Abstract:
There is described an input stage for an A/D converter, comprising a transconductance element adapted to receive, at a first input of the transconductance element, an analog input signal that is to be converted to a digital signal by the A/D converter, a feedback path for providing an analog feedback signal to a second input of the transconductance element, the analog feedback signal being based on a digital output signal of the A/D converter, and an integrator for integrating an output current of the transconductance element, wherein the integrating element is adapted to generate an integrator output signal representative of the integrated output current. There is also described an A/D converter comprising such an input stage and a system comprising a plurality of such A/D converters.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority under 35 U.S.C. §119 of European patent application no. 13197951.0, filed on Dec. 18, 2013, the contents of which are incorporated by reference herein. 
       FIELD 
       [0002]    The present invention relates to the field of A/D converters, in particular to input stages for such converters. 
       BACKGROUND 
       [0003]    In A/D converters (analog to digital converters), the input stage frequently is the most critical part of the converter in terms of noise and linearity. To increase linearity, degeneration of the input transistor pair is often used, but this at the same time increases noise. Another solution is to use an input stage which has feedback to input of the amplifier (like an inverting amplifier), but needs a resistive input, and therefore the input impedance of the A/D converter will be finite, which is not always desirable, in particular when the A/D converter needs to interface with a sensor. Furthermore, such feedback increases power consumption. 
         [0004]    Furthermore, in sensor applications it is often required to have signal processing paths that provide adequate gain matching between the different channels. For instance, the output signals of a magnetic angular sensor are respectively proportional to the sine and cosine of the angle to be measured, the ratio of which can be processed by applying the arctangent function to give the angle of a magnetic field. Amplitude differences in the sine and cosine signals caused by mismatch in the independent signal processing paths give rise to angular errors. Therefore, gain matching between the signal processing paths is essential to achieve good performance. 
         [0005]      FIG. 1  shows a schematic diagram of a sigma delta A/D converter  100 , which achieves high resolution by oversampling noise shaping. More specifically, the converter  100  comprises an adder  104 , a filter  106 , a quantizer  108  and a feedback D/A converter  114 . In operation, an analog input signal  102  is fed as one input to adder  104 . The output of the adder  104  is fed to the filter  106 , which shapes the quantization noise introduced by the following quantizer  108  running at high speed and providing digital output signal  110 . The digital output signal  110  is also supplied as a digital feedback signal  112  to the feedback D/A converter  114  which forwards a resulting analog feedback signal  116  a second input of adder  104  in order to allow the filter  106  to take any error (i.e. difference between analog input signal  102  and analog feedback signal  116 ) into account. The filter  106  may be an integrator stage. The output of the converter  110  is converted into the analog domain by DA converter  114 . The output of the DA converter  114  is compared with the input signal  102  and the error is fed back into the loop. Critical part of the converter  100  is the input stage, which determines the error of the digital representation of the analog input signal. An error made by this input stage itself therefore directly transforms into an error in the digital representation of the input signal which is unwanted. It is noted that SAR (successive approximation) AD converters have a similar issue. Prior art implementations use transconductance (gm) stages that independently convert the input signal and the DA converter signal. 
         [0006]      FIG. 2  shows an example of a known input stage  200  for a differential A/D converter. The input stage  200  comprises a first transconductance element  206 , a second transconductance element  210 , an integrating capacitor  208  and a resistor ladder  216  (forming the feedback D/A converter) capable of providing an output signal value between +Vref and −Vref. The differential analog input signals are supplied to the inputs  202  and  204  of the first transconductance element  206  which generates a first current corresponding to the difference between the two input signals. Similarly, the second transconductance element  210  receives the analog feedback signals from the D/A converter  216  at its inputs  212  and  214  and generates a second current (opposing the first current) corresponding to the difference between the two analog feedback signals. The resulting current, i.e. the sum of the first current and the second current, is integrated on the capacitor  208  and the resulting voltage Vout across the capacitor  208  is provided for additional filtering (in case of a loop filter of higher order) or further processing in the quantizing stage of the converter, e.g. the quantizer  108  shown in  FIG. 1 . In other words, the output current of the first and second transconductance elements  206  and  210  are subtracted in the current domain, and integrated on the capacitor  208 . Referring again to  FIG. 1 , the capacitor  208  may form the entire or a first part of the filter  108 . A disadvantage of this implementation is that both transconductance elements have to convert the full swing signals (either the input signal or the DA converter output signal), which will require a highly linear stage to avoid distortion. Normally this linearity is obtained by degeneration of the transconductance stages, but obviously this increases noise (due to the added resistance), and therefore, power needs to be increased in order to achieve the desired noise floor. 
         [0007]    There may thus be a need for an input stage for an A/D converter which is capable of providing high linearity and low noise at a lower power consumption, and which is capable of providing gain matching between multiple channels in a simple and reliable manner. 
       SUMMARY 
       [0008]    This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the present invention are set forth in the dependent claims. 
         [0009]    According to a first aspect, there is provided an input stage for an A/D converter, the input stage comprising (a) a transconductance element adapted to receive, at a first input of the transconductance element, an analog input signal that is to be converted to a digital signal by the A/D converter, (b) a feedback path for providing an analog feedback signal to a second input of the transconductance element, the analog feedback signal being based on a digital output signal of the A/D converter, and (c) an integrator for integrating an output current of the transconductance element, wherein the integrating element is adapted to generate an integrator output signal representative of the integrated output current. 
         [0010]    This aspect is based on the idea that by supplying the analog input signal and the analog feedback signal to the respective inputs of the same transconductance element, the swing of the input signal to the transconductance element (i.e. the difference between the signals input to the transconductance element) is significantly reduced. Therefore, the linearity requirement for the transconductance element is correspondingly reduced and the addition of noisy and power consuming additional resistors is not necessary. 
         [0011]    In the present context, the term “transconductance element” may particularly denote a circuit element that generates an output current that is directly dependent on a voltage difference across the first and second input of the transconductance element, i.e. i=gm*v, where gm is a constant factor (the transconductance) specific for the element. 
         [0012]    Accordingly, the output current of the transconductance element is representative of the difference between the signals input to the transconductance elements. In other words, the integrated output current represents an integrated difference between the analog input signal to the AD converter and the analog feedback signal provided by the feedback path. Thus the integrator output signal, which is representative of the integrated output current, is a measure for the conversion error of the AD converter. 
         [0013]    Assuming that the conversion error of the AD converter is relatively small, it can be realized that the difference between the signals input to the transconductance element is correspondingly small. Thus, the input stage according to this aspect is capable of functioning very well even without a highly linear transconductance element. 
         [0014]    According to an embodiment, the input stage further comprises (a) a further transconductance element adapted to receive, at a first input of the further transconductance element, a further analog input signal, and (b) a further feedback path for providing a further analog feedback signal to a second input of the further transconductance element, the further analog feedback signal being based on the digital output signal of the A/D converter, wherein (c) the integrator is adapted to integrate a sum of the output current of the transconductance element and the output current of the further transconductance element, and wherein the integrator output signal is representative of the integrated sum of output currents. 
         [0015]    In this embodiment, the input stage comprises a further transconductance element working in a similar manner as the transconductance element described above to generate a current based on a difference between the further analog input signal and the further analog feedback signal. The current generated by the further transconductance element is integrated by the integrator together with the current generated by the transconductance element. Thereby, the integrator output signal is representative of the integrated sum of output currents. 
         [0016]    The transconductance (gm) of the further transconductance element may be equal to or different from the transconductance of the transconductance element. 
         [0017]    According to a further embodiment, the analog input signal and the further analog input signal are analog input signals for a differential A/D converter. 
         [0018]    In the present context, the term “differential A/D converter” is an A/D converter capable of generating a digital representation of a difference between two analog input signals. 
         [0019]    It is noted that the further analog input signal and the further analog input signal have the same polarity, i.e. they are in phase. Similarly, the analog input signal has the same polarity (i.e. phase) as the analog feedback signal. However, the analog input signal has the opposite polarity as the further analog input signal. 
         [0020]    According to a further embodiment, the first input of the transconductance element and the first input of the further transconductance element have opposite polarity. 
         [0021]    Accordingly, also the second input of the transconductance element and the second input of the further transconductance element have opposite polarity. 
         [0022]    Thereby, a positive conversion error for the analog input signal will add a positive value to the integrated sum of output currents, while a positive conversion error for the further analog input signal will add a negative contribution to the integrated sum of output currents. 
         [0023]    According to a further embodiment, the input stage further comprises a chopper arranged between the output of the transconductance element and the integrator, the chopper being operable to reverse a polarity of the output of the transconductance element. 
         [0024]    In the present context, the term “chopper” may in particular denote a switching element capable of connection a first input terminal and a second input terminal with a first output terminal and a second output terminal in two ways, depending on a control signal provided to the chopper: For one value of the control signal, the chopper connects the first input terminal with the first output terminal and the second input terminal with the second output terminal, and for a second value fo the control signal, the chopper connects the first input terminal with the second output terminal and the second input terminal with the first output terminal. 
         [0025]    By operating the chopper to reverse the polarity of the output current from the transconductance element, the input stage may change from a differential mode of operation to a common-mode of operation. 
         [0026]    Alternatively, the chopper may be arranged between the output of the further transconductance element and the integrator. 
         [0027]    According to a further embodiment, the integrator comprises a capacitor, and the integrator output signal is a voltage across the capacitor. 
         [0028]    It should be noted that although the above aspect and embodiments have been described with reference to an A/D converter, the described input stage could also be used in other circuit structures involving a feedback path, such as e.g. a feedback amplifier. 
         [0029]    Furthermore, the function of the integrating capacitor, i.e. to provide loop gain, may be achieved by a resistor. 
         [0030]    According to a second aspect, there is provided an A/D converter, comprising (a) an input stage according to the first aspect or any of the above embodiments, (b) a quantizing stage adapted to receive the integrator output signal and to generate a digital output signal, and (c) a feedback D/A converter adapted to generate the analog feedback signal by converting the digital output signal to an analog signal and to feed the analog feedback signal to the feedback path. 
         [0031]    This aspect is based on substantially the same idea as the first aspect discussed above. In particular, an A/D converter according to the second aspect is cheap and easy to manufacture and nevertheless capable of providing high precision analog to digital conversion with low noise and low power consumption. Furthermore, when implemented with multiple channels, e.g. as a differential A/D converter, gain matching between the channels can be provided in a simple and cost-efficient manner. 
         [0032]    In the present context, the term “quantizing” may in particular refer to the process of selecting one of a plurality of discrete (quantized) values as a representative value for a given analog signal level. 
         [0033]    In the present context, the term “feedback D/A converter” may in particular refer to a digital to analog converter arranged to convert a digital output signal of the A/D converter back into an analog signal, e.g as it is known in the field of sigma delta A/D converters. 
         [0034]    According to an embodiment, the A/D converter further comprises a filtering stage interposed between the input stage and the quantizing stage. 
         [0035]    The filtering stage may preferably be adapted to perform additional filtering, such as noise shaping, to the filtering performed by the integrator. 
         [0036]    According to a further embodiment, the feedback D/A converter comprises a resistive ladder structure. 
         [0037]    The resistive ladder structure is preferably configured to provide a plurality of equally spaced discrete voltage values between a positive reference voltage (+Vref) and a negative reference voltage (−Vref). 
         [0038]    According to a further embodiment, the chopper is operable to switch between a differential mode of operation and a common mode of operation by reversing the polarity of the output of the transconductance element. 
         [0039]    In the case of a differential A/D converter, this may e.g. be used for calibration at start-up. More specifically, at start-up, the chopper is operated to switch the system to the common-mode of operation and the common-mode difference between an input signal and a corresponding feedback D/A converter output signal is measured. This information may then be used to either adapt the reference voltages for the feedback D/A converter or to adapt the taps of feedback D/A converter (range shifting). This calibration may preferably be improved by adding a dedicated additional set of transconductance elements for measuring the common-mode difference between input signal and feedback D/A converter signal during operation, such that the common-mode may be corrected on the fly. 
         [0040]    According to a third aspect, there is provided a system comprising a plurality of A/D converters according to the second aspect or any of the above embodiments thereof, wherein the feedback D/A converters of the plurality of A/D/ converts share a resistive ladder structure. 
         [0041]    The resistive ladder structure is preferably configured to provide a plurality of equally spaced discrete voltage values between a positive reference voltage (+Vref) and a negative reference voltage (−Vref). 
         [0042]    By sharing the resistive ladder structure in the sense that the feedback D/A converters all use the same resistive ladder structure, each A/D converter in the system may have the same gain characteristics, such that gain matching can be obtained. 
         [0043]    It should be noted that embodiments of the invention have been described with reference to different subject matters. In particular, some embodiments have been described with reference to method type claims whereas other embodiments have been described with reference to apparatus type claims. However, a person skilled in the art will gather from the above and the following description that, unless otherwise indicated, in addition to any combination of features belonging to one type of subject matter also any combination of features relating to different subject matters, in particular a combination of features of the method type claims and features of the apparatus type claims, is considered to be disclosed with this document. 
         [0044]    The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodiment to which the invention is, however, not limited. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]      FIG. 1  shows a schematic diagram of a sigma delta A/D converter. 
           [0046]      FIG. 2  shows a schematic diagram of a prior art input stage for a differential A/D converter. 
           [0047]      FIG. 3  shows a schematic diagram of an input stage for a differential A/D converter in accordance with an embodiment. 
           [0048]      FIG. 4  shows a schematic diagram of an input stage for a differential A/D converter in accordance with a further embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0049]    The illustration in the drawing is schematic. It is noted that in different figures, similar or identical elements of the respective embodiments are provided with the same reference signs or with reference signs, which differ only within the first digit. 
         [0050]    With regard to  FIGS. 1 and 2 , reference is made to the corresponding descriptions thereof given above. 
         [0051]      FIG. 3  shows a schematic diagram of an input stage  300  for a differential A/D converter in accordance with an embodiment. 
         [0052]    In general, the structure of the input stage  300  differs from the structure  200  of prior art input stage  200  shown in  FIG. 2  in that each transconductance element of input stage  300  receives an analog input signal and an analog feedback signal of the same polarity, such that each transconductance element of input stage  300  is exposed to much less signal swing than the transconductance elements of the prior art input stage  200  of  FIG. 2 . 
         [0053]    More specifically, as shown in  FIG. 3 , the input stage  300  comprises a first transconductance element  306  having a first input  302  and a second input  304 , a second transconductance element having a first input  312  and a second input  314 , a resistor ladder  310  (constituting a feedback D/A converter), and a capacitor  308  coupled to integrate a sum of the output currents from the first transconductance element  306  and the second transconductance element  316 . 
         [0054]    The first input  302  of the first transconductance element  306  receives a first analog input signal (e.g. the positive analog input for a differential A/D converter) and the second input  304  of the first transconductance element  306  receives a first analog feedback signal from the resistor ladder  310 . Similarly, the first input  312  of the second transconductance element  316  receives a second analog input signal (e.g. the negative analog input for a differential A/D converter) and the second input  314  of the second transconductance element  316  receives a second analog feedback signal from the resistor ladder  310 . The positive output terminals of both transconductance elements  306  and  316  are interconnected and connected to one terminal of the integrating capacitor  308 . Similarly, the negative output terminals of both transconductance elements  306  and  316  are interconnected and connected to the other terminal of the integrating capacitor  308 . Accordingly, the capacitor  308  integrates the sum of currents output by the transconductance elements  306  and  316  (e.g. from the respective positive output terminals) such that the voltage Vout across the capacitor  308  is indicative of the integrated sum of output currents. 
         [0055]    As shown in  FIG. 3  the polarity of the first input  302  of the first transconductance element  306  is the opposite to the polarity of the first input  312  of the second transconductance element  316 . Accordingly, a positive difference between the respective voltages at the first and second input terminals  302 ,  304  of the first transconductance element  306  will result in a positive output current from the positive output terminal (+) of the first transconductance element  306  and thus a positive contribution to the sum integrated by the capacitor  308 . Similarly, a negative difference between the respective voltages will provide a negative contribution to the sum integrated by the capacitor  308 . On the contrary, a positive difference between the respective voltages at the first and second input terminals  312 ,  314  of the second transconductance element  316  will result in a negative output current from the positive output terminal (+) of the second transconductance element  316  and thus a negative contribution to the sum integrated by the capacitor  308 . Similarly, a negative difference between the respective voltages will provide a positive contribution to the sum integrated by the capacitor  308 . 
         [0056]    The input stage  300  shown in  FIG. 3  has several specific advantages: By feeding the input and feedback signals of the same polarity into the respective transconductance elements  306 ,  316 , subtraction effectively takes place while still in the voltage domain. This greatly reduces the differential swing at the inputs of the transconductance elements  306 ,  316 , reducing the demands on their differential mode linear range, avoiding the need for degeneration, and resulting in a significant power saving. 
         [0057]    Another advantage of the input stage  300  is that the noise of the reference D/A converter reference sources is common mode. Also, for a near-midscale D/A converter output, most of the thermal noise of the resistor ladder is common-mode. In single-ended implementations (using only one transconductance element and a single ended input signal), the advantage of the noise of the D/A converter being common-mode is obviously lost. 
         [0058]    An additional advantage of the input stage  300  is that if an A/D converter pair is required that requires gain matching between the two signal conversion paths, the reference ladder  310  can be re-used between the channels. Provided that the transconductance elements of each A/D converter match each other, this re-use of the reference ladder  310  gives inherent gain matching between the two channels. Obviously, the reference ladder  310  can be re-used for any number of channels, i.e. for more than two channels. In cases where the transconductance elements of each A/D converter are not matching, gain matching may be obtained repetitively interchanging the transconductance elements during signal conversion. 
         [0059]      FIG. 4  shows a schematic diagram of an input stage  400  for a differential A/D converter in accordance with a further embodiment. 
         [0060]    As can be seen from  FIG. 4 , the structure of input stage  400  is, with exception of the additional chopper  420 , identical to the structure of input stage  300  shown in  FIG. 3  and discussed above. Thus, a detailed discussion of the similar elements will be dispensed with. As already noted, input stage  400  differs from input stage  300  only in that a chopper  420  is provided between the output terminals of the second transconductance element  416  and the integrating capacitor  408 . The chopper  420  is capable of, in dependency of a control signal applied to control terminal  422 , swap the connections of the output terminals of the second transconductance element  416 . That is, when a predetermined control signal is applied to the control terminal  422 , the chopper will connect the positive output terminal (+) of the second transconductance element  416  to the negative output terminal (−) of the first transconductance element  406  and the negative output terminal (−) of the second transconductance element  416  to the positive output terminal (+) of the first transconductance element  406 . Thereby, the mode of operation is switched from a differential mode (corresponding to the structure shown in  FIG. 3 ) to a common-mode of operation. In the common-mode of operation, a positive voltage difference between the first input terminal  412  and the second input terminal  414  of the second transconductance element  416  will also result in a positive contribution to the sum of currents integrated by the capacitor  408 . 
         [0061]    More specifically, when operating in differential mode (corresponding to  FIG. 3 ), the sum i out  of currents integrated by capacitor  408  is given as: 
         [0000]        i   out =gm(( V   g   +   −V   g   − )−( V   DAC   +   −V   DAC   − ))
 
         [0062]    On the other hand, when operating in common-mode, the sum tout of currents integrated by capacitor  408  is given as: 
         [0000]        i   out =gm(( V   g   +   +V   f   − )−( V   DAC   +   +V   DAC   − ))
 
         [0063]    In the above equations, Vs denote the (positive and negative) analog input signals and V DAC  denote the (positive and negative) analog feedback signals. 
         [0064]    The common-mode may be used to calibrate the system at start-up as follows: At start-up, the chopper  420  is set to measure the common-mode difference between the input signal and feedback D/A converter output signal. This information is then used to either adapt the reference voltages Vref, or to adapt the taps of the D/A converter ladder used (range shifting). 
         [0065]    The input stages  300  and  400  as well as further embodiments may particularly and beneficially be implemented in sigma delta and SAR analog to digital converters, especially, A/D converters that require high linearity and low power consumption. When used in multi-channel systems, the input stages  300  and  400  enable perfect signal processing path gain matching between the multiple channels. Furthermore, the input stages  300  and  400  may advantageously be used in applications requiring a high input impedance, such as applications involving sensors, e.g. magnetic sensor, optical sensors, acoustic sensors or other sensors. 
         [0066]    It is noted that, unless otherwise indicated, the use of terms such as “upper”, “lower”, “left”, and “right” refers solely to the orientation of the corresponding drawing. 
         [0067]    It should be noted that the term “comprising” does not exclude other elements or steps and that the use of the articles “a” or “an” does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.