Abstract:
An analog signal input circuit having sample-hold circuit that is constituted by a switched capacitor amplifier for which the gain is controlled according to the capacitance ratio of the plurality of capacitors connected with a switch group, for which the opening and closing is controlled according to the amplification rate setting command. The clamping voltage of a clamping circuit included in the analog signal input circuit is established in compliance with an amplification rate setting command.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an analog input circuit to be used as an input stage of an analog signal processing apparatus such as a display device for displaying a video image by receiving a video signal. 
     2. Description of the Related Art 
     A conventional video input circuit for a video signal processing apparatus, as an analog signal input circuit, generally has the constitution shown in a block diagram in FIG.  1 . In other words, such an analog/digital converting circuit comprises a clamping circuit  11 , amplifying circuit  12 , low pass filter  13 , and sample-hold and analog/digital converter  14 . 
     An input video signal (VIN) and signals for the clamp setting voltage (VRCL) and clamp pulse (CLP) are input to the clamping circuit  11 . Moreover, the input video signal (VIN) is supplied via a capacitor (not shown) to the clamping circuit  11 . 
     The clamping circuit  11  is a circuit for clamping the input video signal to a certain voltage for a prescribed period. In other words, the clamping circuit  11  outputs the input video signal (VIN) as a video signal clamped to the clamp setting voltage (VRCL) just for the period designated by the clamp pulse (CLP). 
     The video signal clamped by the clamping circuit  11  is applied to the non-inverting input terminal of the amplifying circuit  12 . The amplifying circuit  12  comprises an operational amplifier and a plurality of resistors and switches. One end of each of the plurality of resistors is connected to the clamp setting voltage (VRCL) through the switches. Moreover, the clamp setting voltage (VRCL) is generated by the resistance division of two reference voltages, not shown, for determining the conversion range of the sample-hold and analog/digital converter  14 . The plurality of resistors and switches have the function of adjusting the amplification rate of the amplifying circuit  12 . The output of the amplifying circuit  12  is supplied to the sample-hold and analog/digital converter  14  through the low pass filter  13 . 
     The sample-hold and analog/digital converter  14  is a circuit for sample holding the analog signal supplied from the low pass filter  13  in response to a prescribed sampling clock train, and then generating digital data expressing the amplitude of the sample-held analog signal. 
     It is to be understood the amplitude of a video signal varies depending on its circumstance such as TV tuner, VTR and disc player. However, the analog/digital conversion range of the sample-hold and analog/digital converter  14  is self-determined. In the analog/digital converting circuit of FIG. 1, the level of amplification A becomes no more than one because the level of amplification A of the amplifying circuit  12  is determined with the following relationship according to resistance values R 1  and RN constituting the resistor group. 
     
       
           A= 1+( R   1 / RN ) 
       
     
     Consequently, it is a problem a video signal having an amplitude of, for example, 1.5 V or greater cannot be processed with a conventional input circuit when the analog/digital conversion range of the sample-hold and analog/digital converter  14  is up to 1.5 V. Especially for new image recording media such as DVD (digital versatile disc), the amplitude of the video signal tends to be diversified and it is necessary to contrive countermeasures for that. 
     For the so-called pipeline analog/digital converter, which is noteworthy as the latest analog/digital converter for video signal processing, it is often the case that the analog/digital converting range is 1.0 V. Meanwhile, the amplitude of conventional, standard video signals is about 1.3 V and therefore a problem is that those conventional, standard video signals cannot be connected to the pipeline analog/digital converter without an appropriate level adjustment. 
     Furthermore, in a conventional analog/digital converting circuit, the clamp setting voltage (VRCL) is used as the reference voltage for the amplifying circuit  12  and that voltage and the amplified output of the video signal are connected through the resistor group of the amplifying circuit  12 . As a result, it is a problem that changes in the amplified output of the video signal influences the clamp setting voltage (VRCL) and the clamp level in the clamping circuit  11  changes. 
     SUMMARY OF THE INVENTION 
     The present invention was made in order to solve such problems, and it is an object of the present invention to provide an analog signal input circuit for analog signal processing with which the amplification rate of the analog signal can be set to one or less and a stabilized clamping voltage is attained. 
     The analog input circuit according to the present invention comprises: a clamping circuit for clamping the amplitude of the input analog signal according to a clamping voltage setting signal; a sample-hold circuit for amplifying, and sampling and holding the output signal from the above-mentioned clamping circuit at the prescribed amplification rate; and an analog/digital converter for generating digital data expressing the amplitude of the output signal from the above-mentioned sample-hold circuit; 
     wherein the above-mentioned sample-hold circuit comprises a switch capacitor type of amplifier provided with a plurality of capacitors connected with a switch group, for which the opening and closing is controlled according to an amplification rate setting command, provided as input side capacitors or feedback side capacitors; and the above-mentioned prescribed amplification rate is established according to the capacitance ratio of the above-mentioned input side capacitors and feedback side capacitors based on the above-mentioned setting command; and 
     wherein the above-mentioned clamping circuit generates the above-mentioned clamping voltage setting signal, so that the central value of the amplitude of the output signal of the above-mentioned sample-hold circuit becomes the central voltage of the conversion range of the above-mentioned analog/digital converter according to the above-mentioned setting command. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the constitution of a conventional analog signal input circuit; 
     FIG. 2 is a block diagram showing an embodiment of the analog input circuit for analog signal processing according to the present invention; 
     FIG. 3 is a circuit diagram showing the constitution of a chopper-type sample-hold circuit in the embodiment shown in FIG. 2; 
     FIGS. 4A through 4G are timing charts showing the control sequence in the chopper-type sample-hold circuit shown in FIG. 3; and 
     FIG. 5 is a circuit diagram in the case where a single input amplifying circuit is used as the chopper-type sample-hold circuit in the embodiment shown in FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 is a block diagram showing an embodiment of an analog signal circuit such as an analog video signal input circuit according to the present invention. 
     An analog video signal input circuit according to the present invention comprises a clamping circuit  21 , a low pass filter  22 , a chopper amp sample-hold circuit (hereinafter “sample-hold circuit”)  24 , an analog/digital converter  25 , and a clamping voltage generating circuit  26 . 
     An input composite video signal (VIN) and clamp pulses (CLP) are respectively supplied to the clamping circuit  21 . Moreover, the input composite video signal (VIN) is supplied to the clamping circuit  21  via a capacitor (not shown). 
     In the clamping circuit  21 , the synchronizing signal tip level (tip of sync level) and the pedestal level (pedestal level) of the input video signal (VIN) are clamped to a clamp setting voltage (VRCL) for the period when the clamp pulse (CLP)takes a high level. In effect, the clamping circuit  21  produces the video component of the composite video signal (VIN) as differences from the above-mentioned clamp setting voltage (VRCL). This clamp setting voltage (VRCL) is generated based on the amplification rate selection signal (GCTL) in the clamping voltage generating circuit  26 . Moreover, the operation of the clamping voltage setting circuit  26  is discussed in detail hereinafter. 
     The output signal from the clamping circuit  21  is supplied to the sample-hold circuit  24  through the low pass filter  22 . The sample-hold circuit  24  comprises an operational amplifier and an adjustable input capacitor and a feedback capacitor. The constitution is such that the amplification rate of the operational amplifier included in the sample-hold circuit  24  is adjusted by adjusting the capacitance of the input adjustable capacitor in response to the amplification rate selection signal (GCTL). 
     After being amplified at the prescribed amplification rate, the signal supplied to the sample-hold circuit  24  is sampled and held in response to the sampling clocks ö and is supplied to the analog/digital converter  25 . The analog/digital converter  25  generates a digital signal of 12 bit or 16 bit, for example, representing the amplification of such an analog signal and transfers the digital signal to a video signal processing circuit (not shown) connected to this circuit. 
     The details of the constitution of the sample-hold circuit  24  will be explained hereinbelow with reference to the circuit diagram in FIG.  3 . 
     As shown in the drawing, the sample-hold circuit  24  comprises input terminals IN 1  through IN 5 , output terminals OUT 1  and OUT 2 , switches S 1  through S 10 , switch groups SWG, capacitors C 1 A through CnA, capacitors C 1 B through CnB, and a dual input/output operational amplifier AMP. The operational amplifier AMP is, for example, constituted by a pair of parallel operational amplifiers and amplifies input voltages supplied to the input terminals IN(+) and IN(−), respectively. 
     The video signals sent through the low pass filter  22  are complementary to each other and applied across the input terminals IN 1  and IN 2 , i.e., VIN(+) and VIN(−), respectively. The so-called common mode voltage (CM) is applied to the input terminals IN 3  and IN 5 . The common mode voltage (CM) is the value showing the central voltage value of the analog/digital conversion range in the succeeding analog/digital converter  25 . Furthermore, the amplification rate selection signal (GCTL) is supplied to the input terminal IN 4 . The amplification rate selection signal (GCTL) is the signal for individually controlling each of the gates of the switch group SWG and is supplied from the control circuit (not shown) so that the sample-hold circuit  24  ensures the appropriate amplification rate. 
     The amplified outputs of the operational amplifier AMP appear at the output terminals OUT 1  and OUT 2  as VOUT(+) and VOUT(−) respectively. 
     The switches S 1  through S 10  are analog switches such as MOSFETs each receiving an ON/OFF control signal at its gate. The sampling clock train ö or ö[overbar] is supplied from the control circuit, not shown, as a control signal for these switches S 1  through S 10 . Each switch becomes on when the clock pulse ö or ö [overbar] takes a high level and becomes off when those signals take low level. The switch group SA 1 -SAn and SB 1 -SBn are similar switching elements and each of the switches becomes on and off according to the amplification rate selection signal (GCTL) supplied to each gate. 
     The capacitors C 1 A through CnA and the capacitors C 1 B through CnB are combined with the operational amplifier AMP and constitute a so-called switch capacitor amplifying circuit. In embodiments, various values may be used as the capacitance of these capacitors. 
     Moreover, in the switched capacitor amplifying circuit, the capacitor C 1 A or C 1 B corresponds to a capacitor constituting a so-called feedback branch (hereinafter, simply “feedback side capacitor”). The capacitors C 2 A through CnA or the capacitors C 2 B through CnB correspond to capacitors constituting a so-called input branch (hereinafter, simply “input side capacitors”). The amplification rate of the switched capacitor amplifying circuit is then defined as C 2 A through CnA/C 1 A, or C 2 B through CnB/C 2 A. 
     The connections of each portion of the circuit in FIG. 3 are explained hereinbelow. 
     The input terminal IN 1  is connected to one end of the switch S 1 ; the input terminal IN 2  is connected to one end of the switch S 4 ; and the input terminal IN 3  is connected to one end of the switch S 2  and switch S 3 . 
     The other end of the switch S 1  is connected to all of the other ends of the switch group SA 2 -SAn connected to the capacitors C 2 A through CnA and further to one end of the switch S 5 . The other end of the switch S 5  is connected to the other end of the switch S 4  and further to all of the other ends of the switch group SB 2 -SBn connected to the capacitors C 2 B through CnB. 
     The other ends of the capacitors C 2 A through CnA which are not connected to the switch group SWG are all connected to one end of the capacitor C 1 A and the normal phase input terminal of the operational amplifier AMP. Likewise, the other ends of the capacitors C 2 B through CnB which are not connected to the switch group SBn are all connected to one end of the capacitor C 1 B at the inverted phase input terminal of the operational amplifier AMP. 
     The other end of the capacitor C 1 A is connected to one end of each of the switch S 2  and the switch S 6 ; the other end of the capacitor C 1 B is connected to one end of each of the switch S 3  and the switch S 7 . 
     The other end of the switch S 6  is connected to the normal-phase output terminal of the operational amplifier AMP, one end of the switch S 8 , and the output terminal OUT 1 . The other end of the switch S 7  is connected to the inverted-phase output terminal of the operational amplifier AMP, the other end of the switch S 8 , and the output terminal OUT 2 . 
     The normal-phase input terminal of the operational amplifier AMP is connected to one end of the switch S 9 . The other end of the switch S 9  is connected to the input terminal IN 5 . Also, the inverted phase input terminal of the operational amplifier AMP is connected to one end of the switch S 10 . The other end of the switch S 10  is also connected to the input terminal IN 5 . 
     Furthermore, the signal lines whereon the amplification rate selection signal (GCTL) from the input terminal IN 4  is supplied are connected to all the gates of the switch group SA 2 -SAn and SB 2 -SBn. 
     The operation of the sample-hold circuit  24  is explained hereinbelow while referring to the circuit diagram in FIG.  3  and the timing charts shown in FIGS. 4A through 4G. 
     When the sampling clock ö takes a high level, the switches S 1 , S 2 , S 3 , S 4 , S 8 , S 9 , and S 10  in the circuit diagram in FIG. 3 become on. Because the clock ö[overbar], of the clock ö inverted, is supplied to the gates of the switches S 5 , S 6 , and S 7 , these switches become off went the sampling clock ö takes the high level. 
     At this moment, the ends of the capacitors C 1 A and C 1 B are connected to the input terminal IN 3  through the switch S 2  and S 3  respectively. Accordingly, the voltage at one end of the capacitors C 1 A and C 1 B becomes the common mode voltage (CM) that is the central potential in the analog/digital conversion range applied to the input terminal IN 3 . Meanwhile, the other ends of the capacitors C 1 A and C 1 B are connected to each of the input terminals of the operational amplifier AMP. Assuming the properties of the operational amplifier to be ideal, those two input terminals are regarded as imaginally shorted together and therefore the other ends of the capacitors C 1 A and C 1 B become the same potential. 
     It is now to be understood that those capacitors for which the switch group SAn and SBn connected serially to each capacitor becomes on among the capacitors C 2 A through CnA and the capacitors C 2 B through CnB are capacitors which are selected by the amplification rate selection signal (GCTL) so that the switched capacitor amplifying circuit achieves the prescribed amplification rate. 
     Accordingly, one end of each capacitor selected from among the capacitors C 2 A through CnA is connected through the SAn and SBn and switch S 1  to the input terminal IN 1 . Likewise, one end of each capacitor selected from among the capacitors C 2 B through CnB connected through each of the SAn and SBn and switch S 4  to the input terminal IN 2 . Accordingly, those capacitors selected from the capacitors C 2 A through CnA and the capacitors C 2 B through CnB are charged to VIN(+) or VIN(−), which are the voltages of the input video signals complementary to each other. 
     In this instance, it is to be understood that a capacitor provided at the input side of the amplifier AMP is selectively made effective when a switch corresponding to the capacitor is closed to connect the capacitor to the amplifier. 
     Furthermore, the two output terminals of the operational amplifier AMP are shorted by the switch S 8  and held at the common mode voltage (CM) by the common mode feedback circuit, not shown. Likewise, the two input terminals of the operational amplifier AMP are also held at the common mode voltage (CM) by the switch S 9  and switch S 10 . 
     The state in the proceeding explanation wherein an instance á appears when the sampling clock ö takes a high level in the timing charts in FIGS. 4A through 4G. 
     FIG. 4A shows the sampling clock ö, FIGS. 4B and 4C show the voltages of VIN(+) and VIN(−) which are the input signals to the input terminals IN 1  and IN 2  which are complementary to each other. These complementary input signals are the video signals output from the preceding low pass filter  22  and may take on various values, changing over time according to the prescribed video signal format. In the example shown in FIG. 4B, a signal waveform is assumed wherein VIN(+) is a voltage lower than the common mode voltage (CM) before the time ã and becomes a voltage greater than the common mode voltage (CM) after the passage of time ã. Moreover, VIN(−) shown in FIG. 4C has a waveform that is simply the inverse of VIN(+). 
     FIGS. 4F and 4G show the voltages of the VOUT(+) and VOUT(−) which are the output signals appearing at the output terminals OUT 1  and OUT 2 . Furthermore, FIGS. 4D and 4E show the AMPIN(+) which is the voltage of the normal-phase input terminal and AMPIN(−) which is the voltage of the inverse-phase input terminal of the operational amplifier. 
     As discussed above, at an instance á when ö is at a high level, the voltages of AMPIN(+), AMPIN(−), VOUT(+), and VOUT(−) all become the common mode voltage (CM) because of the operation of the respective switches. 
     Next, it is assumed that the sampling clock ö has become low level. In this case, the switches S 1 , S 2 , S 3 , S 4 , S 8 , S 9 , and S 10  in the circuit in FIG. 3 become off and the switches S 5 , S 6 , and S 7  become on. 
     As a result, from among the capacitors C 2 A through CnA and the capacitors C 2 B through CnB, the input terminal side electrode of the capacitors selected by the amplification rate selection signal (GCTL) are shorted by the switch S 5 . As discussed above, the capacitors C 2 A through CnA are charged to VIN(+) which is the input signal voltage applied to the input terminal IN 1 . Meanwhile, the capacitors C 2 B through CnB are charged to VIN(−) which is the input signal voltage applied to the input terminal IN 2 . Accordingly, the mid-point voltage between VIN(+) and VIN(−) appears between the input terminal side electrodes of the capacitors shorted by the switch S 5 . 
     Consequently, such complementary input voltages VIN(+) and VIN(−) become the inputs for the amplifier, assuming a switched capacitor amplifier comprising a dual input operational amplifier AMP, input side capacitors C 2 A through CnA (C 2 B through CnB), and a feedback side capacitor C 1 A (C 1 B). The input voltages VIN(+) and VIN(−) are amplified at the prescribed amplification rate and an output voltage appears between VOUT(+) and VOUT(−) which are the output terminals of the operational amplifier AMP. 
     When the value of the output voltage is defined as the voltage difference between VOUT(+) and VOUT(−), the amplification rate A can be expressed with the following formula according to the properties of the switched capacitor amplifier discussed above. 
     
       
           A =( V OUT(+)− V OUT(−))/( V IN(+)− V IN(−))=[ C   2 + C   3 + . . . + C n] sel   /C   1   
       
     
     In this formula, [C 2 +C 3 + . . . +Cn] sel  means an operator which is the sum of the capacitances of the capacitors, selected by the amplification rate selection signal (GCTL) from the capacitors C 2 A through CnA (C 2 B through CnB). 
     In effect, in the circuit shown in FIG. 3, the amplification rate for the video signal is freely adjusted by appropriately establishing the capacitances of the capacitors C 2 A through CnA (C 2 B through CnB) and appropriately selecting the connected capacitances by the amplification rate selection signal (GCTL). 
     When, especially, the total capacitance of the capacitors C 2 A-CnA( 2 B-CnB) is made equal to or smaller than the capacitance of the capacitor ClA(ClB), the amplification A does never exceed 1. Under such condition, the analog input signal circuit of the present invention makes possible to supply the conventional stand video signal or DVD signal having amplitude voltage equal to or larger than 1.3V to an A/D converter of pipeline type. 
     The state wherein the sampling clock ö is at a low level as explained above is shown, and this is an instance â in the timing charts in FIGS. 4A through 4G. In this drawing, the voltages of the input signals AMPIN(+) and AMPIN(−) are shown in FIGS. 4D and 4E and the signals amplified at the above-mentioned amplification rate A become VOUT(+) and VOUT(−), which are the output signals shown in FIGS. 4F and 4G respectively. 
     The sample-hold circuit  24  holds the voltages VOUT(+) and VOUT(−) at such a time. The analog/digital converter  25  then performs analog/digital conversion of these held voltages. 
     The clamping voltage generating circuit  26  is explained hereinbelow. This circuit is a circuit for generating the clamp setting voltage (VRCL) corresponding to the amplification rate selection signal (GCTL) establishing the amplification rate of the switched capacitor amplifier in the sample-hold circuit  24 . 
     For the sake of the explanation of this circuit, the amplitude of the input video signal (VIN), meaning the transitional amplitude from peak to peak in the signal wave form, is assumed to be 0.5 V, for example. Also, the conversion range of the analog input voltage of the analog/digital converter  25  (hereinafter, simply “conversion range”) is assumed to be from 1.0 V to 2.0 V. 
     In this case, the common mode voltage (CM) showing the central or midpoint potential of the conversion range of the analog/digital converter  25  becomes 1.5 V. The amplification process in the above-mentioned sample-hold circuit  24  is then carried out with the common mode voltage (CM) as the center. Accordingly, the input video signal (VIN) to the sample-hold circuit  24  is preferably a varied signal having the 1.5 V direct current voltage as the center. Also, in order to perform the appropriate. analog/digital conversion operation, it is necessary that the value of A, the amplification rate of the sample-hold circuit  24 , multiplied by the difference between the input signal and the common mode voltage (CM) fall within the conversion range. 
     In order to fit this numerical example, the amplitude of the input video signal (VIN) is equal to 0.5 V and the conversion range of the analog/digital converter  25  is a range of 1.0 V to 2.0 V, and therefore it is preferable that the amplification rate A be A=2. Also, the input signal of the analog/digital converter  25  is preferably the signal waveform with the maximum value of 2.0 V and minimum value of 1.0 V, wherein the 1.5 V direct current voltage is the center. Consequently, the input signal of the sample-hold circuit  24  satisfies such conditions if it has a signal waveform with a maximum value of 1.75 V and minimum value of 1.25 V, wherein the 1.5 V direct current voltage is the center. 
     In other words, the clamping circuit  21  works to clamp the input video signal (VIN) with the prescribed clamp setting voltage (VRCL) and keep appropriate the signal wave form. For example, in the case of clamping at the leading level of the synchronizing signal included in the input video signal (VIN), meaning the synch tip level that is the minimum level of the video signal, the 1.25 V which is the minimum value of the abovementioned signal waveform becomes the clamp setting voltage (VRCL). The clamping voltage generating circuit  26  generates the clamp setting voltage (VRCL) based on the amplification rate selected signal (GCTL) and supplies this to the clamping circuit  21 . 
     For example, the clamping voltage generating circuit  26  comprises a digital/analog converting circuit (hereinafter, simply “D/A converting circuit”), a memory circuit, and a microcomputer for controlling these (none of these is shown). Information, such as information relating to the clamping operation in the clamping circuit  21 , such as carrying out clamping at the pedestal level or synch tip level, and the conversion range of the analog/digital converter  25 , is stored in the memory circuit. 
     When the amplification rate selecting signal (GCTL) is input from the control circuit, not shown, to the clamping voltage generating circuit  26 , the above-mentioned microcomputer calculates the amplification rate in the sample-hold circuit  24  from this signal. Furthermore, this microcomputer adds the conversion range of the analog/digital converter  25  and the operating conditions of the clamping circuit  21  stored in the memory circuit, and calculates the optimum clamp setting voltage and supplies this to the D/A converting circuit. The D/A converting circuit converts this digital value to an analog direct current voltage, generates the clamp setting voltage (VRCL), and supplies this to the clamping circuit  21 . 
     Moreover, the clamping voltage generating circuit  26  is not limited to the above-mentioned constitution. For example, a D/A converting circuit may be established in the clamping circuit  21 , and only the digital signal showing the clamp setting voltage (VRCL) output from the clamping voltage generating circuit  26  to the clamping circuit  21 . 
     The present invention is not limited to the embodiments discussed above and may also use a sample-hold circuit, with the so-called single input system, as shown in FIG. 5 as the sample-hold circuit  24 . In addition, the circuit shown in FIG. 5 is different only with regards to being connected to the input terminals IN 2  and IN 3  in the circuit shown in FIG.  3  and these being common mode voltage (CM) input terminals. An explanation thereof is accordingly omitted. 
     In the present embodiment, in the switched capacitor amplifier in the sample-hold circuit  24 , the constitution is such that the input side capacitors are variable as means for adjusting the amplification rate, but the present invention is not limited to such a constitution. In other words, the constitution may also be such that in the circuit diagrams shown in FIG. 2 or  5 , the input side capacitors C 2 A through CnA (C 2 B through CnB) are fixed and the feedback side capacitor C 1 A (C 1 B) is varied by the amplification rate selection signal (GCTL). 
     In the present embodiment, the explanation was made while using an analog/digital converting circuit for video signal processing, but the present invention is not limited to such a case. Needless to say, it may be used an analog signal input circuit, treating an analog signal such as an audio signal. 
     As discussed in detail above, the present invention makes possible the provision of an analog input circuit which can freely adjust the amplification rate, even when one or less, and generate the appropriate and stable clamping voltage corresponding to that amplification rate.