Abstract:
A sigma-delta (ΣΔ) analog to digital converter with internal synchronous demodulation responsive to a sample clock, reference clock and conversion clock including a sample switching circuit responsive to an AC input to sample the AC input at the sample clock rate; the sample switching circuit including first and second input switches responsive to the reference clock for selectively, alternately sampling the positive and the negative AC input at the reference clock rate; and an inverter circuit responsive to the reference clock and the sample clock for reversing the polarity of signals from the sample clock in synchronism with the reference clock to reverse the sense of the input switches and synchronously demodulating the AC input within the converter.

Description:
FIELD OF THE INVENTION 
     This invention relates to sigma-delta (ΣΔ) analog to digital converter with synchronous demodulation. 
     BACKGROUND OF THE INVENTION 
     Alternating current (AC) signals such as those from inductive or capacitive sensors benefit greatly from the use of phase sensitive synchronous demodulation to obtain amplitude information. This method provides a high degree of filtering of extraneous noise outside the frequency and phase of the required signal. A typical input signal chain is compromised of three main elements: synchronous demodulator, filter and analog to digital converter. Although there are several variations, the classic method of implementing a demodulator is to use a switch controlled by the reference oscillator. This switch switches alternately between the input signal and its inverse to produce a synchronously rectified result. The phase timing of this switch is set to produce a full or half wave rectified version of the AC signal. Phase adjustment maybe included to enhance signal fidelity such as removing unwanted phase shifts in the signal path. Switching must be fast, exact and the effects of switch resistance or amplitude imperfections may degrade overall system performance. 
     Filtering of the rectified signal has to maintain fidelity and bandwidth for the signal of interest, while ripple must be minimized, consequently the analog filter circuit is often complex and limits the overall performance of the measurement system. 
     Alternate methods have been used where a digital signal processor is used to digitize the AC signals and then perform demodulation and filtering in software. This overcomes some of the filtering limitations but in doing so requires the use of more complex and costly high speed digital to analog converters and digital signal processor (DSP) hardware. 
     SUMMARY OF THE INVENTION 
     An improved sigma-delta (ΣΔ) analog to digital converter with internal synchronous demodulation in accordance with this invention is easily implemented internally in the converter. The ΣΔ analog to digital converter is simple and inexpensive to implement and fabricate and avoids the need for a preliminary demodulator or low pass filter. The ΣΔ analog to digital converter uses a small but elegant change in operation of the converter input switches to effect the synchronous demodulation and permits a complete filtering of the AC ripple by the conventional digital filter associated with the converter. 
     A ΣΔ analog to digital converter can be accomplished simply and effectively within the converter by periodically reversing the sense of the converter input switches in time with the reference clock to synchronously demodulate the AC input. 
     The subject invention, however, in other embodiments, need not achieve all these objectives and the claims hereof should not be limited to structures or methods capable of achieving these objectives. 
     This invention features a ΣΔ analog to digital converter with internal synchronous demodulation responsive to a sample clock, reference clock and conversion clock. A sample switching circuit is responsive to an AC input to sample the AC input at the sample clock rate. The sample switching circuit includes first and second input switches responsive to the reference clock for selectively, alternately sampling the positive and the negative AC input at the reference clock rate. An inverter circuit responsive to the reference clock and the sample clock reverses the polarity of signals from the sample clock in synchronism with the reference clock to reverse the sense of the input switches and synchronously demodulate the AC input within the converter. 
     In a preferred embodiment the inverter may include an exclusive OR gate whose inputs are the reference clock and the sample clock. The sample clock may have a higher rate than the reference clock and the reference clock may have a higher rate than the conversion clock. The conversion clock rate may be a whole number of half cycles of the reference clock. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which: 
         FIG. 1  is a schematic block diagram of a prior art synchronous demodulation analog to digital conversion system; 
         FIG. 2  illustrates a typical AC input V in(+) , V in(−)  to the demodulator of  FIG. 1 ; 
         FIG. 3  illustrates a typical rectified waveform produced by the demodulator for the AC input in  FIG. 2 ; 
         FIG. 4  is a schematic block diagram of a ΣΔ analog to digital converter system with internal synchronous demodulation according to this invention; 
         FIG. 5  is a schematic block diagram of one implementation of the inverter of  FIG. 4 ; and 
         FIG. 6  illustrates the inherently rectifying effect of the inverted operation of the input switches of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer. 
     There is shown in  FIG. 1  a conventional analog to digital converter system  10  with synchronous demodulation typically includes a demodulator  12 , low pass filter  14 , and analog to digital converter  16 . The AC inputs V in(+)  and V in(−)  at  18  and  20  are fed to demodulator  12  where they are synchronously demodulated or rectified with respect to the reference clock  13  and the rectified output appears at output  22 . Typically the input AC waves V in(+)  and V in(−)  are 180° out of phase as shown in  FIG. 2 . After demodulation or rectification they appear as shown by the demodulated waveform  24 ,  FIG. 3  and after passing through low pass filter  14  the output  24  to analog to digital converter  16 ,  FIG. 1 , appears as a low ripple DC level  26 ,  FIG. 3 . This conventional approach requires a separate demodulator, and analog low pass filter to prepare the AC input signal for submission to the analog to digital converter  16 . Low pass filter  14  removes some but not all of the ripple from the output signal. 
     A ΣΔ analog to digital converter system with internal synchronous demodulation  30 ,  FIG. 4 , includes an analog to digital converter which is shown as a ΣΔ converter  16   a  with conventional components including a summing circuit  32 , integrator circuit  34 , quantizer  36 , digital filter  38 , and feedback loop  40  which includes one bit DAC  42 . There is also shown in system  30  a sample switching circuit  44  which includes input switches  46  and  48  and output switches  50  and  52 . In this case the input is capacitively coupled using capacitor  55 . Also conventionally associated with converter  16   a  are a sample clock  54 , reference clock  56 , and conversion clock  58 . Sample clock  54  is a high rate clock, e.g. 3 MHz which operates switches  46 ,  48 ,  50 , and  52  and defines the sampling rate of the converter system. Reference clock  56  is a lower frequency clock, for example, 5 kHz. Conversion clock  58  is an even lower rate clock, for example, 1000 Hz which defines the period of the conversion by sampling the output of the digital filter  38 . 
     In conventional operation sample clock  54  operates switches  46 ,  48 ,  50  and  52  by, for example, opening switches  48  and  50  and closing switches  46  and  52  to charge capacitor  55  to V in(+)  and then opening switches  46  and  52  and closing switches  48  and  50  to connect capacitor  55  to V in(−)  and through switch  50  to input of summing circuit  32 . Converter  16   a  operates in the conventional manner whereby the difference between the output of one bit DAC  42  and the input from switch  50  is integrated in integrator  34  then compared to some reference level in quantizer  36 . For example, if it is above the level, a one appears on line  60 ; if it is below that predetermined level a zero appears there. That one or zero is fed back to one bit DAC  42  and also forward to digital filter  38 . Conversion clock  58  is set by the decimation ration of the converter and determines how many of these filtered one/zero outputs will be regarded as a valid representation of the AC input. 
     In accordance with this invention without using a demodulator, such as demodulator  12  in  FIG. 1 , or a low pass filter such as low pass filter  14  in  FIG. 1 , the same demodulation can be effected simply by using an inverter  70 ,  FIG. 4 , which periodically changes the sense of input switches  46  and  48  according to the state of the reference clock  56 . This can be done in the embodiment of  FIG. 4  by inverting the polarity of the sample clocks upon the occurrence of a reference clock. Remember the sample clock occurs at 3 MHz in this specific example and the reference clock occurs at 5 kHz. But the simple expedient of reversing the sense of switches  46  and  48  responding to the sample clock each time a reference clock occurs, the AC inputs V in(+) , V in(−)  shown again in  FIG. 5  are sampled in a selectively alternative fashion. For simplicity the frequency of V in(+)  and V in(−)  in  FIG. 5  is shown as the reference clock frequency in a specific example 5 kHz. Without more, the switching would enable the positive portion  80  of V in(+)  to be sampled and then the negative portion  82  of V in(−)  to be sampled. Then the negative portion  84  of V in(+)  and the positive portion of V in(−)    86 . The result would be that the average value over time is zero, not a workable input for the converter. However, by inverting the sample clock each half cycle of the reference clock this can be avoided. Each time the inputs, which for simplicity have the same reference as the reference clock  56 , go through zero such as at  88  and  90  the inverter  70  inverts the sample clock polarity. This reverses the sense of switches  46  and  48 . Thus, after sampling a portion  80 , instead of sampling portion  82  because of the inversion, it is the positive portion  86  of V in(−)  that is sampled. So too at zero crossing  92  and  94  the sample portion is  96 , a positive portion of V in(+) , after zero crossing  98  and  100  the sample portion is the positive portion  102  of V in(−) . Thus, by inverting the sample clock polarity every time a reference clock occurs a demodulated or rectified output occurs which has the same profile as at  24  in  FIG. 3 . And so the demodulation has been achieved with far fewer and less complex components. In addition this signal in a digital domain can be dealt with by digital filter  38  so that the ripple is not just substantially filtered out but can be actually completing filtered out. This can be done simply by making sure that the conversion clock  58  is a whole number multiple of a half cycle of reference clock  56 . Since for this particular example sample clock  54  is 3 MHz, reference clock  56  is 5 kHz and conversion clock  58  is 1,000 Hz this is the case and the added advantage of the highly effective digital filtration is achieved. In fact the connection  110 ,  FIG. 4 , shown from the sample clock  54  to reference clock  56  and the connection  112  from reference clock  56  to conversion clock  58  represents that each lower clock is a reduction from the next higher clock and they are indeed whole multiples of one another. 
     Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments. 
     In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant can not be expected to describe certain insubstantial substitutes for any claim element amended. 
     Other embodiments will occur to those skilled in the art and are within the following claims.