Abstract:
A Delta-Sigma DAC is provided, comprising an interpolator, a Delta-Sigma modulator, a FIR filter and an analog filter. The interpolator oversamples a n-bit digital signal to generate a n-bit oversampled signal. The Delta-Sigma modulator coupled to the output of interpolator shapes the n-bit oversampled digital signal to generate a shaped digital signal. The FIR filter coupled to the Delta-Sigma modulator filters the shaped digital signal to generate an analog audio signal. The analog filter coupled to the FIR filter amplifies the analog audio signal to generate a audible signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/755,355, filed on Dec. 30, 2005. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to audio devices, and in particular, to anti-glitch devices in audio DACs. 
   2. Description of the Related Art 
     FIG. 1  shows a conventional Delta-Sigma DAC for converting a 16-bit digital signal to an audible signal Vout. The Delta-Sigma technique is popular because it achieves high resolution and quality with effective hardware implementations. An interpolator  102  receives an n-bit digital signal at a first sampling rate, and performs an interpolation to generate an n-bit output signal at a second, higher sampling rate. A Delta-Sigma modulator  104  receives the output signal from the interpolator  102  and shapes the quantization noises therein, thereby generating a shaped signal as a substantially linear analog representation of the 16-bit digital signal within a pass band. A DAC  106  then converts the shaped signal to an analog form, and a filter  110  filters high frequency noises therein to output the audible signal Vout. 
   When powering up, a system clock (not shown) requires a period of transient time to settle, and the Delta-Sigma modulator  104  also takes time to converge to stability. Random digital signals may be generated during the period, and amplified by the DAC  106  to output glitch noise. In the filter  110 , an inverter  120  is conventionally implemented to avoid power-up glitches. A logic high signal is input to the inverter  120  when powering up, thus the inverter  120  enters a high impedance (High-Z) mode that forms an equivalent open circuit for the output node A. In this way, the power-up glitches are not passed to the output of filter  110 . When the Delta-Sigma modulator  104  completes initialization, a zero pattern is output, and the inverter  120  returns to normal mode from the High-Z mode, passing the zero pattern to the filter  110 . The zero pattern does not generate audible sounds through the filter  110 . Additionally, a reference voltage Vref for the operational amplifier OP 1  is coupled to ground by a switch  112  according to the control signal #ctrl when powering up, and the filter  110  forms a unity gain buffer that is also capable of avoiding power-up glitches. The reference voltage Vref is typically cascaded to a large capacitor (not shown) to obtain higher SNR. When the inverter  120  returns from High-Z mode to normal mode, the control signal #ctrl simultaneously switches to a logic low, such that the reference voltage Vref gradually increases to its operating point according to the RC constant. 
   In  FIG. 1 , an alternative implementation provides an output switch  114  coupled to the output of the operational amplifier OP 1 . The output switch  114  as well as the switch  112 , may be a NMOS. When powering up, a control signal #ctrl of logic high is sent to the output switch  114 , coupling the audible signal Vout to ground. The power-up glitches output from the operational amplifier OP 1  are thus instantly avoided. 
   The High-Z mode solution, however, cannot be applied to finite impulse response (FIR) based Delta-Sigma DACs or switched capacitor architectures. Additionally, the zero patterns generated from the Delta-Sigma modulator  104  may still render glitches since the duty cycle transient of the zero pattern is unpredictable for the filter  110 . The output switch  114  may not effectively pull the audible signal Vout to ground because the operational amplifier OP 1  may output a significantly large loading. An improved anti-glitch circuit is therefore desirable. 
   BRIEF SUMMARY OF THE INVENTION 
   A detailed description is given in the following embodiments with reference to the accompanying drawings. 
   A Delta-Sigma DAC is provided, comprising an interpolator, a Delta-Sigma modulator, a FIR filter and an analog filter. The interpolator oversamples a n-bit digital signal to generate a n-bit oversampled signal. The Delta-Sigma modulator coupled to the output of interpolator, shapes the n-bit oversampled digital signal to generate a shaped digital signal. The FIR filter coupled to the Delta-Sigma modulator filters the shaped digital signal to generate an analog audio signal. The analog filter coupled to the FIR filter, amplifies the analog audio signal to generate a audible signal. 
   When the Delta-Sigma DAC powers up, a mute signal is enabled to disable the analog filter, thus the audible signal is not output. When the shaped digital signal comprises a zero pattern, the mute signal is disabled, and the analog filter is enabled to output the audible signal. 
   The analog filter may comprise an operational amplifier, a passive component, and a first switch. The operational amplifier controlled by the mute signal, comprises a first input node receiving the analog audio signal, a second input node receiving a reference voltage, and an output node outputting the audible signal. The passive component is coupled to the output node and first input node of the operational amplifier. The first switch is coupled to the second input node of the operational amplifier, receiving a control signal. When the Delta-Sigma DAC powers up, the control signal is enabled, such that the first switch couples the reference voltage to a relative ground, and the mute signal is simultaneously enabled, such that the output node of operational amplifier is coupled to the relative ground. When the shaped digital signal comprises a zero pattern, the control signal is disabled, and the reference voltage is input to the second input node. A second switch may further be coupled to the output node, controlled by the control signal. When the control signal is enabled, the second switch couples the output node of operational amplifier to the relative ground. 
   The operational amplifier comprises three stages. A differential input stage having the first and second input nodes, receives the analog audio signal and the reference voltage. A gain stage coupled to the differential input stage, adjusts gain of the output therefrom. An output stage coupled to the gain stage, has the output node that renders the audible signal. The mute signal may be sent to the output stage. When the mute signal is enabled, the output stage couples the output node to the relative ground. 
   The output stage may comprise at least three MOS devices. A first PMOS has a source coupled to a power supply, and a drain coupled to the output node. A first NMOS has a drain coupled to the output node, and a source coupled to a power sink. A second PMOS has a source coupled to the power supply, a drain coupled to the gate of first NMOS, and a gate coupled to the mute signal. When the mute signal is enabled as a logic low, the second PMOS and first NMOS are activated, and the output node is pulled to the power sink as the relative ground. Alternatively, the second PMOS may be substituted by a second NMOS. Thus when the mute signal is enabled as a logic high, the second NMOS and first NMOS are activated, and the output node is pulled to the power sink as the relative ground. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
       FIG. 1  shows a conventional Delta-Sigma DAC; 
       FIG. 2  shows an embodiment of a Delta-Sigma DAC according to the invention; 
       FIG. 3  shows an embodiment of the analog filter  320  according to  FIG. 2 ; 
       FIGS. 4 and 5  show embodiments of the output stage  430  in  FIG. 3 ; and 
       FIGS. 6   a  and  6   b  show embodiments of the band-gap circuit  312  in  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
     FIG. 2  shows an embodiment of a Delta-Sigma DAC according to the invention. The Delta-Sigma DAC comprises an interpolator  102 , a Delta-Sigma modulator  104 , a FIR filter  210  and an analog filter  320 . The analog filter  320  comprises a passive component  302 , an operational amplifier  304 , a first switch  306  and a second switch  308 . The passive component  302  is coupled to the output node and first input node of the operational amplifier  304 . The first switch  306  is coupled to the second input node of the operational amplifier  304 , receiving a control signal #ctrl. The operational amplifier  304  comprises a first input node, a second input node, and an output node. The first input node receives the analog audio signal, the second input node receives a reference voltage Vref, and the output node outputs the audible signal. The operational amplifier  304  is a modified version that outputs no signal when powering up. When the Delta-Sigma DAC powers up, a mute signal #en is sent to the analog filter  320 , and the output of operational amplifier  304  is disabled accordingly, thus no audible signal Vout is output. Specifically, the mute signal #en disables the operational amplifier  304  by coupling the output node of operational amplifier  304  to the relative ground. Simultaneously, the control signal #ctrl is enabled when the Delta-Sigma DAC powers up, such that the first switch  306  couples the reference voltage Vref to the relative ground. In this way, the operational amplifier  304  functions as an unity gain buffer that economizes unnecessary driving power. When the Delta-Sigma DAC completes the power up initialization, the shaped digital signal generated from the Delta-Sigma modulator  104  comprises a zero pattern, such that the control signal #ctrl and the mute signal #en are disabled, and the reference voltage Vref is sent to the analog filter  320  via the second input node, making the analog filter  320  operative to output the audible signal Vout. 
     FIG. 3  shows an embodiment of the analog filter  320  according to  FIG. 2 . The Delta-Sigma DAC may further comprise a second switch  308  coupled to the output node, controlled by the control signal #ctrl that controls the first switch  306 . When the control signal #ctrl is enabled, the second switch  308  couples the output node of operational amplifier  304  to the relative ground, providing a further guarantee to avoid power-up glitches. The first switch  306  and second switch  308  may be simultaneously NMOS, and the control signal #ctrl is enabled as a logic high. Otherwise, if the first switch  306  and second switch  308  are identically implemented by PMOS, the control signal #ctrl is enabled as a logic low. 
   In  FIG. 3 , the operational amplifier  304  comprises a differential input stage  410 , a gain stage  420  and an output stage  430 . The differential input stage  410  has the first and second input nodes, receiving the analog audio signal and the reference voltage Vref. The gain stage  420  is coupled to the differential input stage  410 , adjusting gain of the output therefrom. The output stage  430  is coupled to the gain stage  420 , having the output node that renders the audible signal. The three stages are based on conventional operational amplifier architecture, and the embodiment provides a modified output stage  430  to avoid power-up glitches. The mute signal #en is sent to the output stage  430 . When the mute signal is enabled, the output stage  430  couples the output node to the relative ground. The passive component  302  may be a RC circuit comprising a capacitor C 1  and a resistor R 1  cascaded in parallel. The operational amplifier  304  is powered by a power supply +Vdd and a power sink power sink −Vdd, whereas the power sink power sink −Vdd may also be referred to as the relative ground. 
     FIGS. 4 and 5  show embodiments of the output stage  430  in  FIG. 3 . In  FIG. 4 , the output stage  430  comprises a first PMOS MP, a first NMOS MN and a second PMOS M 1 . The first PMOS MP has a source coupled to power supply +Vdd, and a drain coupled to the output node. The first NMOS MN has a drain coupled to the output node, and a source coupled to power sink power sink −Vdd. The second PMOS M 1  has a source coupled to power supply +Vdd, a drain coupled to the gate of first NMOS MN, and a gate coupled to the mute signal #en. When the mute signal #en is enabled as a logic low, the second PMOS M 1  and first NMOS MN are activated, and the audible signal Vout is pulled to power sink −Vdd. 
     FIG. 5  shows an alternative implementation of the output stage  430 . The second PMOS M 1  is substituted by a second NMOS M 2  having a drain coupled to power supply +Vdd, a source coupled to the gate of first NMOS MN, and a gate coupled to the mute signal. When the mute signal is enabled as a logic high, the second NMOS M 2  and first NMOS MN are activated, and the audible signal Vout is pulled to the power sink −Vdd. 
   In  FIGS. 4 and 5 , the first NMOS MN and first PMOS MP form a push-pull circuit in the output stage of the operational amplifier, and the gates thereof may couple to a PMOS MB for bias control. The implementation varies from different operational amplifiers, thus the major concept of the invention is to provide a modified logic that pulls the audible signal Vout of the operational amplifier  304  to ground. Alternatively, the pull down mechanism may be implemented in other stages of the operational amplifier. 
     FIGS. 6   a  and  6   b  show embodiments of the band-gap circuit  312  in  FIG. 3 . In  FIG. 6   a , the band-gap circuit  312  comprises a first resistor Ra and a first capacitor Ca, parallel coupled to a reference node and the relative ground. A second resistor Rb is coupled to a band-gap voltage source and the reference node. The reference voltage Vref is output from the reference node. In  FIG. 6   b , a band-gap current source, rather than the second resistor Rb, is coupled to the reference node, providing the reference voltage Vref through the reference node. 
   While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.