109  and  111 .  
         [0050]     The analogue feedback signal is a rectangular pulse of duration τ and with a delay t d  with respect to the quantizer clock. In the case of the second-order modulator of  FIG. 3 , clock jitters modulating the pulse duration τ result in an output spectrum containing both white and first order shaped noise:  
                 S     ɛ   τ       ⁡     (   z   )       =         (       σ   τ   2       τ   2       )     ⁡     [     1   +     α   ⁡     (     1   -     z     -   1         )         ]       ⁡     [     1   +     α   ⁡     (     1   -   z     )         ]               Equation   ⁢           ⁢   8             
 
 where 
 
         [0052]     σ τ   2  is the variance of the effect of the clock jitter on the pulse width τ,  
         [0053]     α=−2+τ/(2T) and  
         [0054]     T is the clock period.  
         [0055]     The result of pulse duration jitters in the output spectrum is both a white noise component and a first order shaped noise, the latter being of substantially reduced importance, due to the output filters.  
         [0056]     The effect of clock jitters on the pulse delay, on the other hand, give rise to a first order shaped noise spectrum:  
                 S     ɛ     t   d         ⁡     (   z   )       =         (       σ     t   d     2       T   2       )     ⁡     [     1   -     z     -   1         ]       ⁡     [     1   -   z     ]               Equation   ⁢           ⁢   9             
 
         [0057]     The white noise component can be represented by an error ε 2 =σ τ /τ added at the input of the first integrated stage and the shaped noise components correspond to errors ε 1 =ασ τ /τ+σ td /T added at the input of the second integrator stage. It follows that the first stage is the most critical for the deterioration of the performance of the modulator in respect of clock jitters. It also follows that increasing the integration order of a continuous time modulator of this type does not significantly reduce the effect of clock jitters.  
         [0058]      FIG. 8  shows a continuous-time sigma-delta modulator in accordance with another embodiment of the present invention. In the modulator of  FIG. 8 , the feedback path  101  includes FIR filters  115 ,  116 , and  117  having analogue-to-digital transfer functions H DA (S) shown at  127 ,  128  and  129  that supply respective analogue feedback signals to the adders  109 ,  111  and  113  with respective coefficients. The modulator of  FIG. 8  offers a substantial improvement in respect of sensitivity to clock jitters compared to the modulator of  FIG. 3 .  
         [0059]     Choice of the coefficients of the transfer function of the first FIR filter for the first integrator stage  110  of the modulator of  FIG. 8  depends on the nature of the input signal. Once the coefficients of the first FIR filter  115  have been obtained, the coefficients of the other filters  116  and  117  can be calculated to reach the desired noise transfer function, as before. The loop transfer function of the continuous-time modulator may be equated with that of a corresponding discrete-time modulator for the purposes of calculating the coefficients.  
         [0060]     A satisfactory solution when the input signal is unknown or when the optimum spread of filter coefficients is troublesome for analogue implementation is again the use of a comb filter, as described above for the discrete-time modulator.  
         [0061]      FIG. 9  shows a second order continuous time modulator implementation in which the input signal V IN  is supplied to the input  124  of the analogue amplifier of the first integrator stage  110  through a resistor  130  and the output of the amplifier of the first integrator stage  110  is supplied to the input of the amplifier of the second integrator stage  112  through a resistor  131 . The low pass FIR filters of order M are formed by current switch circuits triggered by the corresponding stages of an M-stage shift register  118 , the output currents from the stages of each FIR filter being summed and supplied to the input  124  of the corresponding integrator amplifier. The structure and operation of this continuous-time modulator resembles thus far that of the discrete-time modulator of  FIG. 7 .  
         [0062]     A preferred embodiment of the nth stage of an FIR filter is shown in  FIG. 10 . The D input of the stage receives the Q signal from the corresponding stage of the shift register  118 . The input D is applied to the gates of a differential pair of transistors  132  and  133 , the gate of transistor  133  being supplied through an inverter  134 . The co-efficient of this stage of the filter A M  is defined by a transistor  135  connected in series between the common source of transistors  132  and  133  and ground. A current source transistor  136  is connected in series between the drain of the transistor  133  and the voltage supply and the junction between transistors  133  and  136  supplies the output current of the stage to the amplifier input  124 .  
         [0063]     In both CT and DT modulators, the first stage is the most critical. In fact, the circuit imperfections are dominated by those generated by the first stage. These imperfections are essentially thermal noise, harmonic distortion and jitter-induced noise. For this reason the first stage is the most power and area consuming block.  
         [0064]     Yet another embodiment of the present invention, shown in  FIG. 11 , and including FIR filters in the feedback path, is a mixed-time (MT) sigma-delta modulator, that is to say that the first integrator stage of the modulator is a continuous-time integrator and the other stages are discrete-time integrators. The first CT-integrator stage reduces drastically the power consumption and silicon area of the integrated circuit and the FIR filtering in the feedback path reduces its sensitivity to clock jitters; the subsequent DT integrator stage(s) are relatively insensitive to clock jitters so that, overall, the MT modulator is much less sensitive to clock jitters.  
         [0065]     In a practical implementation of the mixed-time sigma-delta modulator of the kind shown in  FIG. 11 , the first stage of the modulator, comprising adder  109 , integrator  110  and FIR filter  115 , having digital-to-analogue transfer function  127  is similar to the first stage of the CT-modulator described with reference to  FIGS. 8, 9  and  10 . A sampling switch  226  samples the output of the first integrator stage  110 . The following stages are discrete-time integrator stages comprising adders  11  and  13  and integrator stages  12  and  14  together with feedback paths  16  and  17 , similar to the second and higher order integrator stages and associated feedback stages described with reference to FIGS.  4  to  7 . The FIR filters  115 ,  16  and  17  in the preferred embodiment are again semi-digital low-pass filters. In a sub-ideal embodiment, the FIR filter  115  for the first integrator stage conveniently may be implemented as a comb filter.  
         [0066]     In a practical second order modulator of this kind, with gain of each of the integrator stages  110  and  12  of 0.5, and with approximately the ideal FIR filter coefficients, but enabling use of unit capacitors in simplified circuits, optimum values of the coefficients were calculated as follows, the figures being only an example of the choice made for one particular modulator:  
         [0067]     for the first stage FIR filter  115   
                                               a 0     a 1     a 2     a 3     a 4                     4/40   10/40   12/40   10/40   4/40                  
 
         [0068]     for the second stage FIR filter  16   
                                               b 0     b 1     b 2     b 3     b 4                     1   80/200   40/200   40/200   0                  
 
         [0069]     In calculating the improvement obtained with this type of circuit compared to the modulators of FIGS.  1  to  3 , the effect of clock jitters may be estimated, to a close approximation, by calculating the effect of pulse width jitters on the first stage. The transfer function of the filter can be written as:  
               F   N     =       ∑     m   =   0     M     ⁢           ⁢       f   m     ⁢     z     -   m                   Equation   ⁢           ⁢   10             
 
         [0070]     It can be shown that the white noise spectrum that results at the output from clock pulse width jitters at the first stage is:  
                 S   ɛ     ⁡     (   z   )       =       (       σ   τ   2       τ   2       )     ⁢     (       ∑     p   =   0     M     ⁢       ∑     q   =   0     M     ⁢       f   p     ⁢     f   q     ⁢       ℛ   y     ⁡     [     p   -   q     ]             )               Equation   ⁢           ⁢   11             
 
         [0071]     where R y  is the output auto-correlation function of the modulator. The improvement in the result at the modulator output of clock pulse width jitters at the first integrator stage compared to the modulator of  FIG. 3  is a factor IP of  
         (       ∑     p   =   0     M     ⁢       ∑     q   =   0     M     ⁢       f   p     ⁢     f   q     ⁢       ℛ   y     ⁡     [     p   -   q     ]             )     .       
 
         [0072]     To a first approximation, the modulator output spectrum due to clock pulse width jitters at the first stage is less than:  
                 S   ɛ     ⁡     (     ⅇ   jω     )       =         (       σ   τ   2       τ   2       )     ⁢     (       A   2     /   2     )       +   Θ             Equation   ⁢           ⁢   12             
 
         [0073]     where A is the amplitude of the input signal applied to the modulator and Θ is a factor related to N, the number of integrator stages, and M, the order of the FIR filters.  
         [0074]     Calculation shows that the worst case for the effect of clock jitters on the modulator output is when the input signal frequency is in the pass band of the low pass FIR comb filter  115  of the first integrator stage  110 . Two extreme cases can be distinguished: the noise tends towards a constant value for small signals, the noise value being smaller for higher order FIR filters. On the other hand, for large amplitude input signals, the output noise is signal dependent.  
                 S   ɛ     ⁡     (     ⅇ   jω     )       =     {               (       σ   τ   2       τ   2       )     ⁢     (   Θ   )       ⁢                   A   →   0                 (       σ   τ   2       τ   2       )     ⁢     (       A   2     /   2     )             A   →     A   max                       Equation   ⁢           ⁢   13             
 
         [0075]     The calculated values of S ε (e jω ) are shown for an example of a second order modulator in  FIG. 12 . At least an improvement of 3 dB may be obtained relative to the modulators of FIGS.  1  to  3 , even at large-signal amplitudes. For an input signal 3 dB less than the maximum of the input signal range, the improvement is 6 dB. If the input signal frequency falls in the stop band of the low pass filter, the improvement in large-signal performance of the system is higher.  
         [0076]     Ignoring quantization noise, the output signal-to-noise ratio of the modulator is given by:  
             SNR   =         (   OSR   )     ⁢       A   2     /   2         [         (       σ   τ   2     /     τ   2       )     ⁢     (         A   2     /   2     +   Θ     )       +     σ   th   2       ]               Equation   ⁢           ⁢   14             
 
         [0077]     where σ th   2  is the noise power and OSR is the over sampling ratio. The calculated values of SNR are shown in  FIG. 13  for the same modulator as  FIG. 12  second-order system for different values of the filter order M.  
         [0078]     A major advantage of this embodiment of the invention appears especially for small signals, which significantly extends the modulator dynamic range. The dynamic range represents the capability of the system in capturing weak signals and in many applications it is more critical than SNR max . The factor Θ with the ideal FIR filter coefficients in an example of a modulator with fourth order filters and an over sampling ratio of 32 is estimated at better than −17 dB.  
         [0079]     In addition to reducing the effect of clock jitters at the first, continuous-time integrator stage, the low pass FIR filters have a smoothing effect that reduces voltage variations at the output of the first integrator  110  and further improves linearity and bias current requirements in the subsequent discrete-time stages.  
         [0080]     Once again, it will be appreciated that the description given above with reference to FIGS.  8  to  11  of the accompanying drawings relates to basic sigma-delta modulator functions and that additional functions may be utilised. In particular, double sampling may be used, which consists of applying the feedback signal on both clock phases so as to balance the capacitive load seen by the integrator amplifiers during each clock phase. Once again, since the architecture using FIR filters comprises capacitors split into several capacitor elements, this need not involve the addition of supplementary capacitors, it being sufficient to divide the capacities of each FIR filter into two bunches of capacitor elements controlled by the different clock phases. In addition, feed forward, interpolation and resonance paths may be added.