Digital to analogue converter comprising a third order sigma delta modulator

Abstract: The present invention refers to a digital analogical converter comprising a sigma delta cascade modulator having two outputs, particularly a third order sigma delta modulator 2+1. In an embodiment the digital analogical converter comprises: a sigma delta modulator (1) of the type having two outputs (67, 68) able to supply a first (Y1) and a second (Y2) signal to said two outputs (67, 68); a reconstruction circuit (2) of first said (Y1) and second (Y2) signal able to provide a reconstructed signal (Yout); a filter (3) able to filter said reconstructed signal (Yout); characterized in that said reconstruction circuit (2) combines said first (Y1) and second (Y2) signals according to the following relationship Yout= Y1* (1+ Z -1 ) - Y2* (1- Z -1 ) + Y2* Z -2 * (1- Z -1 ) where Yout corresponds to said reconstructed signal, Y1 corresponds to said first signal, Y2 corresponds to said according to signal, Z corresponds to the Z transform.

Description: The present invention refers to a digital analogical converter comprising a sigma delta cascade modulator having two outputs, particularly a third order sigma delta 2+1 modulator. The two output signals of a third order sigma delta modulator must be combined by means of an appropriate reconstruction transfer function to get a correct output signal. A digital analogical converter comprises a sigma delta modulator having two outputs, which are connected to a reconstruction circuit and then to a levelling filter. The reconstruction circuit transfer function depends on the modulator topology. In the case of a sigma delta modulator of the type 2+1, the reconstruction transfer function is the following: Yout= Y1- (1- Z -1 ) 2 * Y2 where Yout is the combined signal, Y1 is the signal present at an output, Y2 is the signal present at the other output and Z corresponds to the Z transform. The transfer function should be realized at low cost and it should not influence the system performances in terms of signal to noise ratio and distortion. An embodiment of a reconstruction circuit is effected by means of digital circuits. The signal after the reconstruction has a resolution of a thermometric code with 6 bit, so that a digital analogical converter is necessary to send the signal to the levelling filter that normally is analogical. The resolution of this digital analogical converter is equal to that of the whole structure, therefore are necessary linearisation techniques to reach the requested distorsion performances. Another embodiment of the reconstruction circuit is realized by means of analogical circuits. This system is based on the fact that the levelling filter is constituted by a cascade of integrators. In fact if the signal is applied to the input of an integrator inside the cascade, its equivalent transfer function is composed by N derivatives in cascade where N is the number of integrators between the filter main input and this additional input. Since the reconstruction function generally uses only derivatives and multiplier factors to adapt the filter coefficients, by opportunely choosing the additional input point it is possible to realize the desired transfer function. In the case of the use of the transfer function above, the signal Y1 is applied to the filter main input, the signal Y2 to the third integrator input, and the opportune multiplier factors are chosen to get the desired gain. The disadvantages are in the fact that in order to reach the desired reconstruction function structure limitations are necessary and the performances depend on the capacitor tolerances and on the parameters of the operational amplifiers used as integrators, like the finished gain values, the speed and the rise time. The patent US 5,072,219 discloses a digital-analog conversion system which comprises a digital modulator having several quantification levels formed by a second order Delta-Sigma modulator and a digital-analog converter and switched capacitors filter set whose progression law between the different analog levels is independent of the absolute and relative values of the constituent components of the said assembly. The patent application WO9706601 discloses a highly linear switched capacitor digital to analog converter, which includes an array of holding capacitors connected in parallel to an operational amplifier having a finite gain. A conversion period is divided into subintervals and each holding capacitor is associated with one of the subintervals. It also includes switches for transferring a packet of charge to each holding capacitor during the subinterval associated with the holding capacitor. The patent US5477481 discloses a switched capacitor integrator with chopper stabilisation performed at the sampling rate that virtually eliminates the flicker noise and any low frequency interference generated by the amplifier. The integrator samples the input and then passes the sampled input to the feedback capacitor during each chopping phase of the amplifier to thereby provide a double sampled integrator. The output of the integrator is sampled at the end of each cycle of the chopping signal. An object of the present invention is to realize a digital analogical converter comprising a sigma delta modulator of the type having two outputs and a reconstruction circuit which does not require a digital analogical converter, is insensitive to the operational amplifier parameters and does not impose limits in the choice of the levelling filter. In accordance to the present invention, these and other objects are attained by means of a digital analogical converter according to claim 1. Thanks to the present invention it is possible to realize a low cost analogical type reconstruction circuit, simple and with good performances in terms of signal noise ratio and distorsion. The characteristics and the advantages of the present invention will result evident from the following detailed description of one embodiment, illustrated as not limitative example in the joined drawings, in which: Figure 1 shows a digital analogical converter comprising a sigma delta of the type having two outputs modulator; Figure 2 shows a sigma delta modulator of the third order of the type 2+1; Figure 3 shows an integrator with switched capacitors; Figure 4 shows a realization example of a reconstruction circuit of a digital analogical converter in accordance to the present invention. Figure 1 shows a sigma delta modulator 1 having an input signal X and two output signals Y1 and Y2. The signals Y1 and Y2 are supplied to a reconstructed circuit 2 which provides a reconstructed signal Yout on its output obtained by the combination of the Y1 and Y2 signals. The reconstructed signal is then applied to a levelling filter 3. In Figure 2 a sigma delta modulator of the third order of the type 2+1 is reported. It has an input signal X and two output signals Y1 and Y2. It comprises a second order modulator based on the quantizer 10, which produces the output signal Y1, and a first order modulator based on the quantizer 15 which produces the output signal Y2. The input signal X is applied to an adder 14 to which the signals are applied as inputs which are in output from a delay circuit 13 and from a circuit 12 which effects the multiplication of the signal present at its input by a multiplier factor equal to -2. The output signal of the adder node 14 is applied in input to the quantizer 10 which provides the output signal Y1. The quantizer 10 provides another output signal, that is the signal relative to the quantization error e1 which is applied to the input of a delay circuit 11. The output of the delay circuit 11 is applied to the inputs of the delay circuit 13 and of the circuit 12. The signal relative to the quantization error e1 is also applied to an adder node 17 to which is also applied as input the signal outputted from a delay circuit 16 which also effects the inversion of the signal sign. The output signal of the adder node 17 is applied to the input of the quantizer 15 which provides the output signal Y2. The quantizer 15 provides another output signal, that is the signal relative to the quantization error e2 which is applied to the input of the delay circuit 16. According to the present invention the reconstruction transfer function used is the following: Yout= (Y1- (1- Z -1 ) 2 * Y2)* (1+ Z -1 ) where Yout is the combined signal, Y1 is the signal present at a modulator output, Y2 is the signal present at the other modulator output and Z corresponds to the Z transform. In the transfer function (1) a zero (1+ Z -1 ) has been inserted at a half of the sampling frequency to reduce the quantization noise at high frequency. After some simple mathematical calculations the previous transfer function (2) becomes: Yout=Y1* (1+Z -1 )-Y2* (1-Z -1 )+Y2* Z -2 * (1-Z -1 ) In this way the transfer function results the sum of three terms. The signals Y1 and Y2 are signals that can have only two values. For this transfer function implementation delays are requested which are realizable by means of flip-flops and sign inversions that can be realized by means of inverters. Besides there are circuits able to realize the function (1-Z -1 ) like that reported in Figure 3. In Figure 3 it has been shown an operational amplifier 30, which provides an output voltage Vout, which has a capacitor C1 applied between its inverting input and its output. Its not inverting input is connected to a prefixed bias voltage Vp1. At the inverting input a terminal of a switch 36 is also connected, the other terminal of which is connected to a terminal of a switch 35 and to a terminal of a capacitor C2.The other terminal of the switch 35 is connected to a prefixed bias voltage Vp2. The other terminal of the capacitor C2 is connected to a terminal of a switch 33 and to a terminal of a switch 34. To the other terminal of the switch 33 the signal V1 is connected. To the other terminal of the switch 34 the signal V2 is connected. The switches reported in Figure 3 are controlled switches, particularly the switches 33 and 35 are controlled by a same signal having a prefixed frequency; the switches 34 and 36 are controlled by a same signal having a prefixed frequency equal to that used for the switches 33 and 35 but out of phase of half a period. The switch control frequency is equal to the sampling frequency of the input signal. Sampling that is effected at the input of the modulator 1. The transfer function of the circuit reported in Figure 3 is the following: Vout=[(C2/ C1)* (Z -1 * V1- V2)] / (1-Z -1 ) Figure 4 shows an embodiment of a reconstruction circuit of a digital analogical converter in accordance with the present invention. It is noted that the input stage of the levelling filter 3 is shown which comprises an operational amplifier 40 which has a capacitor 41 applied between its inverting input and its output. Its not inverting input is connected to a prefixed bias voltage Vp6. At the inverting input is also connected the signal Yout coming out from the reconstruction circuit 2 and present on the node 42. The reconstruction circuit 2 comprises a first branch that includes a switch 43 connected between the node 42 and a node 58, a switch 44 connected between the node 58 and a prefixed bias voltage Vp3, a capacitor 55 connected between the node 58 and a node 61, a switch 45 connected between the node 61 and a node 67 to which the signal Y1 is applied, a switch 46 connected between the node 61 and the output of an inverter 64 the input of which is applied to the node 67. The reconstruction circuit 2 comprises a second branch that includes a switch 47 connected between the node 42 and a node 59, a switch 48 connected between the node 59 and a prefixed bias voltage Vp4, a capacitor 56 connected between the node 59 and a node 62, a switch 49 connected between the node 62 and a node 69, a switch 50 connected between the node 62 and the node 69. At the node 69 the output of an inverter 65 is applied, the input of which is applied to the output of a double delay circuit 66, the input of which is applied to the node 68 to which the signal Y2 is applied. The reconstruction circuit 2 comprises a third branch that includes a switch 51 connected between the node 42 and a node 60, a switch 52 connected between the node 60 and a prefixed bias voltage Vp5, a capacitor 57 connected between the node 60 and a node 63, a switch 54 connected between the node 63 and a node 68 to which the signal Y2 is applied, a switch 53 connected between the node 63 and the node 68. The switches of Figure 4 are switches controlled by two signals having the same frequency (equal to the sampling frequency of the input signal ) but out of phase of half period. Particularly the switches 44, 45, 48, 49, 52 and 53 are controlled by a signal, and the switches 43, 46, 47, 50, 51 and 54 are controlled by the other signal. The control signals are generated by suitable not shown frequency generators. The circuit of Figure 4 is able to realize the transfer function (3) in an analogical way. It results being sensible to the capacitor tolerances but with the normally attainable tolerance value it is possible to reach the requested performances. For instance with a capacitor construction tolerance of 3 x 1000 it is possible to obtain a signal to noise ratio higher than 100 dB.

Claims: Digital analogical converter comprising: a sigma delta modulator (1) of the type having a first (67) and a second (68) output able to supply a first (Y1) and a second (Y2) signal respectively; a reconstruction circuit (2) of first said (Y1) and second (Y2) signals able to provide a reconstructed signal (Yout); a filter (3) able to filter said reconstructed signal (Yout); characterized in that said filter (3) comprises an operational amplifier (40) having a positive input and a negative input, said negative input is connected to a first node (42); said reconstruction circuit (2) comprises a first switch (43) connected between said first node (42) and a second node (58); a second switch (47) connected between said first node (42) and a third node (59); a third switch (51) connected between said first node (42) and a fourth node (60); a fourth switch (44) connected between said second node (58) and a first prefixed bias voltage (Vp3); a fifth switch (48) connected between said third node (59) and a second prefixed bias voltage (Vp4); a sixth switch (52) connected between said fourth node (60) and a third prefixed bias voltage (Vp5); a first capacitor (55) connected between said second node (58) and a fifth node (61); a second capacitor (56) connected between said third node (59) and a sixth node (62); a third capacitor (57) connected between said fourth node (60) and a seventh node (63); a seventh switch (45) connected between said fifth node (61) and a first input; an eighth switch (49) connected between said sixth node (62) and a third input; a ninth switch (53) connected between said seventh node (63) and a second input; a tenth switch (46) connected between said fifth node (61) and a fourth input; an eleventh switch (50) connected between said sixth node (62) and a third input; a twelfth switch (54) connected between said seventh node (63) and a second input; said first input is applied to said first output (67); said fourth input is applied to said first output (67) inverted; said third input is applied to said second output (68) delayed twice and inverted; said second input is applied to said second output (68) and said reconstruction circuit (2), combines said first (Y1) and second (Y2) signals according to the following relationship Yout= Y1* (1+ Z -1 ) - Y2* (1 - Z -1 ) + Y2* Z -2 * (1-Z -1 ) where Yout corresponds to said reconstructed signal, Y1 corresponds to said first signal, Y2 corresponds to said second signal, Z corresponds to the Z transform. Converter according to claim 1 characterized in that said reconstruction circuit (2) is of analogical type. Converter according to claim 1 characterized by comprising a fourth capacitor (41) connected between said negative input and an output of said operational amplifier. Converter according to claim 1 characterized in that said positive input is connected to a fourth prefixed bias voltage (Vp6). Converter according to claim 1 characterized by comprising a first generator of an opening and closing signal of said fourth (44), fifth (48), sixth (52), seventh (45), ninth (53) and eighth (49) switches. Converter according to claim 1 characterized by comprising a second generator of an opening and closing signal of said first (43), second (47), third (51), eleventh (50), tenth (46) and twelfth (54) switches. Converter according to claim 1 characterized in that the signal present on the first node (42) corresponds to the reconstructed signal (Yout) of said first (Y1) and second (Y2) output signal of said sigma delta modulator (1). Converter according to claim 7 characterized in that said reconstructed signal (Yout) is filtrated by a levelling filter (3) and that said operational amplifier (40) belongs to said filter (3).

Inventors: BASCHIROTTO ANDREA, COLONNA VITTORIO, CUSINATO PAOLO, GANDOLFI GABRIELE, BASCHIROTTO, ANDREA, COLONNA, VITTORIO, CUSINATO, PAOLO, GANDOLFI, GABRIELE
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 2000
Main classification: H03M3
IPC codes: H03M3