Abstract:
Hybrid digital-to-analog converter and method thereof are provided. The hybrid digital-to-analog converter (DAC) includes a data processor, at least one first type DAC, at least one second type DAC, and an output circuit. The data processor processes an input digital signal to output at least one of first and second digital signals which are related to a higher bit portion and a lower bit portion of the input digital signal, respectively. If the data processor outputs the first digital signal to the first type DAC, the first type DAC converts the first digital signal. The at least one second type DAC receives and converts the second digital signal outputted from the data processor. The output circuit receives at least one output signal of the first and the second type DACs to output an output analog signal.

Description:
[0001]    This application claims the benefit of Taiwan application Serial No. 101147348, filed Dec. 14, 2012, the subject matter of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a hybrid digital-to-analog converter and a method thereof. 
         [0004]    2. Description of the Related Art 
         [0005]    Digital-to-analog converters (DACs) play an important role in modern electronic or communication system. The quality of the DAC may intensively affect the whole performance of the electronic or communication system. 
         [0006]    The DAC processes not only digital signals with larger voltage swing, but also small signals. It is inevitable that the currently available DAC is required to handle the issue of non-linear distortion when processing digital signals with large voltage swing, and to be designed considering the issues of power dissipation and circuit area. 
         [0007]    Thus, the disclosure in the following provides a hybrid digital-to-analog converter, concerned with the above issues. 
       SUMMARY OF THE INVENTION 
       [0008]    The disclosure is directed to a hybrid digital-to-analog converter and a method thereof. In an embodiment, large signals are processed mainly by a switched capacitance digital-to-analog converter, which is suitable for processing large signals. In addition, small signals are processed mainly by a current-steering digital-to-analog converter, which is suitable for processing small signals. 
         [0009]    According to an exemplary embodiment of the disclosure, a hybrid digital-to-analog converter is provided. The hybrid digital-to-analog converter includes a data processor, at least one first type digital-to-analog converter, at least one second type digital-to-analog converter, and an output circuit. The data processor processes an input digital signal to output at least one of a first digital signal and a second digital signal, wherein the first digital signal and the second digital signal are related to a higher bit portion and a lower bit portion of the input digital signal, respectively. The at least one first type digital-to-analog converter is coupled to the data processor, wherein if the data processor outputs the first digital signal to the first type digital-to-analog converter, the first type digital-to-analog converter converts the first digital signal. The at least one second type digital-to-analog converter, coupled to the data processor, is used for receiving and converting the second digital signal outputted from the data processor. The output circuit, coupled to the first type digital-to-analog converter and the second type digital-to-analog converter, is used for receiving at least one output signal of the first type and the second type digital-to-analog converters to output an output analog signal. 
         [0010]    According to another exemplary embodiment of the disclosure, a hybrid digital-to-analog conversion method is provided, including the following. (a) An input digital signal is processed to obtain at least one of a first digital signal and a second digital signal, wherein the first digital signal and the second digital signal are related to a higher bit portion and a lower bit portion of the input digital signal, respectively. (b) A first type digital-to-analog conversion is performed on the first digital signal if the step (a) obtains the first digital signal. (c) A second type digital-to-analog conversion is performed on the second digital signal. (d) At least one output signal by the first type and the second type digital-to-analog conversion is received to output an output analog signal. 
         [0011]    The above and other aspects of the invention will become better understood with regard to the following detailed description of the preferred but non-limiting embodiment(s). The following description is made with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a block diagram illustrating a hybrid digital-to-analog converter  100  according to an embodiment of the disclosure. 
           [0013]      FIG. 2  is a functional block diagram illustrating the data processor  110  according to an embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    In an embodiment of the disclosure, a hybrid digital-to-analog converter includes different types of digital-to-analog converters (DACs), such as, without limited to, current-steering DACs and switched capacitance DACs. In the process for large digital signals, a switched capacitance DAC is employed to process large signals so as to achieve high linearity. In addition, in the process for small digital signals, a current-steering DAC is utilized for processing small signals, thus reducing the required number of chips of current source, the required area, and power dissipation. 
         [0015]    Referring to  FIG. 1 , a hybrid digital-to-analog converter  100  is illustrated according to an embodiment in a block diagram. As shown in  FIG. 1 , the hybrid digital-to-analog converter  100  includes: a data processor  110 , a current-steering DAC  120 , a switched capacitance DAC  130 , an operating amplifier OP, a plurality of resistors R F , and a plurality of capacitors C F , wherein V CM  is a common mode voltage, an Vo+ an Vo− are the outputs of the hybrid digital-to-analog converter  100 . The operating amplifier OP, resistors R F , and capacitors C F  can be referred to as an output circuit. The current-steering DAC  120  includes a plurality of switching current units  121 ; and the switched capacitance DAC  130  includes a plurality of switched capacitance units  131 . 
         [0016]    In  FIG. 1 , while the hybrid digital-to-analog converter  100  is exemplified by a fully-differential hybrid digital-to-analog converter for the sake of explanation, it is to be understood that the disclosure is not limited thereto. In other embodiments, the hybrid digital-to-analog converter can be implemented by using a single-ended hybrid digital-to-analog converter. One ordinary skill in the art can derive the implementation of the hybrid digital-to-analog converter with a single-ended hybrid digital-to-analog converter, according to the specification and its spirit. All the embodiments according to the specification and its spirit are included in the scope of the disclosure. 
         [0017]    The data processor  110  is used for processing an input digital signal IN to obtain a higher bit group M and a lower bit group N. Signals Mb and Nb are inverted bits of the higher bit group M and the lower bit group N, respectively. How the data processor  110  processes the input digital signals to obtain the high bit group M and the low bit group N will be described later. For the sake of convenience of naming, the bit group M and Mb are referred to as higher bit groups and the bit group N and Nb are referred to as lower bit groups. 
         [0018]    The current-steering DAC  120  performs digital-to-analog conversion on the lower bit groups N and Nb transmitted from the data processor  110 . In general, the current-steering DAC, when processing signals with larger voltage swing, has its resistance easily varying with the voltage variation due to the high sensitivity of the resistance to voltage, thus causing serious non-linear distortion. In addition, in high signal-to-noise ratio (SNR) applications (SNR is measured in a small signal mode), a current source is required to occupy a lot of circuit area so as to reduce the mismatch of current sources and flicker noise. Further, the current-steering DAC  120  has a circuit bandwidth of 1/(2π*(1/(R F *C F ))). 
         [0019]    The switched capacitance DAC  130 , when processing large signals, has better linearity since the sensitivity of its internal circuitry to voltage is low. However, in high SNR applications, the switched capacitance DAC  130  requires a high cost of circuit area. Since the thermal noise (KT/C noise) relates to the internal capacitance C S  of the switched capacitance DAC  130 , the area of the capacitance C S  has to be larger so as to reduce the thermal noise. Besides, the area of the capacitance C F  has a multiple relationship with the capacitance C S , and the capacitance C F  is far greater than the capacitance C S . The area of the capacitance C F  is then very large, making the whole area fairly large. In addition, the switched capacitance DAC  130  has a bandwidth of k*(C S /C F )*F S , wherein k is a constant, and F S  is a sampling frequency. 
         [0020]    Accordingly, in the embodiment, in the processing of large signals, large signals are processed mainly by the switched capacitance DAC  130 . This is because the capacitance of the switched capacitance DAC  130  has a low sensitivity to voltage, thus achieving the requirement for high linearity. In the embodiment, in the processing of small signals (e.g., measuring SNR in a small signal mode), small signals are processed mainly by the current-steering DAC  120 . It is because the current variation of the small signal is small relatively, the number of chips of current source is small, and the area is also small. 
         [0021]    The capacitance C F  can be regarded as a part of a low pass filter, coupled between the input and output terminals of the operating amplifier OP. 
         [0022]    The structure and operation of the data processor  110  is described according to an embodiment. Referring to  FIG. 2 , the data processor  110  is illustrated according to the embodiment in a functional block diagram. The data processor  110  as illustrated in  FIG. 2 , for example, is applied to an oversampling system, and it is understood the embodiment is not limited thereto. In the embodiment, the data processor  110  includes: a binary-to-thermometer decoder  210 , and a sigma-delta modulator (SDM)  220 , data weighted averaging (DWA) units  230  and  240 . 
         [0023]    As an example, the input digital signal IN indicates 24-bit binary digital data. First, the data processor  24  divides the 24-bit digital signal into two portions: one portion is a digital signal U of 6 bits (which are high bits) and the other portion is a digital signal L of 18 bits (which are low bits). While the input digital signal of other number of bits can be processed and the input digital signal can be divided in different ways, it is understood that the embodiment is not limited thereto. 
         [0024]    The 6-bit digital signal U is decoded by the binary-to-thermometer decoder  210  into a thermometer code T1, wherein the thermometer code T1 can represent 0 to 63 levels, for example, and it is understood that the embodiment is not limited thereto. The binary-to-thermometer decoding is not limited in detail here, and can be implemented in different ways. It is also understood that the number of levels that the thermometer code T 1  can represent is not limited thereto. In addition, the number of the switched capacitance units  131  included in the switched capacitance DAC  130  can be equal to or related to the number of levels that the thermometer code T1 can represent. 
         [0025]    The 18-bit digital signal L is decoded by the SDM  220  into a thermometer code T2, wherein the thermometer code T2 can represent 0 to 3 levels, for example, and it is understood that the embodiment is not limited thereto. The sigma-delta modulation is not limited in detail here, and can be implemented in different ways. It is also understood that the number of levels that the thermometer code T2 can represent is not limited thereto. In addition, the number of the switching current units  121  included in the current steering DAC  120  can be equal to or related to the number of levels that the thermometer code T2 can represent. 
         [0026]    The data weighted averaging units  230  and  240  perform data weighted averaging on the first thermometer code T1 and the second thermometer code T2 to generate the higher bit groups (M and Mb) and the lower bit groups (N and Nb). The data weighted averaging is not limited in detail here, and can be implemented in different ways. After the data weighted averaging, the higher bit groups (M and Mb) can represent 0 to 63 levels and the lower bit groups (N and Nb) can represent 0 to 3 levels, for example. 
         [0027]    Accordingly, the higher bit groups (M and Mb) and the lower bit group (N and Nb) obtained as above can facilitate the whole DAC linearity, so that the hybrid digital-to-analog converter of the embodiment provides better linearity. 
         [0028]    In addition, a converted voltage corresponding to one level in the lower bit groups (N and Nb) is equal to a converted voltage corresponding to one level in the higher bit groups (M and Mb). 
         [0029]    When performing digital-to-analog conversion, the hybrid digital-to-analog converter of the embodiment performs digital-to-analog conversion on the signal N if it is determined that the input signal is a small signal, or performs digital-to-analog conversion on both the signals M and N if it is determined that the input signal is a large signal. 
         [0030]    It will be explained in the following. If the data processor  110  determines that the input digital signal IN is greater than a threshold value, (e.g., the input digital signal is of 24 bits, and the threshold value can be set to 000000111111111111111111), that is, the data processor  110  determines that the input digital signal IN is a large signal, the data processor  110  then outputs the signals M and Mb to the switched capacitance DAC  130 , so that the switched capacitance DAC  130  performs digital-to-analog conversion on the signals M and Mb. In this situation, the data processor  110  sends the signals N and Nb to the current-steering DAC  120 . 
         [0031]    Conversely, if the data processor  110  determines that the input digital signal IN is smaller than or equal to the threshold value, that is, the data processor  110  determines that the input digital signal IN is a small signal, the data processor  110  then outputs the signals N and Nb to the current-steering DAC  120 , so that the current-steering DAC  120  performs digital-to-analog conversion on the signals N and Nb. In this situation, there are no signals M and Mb, and the data processor  110  will not send the signals M and Mb to the switched capacitance DAC  130 . 
         [0032]    As exemplified above, if the data processor  110  determines that the input digital signal IN is smaller than or equal to the threshold value, the input digital signal IN is a small signal; if the data processor  110  determines that the input digital signal IN is greater than the threshold value, the input digital signal IN is a large signal. In other embodiments, if the data processor  110  determines that the input digital signal IN is smaller than the threshold value, the input digital signal IN is a small signal; if the data processor  110  determines that the input digital signal IN is greater than or equal to the threshold value, the input digital signal IN is a large signal. 
         [0033]    The data processor  110  controls the switching current units  121  of the current-steering DAC  120  according to the signals N and Nb. Specifically, the data processor  110  controls, for example, the switching of internal switches of the switching current units  121  of the current-steering DAC  120  according to the signals N and Nb. Likewise, the data processor  110  controls the switched capacitance units  131  of the switched capacitance DAC  130  according to the signals M and Mb. Specifically, the data processor  110  controls, for example, the switching of internal switches of the switched capacitance units  131  of the switched capacitance DAC  130  according to the signals M and Mb. 
         [0034]    The hybrid digital-to-analog converter of the embodiment can be applied to any electronic system with DAC(s), such as, an analog-to-digital converter (ADC) having DAC(s), a communication system, and so on, without limited thereto. 
         [0035]    While the invention has been described by way of example and in terms of the preferred embodiment(s), it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.