Successive approximation type A/D converter

Abstract: A successive approximation type A/D converter of the present invention has a capacitor (C F ) having a pre­ determined capacitance value and arranged between an input terminal of a voltage comparator (A, SW) and an fixed potential terminal having a predetermined poten­ tial. According to the successive approximation type A/D converter of the present invention, the capacitor (C F ) arranged between the input terminal of the voltage comparator (A, SW) and the fixed potential terminal having the predetermined potential prevents a potential at the input node of the voltage comparator (A, SW) from being greatly changed to exceed a power source voltage range due the influences of a local D/A converter (C1, C2, C3, C4, SL1, SL2, SL3, SL4, D) when the sample mode is switched to the approximation mode. Therefore, the leakage of charges stored in the input side of the volt­ age comparator (A, SW) can be prevented. Accordingly, even if the amplitude of an analog input voltage is equal to the amplitude of the power source voltage, high-precision A/D conversion can still be performed.

Description: The present invention relates to an A/D converter (analog-to-digital converter) used in various electronic devices and, more particularly, to a successive approximation type A/D converter formed into an integrated circuit. Generally, a successive approximation type A/D converter samples an analog voltage input, supplies a successive approximation control digital output from a successive approximation type control circuit to a local D/A converter to cause the D/A converter to generate a local analog voltage, compares the sampled and held analog voltage with a local analog voltage, and successively determines the value of each bit of an A/D conversion output on the basis of the relationship in magnitude between the compared voltages, thereby obtaining an A/D conversion output having a plurality of bits from the successive approximation type control circuit. Fig. 1 shows a 3-bit A/D converter as a successive approximation type A/D converter. More specifically, reference symbols C1 and C2 respectively denote capacitors each having capacitance value C; C3, a capacitor having capacitance value 2C; C4, a capacitor having capacitance value 4C; A, an inverting amplifier; and SW, a switch. They constitute a voltage comparator. Reference numeral 1 denotes an analog input terminal; 2, a reference voltage terminal; and 3, a ground terminal. Reference symbols SL1 to SL4 denote selectors whose selective connection states are controlled by successive approximation control digital signals; and B, a successive approximation type control circuit having functions of successively outputting the successive approximation control digital signals, and determining each bit value of an A/D conversion output on the basis of the logical level of an output from inverting amplifier A. The A/D-converting operation of the successive approximation type A/D converter will be described below. In a sample mode, switch SW is set in an ON state, and selectors SL1 to SL4 are controlled in a state for selecting analog input terminal 1. In this case, the potential at input terminal node N of inverting amplifier A becomes threshold voltage Vop. Assuming that an analog input voltage is represented by Vain, then charge Q8 stored in capacitors C1 to C4 can be given as: Q8 = (Vop - Vain)·8C Subsequently, the mode is switched to an approximation mode, wherein switch SW is set in an OFF state, selectors SL1 to SL3 are controlled in a state for selecting ground terminal 3, and selector SL4 is controlled in a state for selecting reference voltage terminal 2. In this case, assuming that the potential at node N is represented by V1, then charge Q1 stored in capacitors C1 to C3, and charge Q2 stored in capacitor C4 are respectively given as: Q1 = (C + C + 2C)V1 Q2 = 4C(V1 - V R ) Then, from the principle of conservation of charges, the following equation can be established at node N: Q8 = Q1 + Q2 Therefore, substitutions of equations (1), (2), and (3) into equation (4) yield: (Vop - Vain)·8C = 4CV1 + 4C(V1 - V R ) = 8CV1 - 4CV R Vop - Vain = V1 - 1 2 V R V1 - Vop = 1 2 V R - Vain In equation (5), since V1 > Vop when 1 / 2 V R > Vain, the output from inverting amplifier A goes to low level. Since V1 < Vop when 1 / 2 V R < Vain, the output from inverting amplifier A goes to high level. Successive approximation type control circuit B determines the value ("1" or "0") of an MSB (most significant bit) of a digital output to be output from successive approximation type control circuit B on the basis of the output from inverting amplifier A, and supplies a control signal to selectors SL2 to SL4 to perform an approximation operation corresponding to the next significant bit. The operation that successive approximation type control circuit B outputs a certain successive approximation control signal, and then determines the value of a certain bit on the basis of an output from the inverting amplifier is repeated a predetermined number of times (three times in the embodiment), thereby determining a 3-bit A/D conversion output. In order to perform A/D conversion with high precision in the successive approximation type A/D converter described above, the charges stored in capacitors C1 to C4 in the approximation mode must be held. In this case, if a MOS transistor is used for the input stage of inverting amplifier A and an input signal is supplied to its gate, no problem is posed because of a very high input impedance. However, in inverting amplifier A whose input terminal is connected to node N, the problem of charge leakage caused by switch SW connected to node N is posed. A case will be taken into consideration, wherein as the switch SW, a CMOS analog switch comprising parallel-connected n- and p-channel transistors TN1 and TP1 having gates for respectively receiving complementary switch control signals  , and  is used, as shown in, e.g., Fig. 2A. Figs. 2B shows an equivalent circuit of this analog switch. Referring to Fig. 2B, reference symbols D P and C P denote diodes and capacitors, both of which have junctions with the source and drain of p-channel transistor TP1; and D N and C N , diodes and capacitors, both of which have junctions with the drain and source of n-channel transistor TN1. In the case wherein the above-described A/D converter is operated at V DD voltage = 5 V and analog input voltage Vain = 0 to 5 V, if selector SL4 is set in a state for selecting reference voltage terminal 2 before selectors SL1 to SL3 are set in a state for selecting ground terminal 3 when the sample mode is switched to the approximation mode, potential V1 at node N becomes Vop + V R - Vain. In this case, normally, if sampling is performed at Vop = 2.5 V, V R = 5 V, and Vain = 0 V, then V1 = 7.5 V. Diodes D P in Fig. 2B are rendered conductive, and the charges stored in capacitors C1 to C4 leak. For this reason, a proper A/D conversion output cannot be obtained. In contrast to this, if selectors SL1 to SL3 are set in a state for selecting ground terminal 3 before selector SL4 is set in a state for selecting reference voltage terminal 2 when the sample mode is switched to the approximation mode, potential V1 at node N becomes Vop - Vain. Thus, if sampling is performed at Vain = 5, then V1 = -2.5 V. As a result, diodes D N are rendered conductive, and the charges at node N leak. Similarly, a proper A/D conversion output cannot be obtained. That is, in the conventional successive approximation type A/D converters described above, if analog input voltage Vain appears near V DD voltage or the ground potential, potential V1 at input terminal node N of inverting amplifier A for comparing voltages may be greatly changed to be higher than V DD power source voltage or to be lower than the ground potential because of the influences of a switching operation of the capacitor array (in other words, a D/A-converting operation for generating a local analog signal) when the sample mode is switched to the approximation mode. In this case, charge leakage occurs in switch SW, and hence a proper A/D-converting operation cannot be performed. For this reason, when the amplitude of analog input voltage Vain is equal to the power source voltage amplitude of the A/D converter, high-precision A/D conversion cannot be performed. Prior art document US-A-4 200 863 discloses an array of binary weighted capacitors wherein an additional capacitor has a capacitance value equivalent to that of the least of the binary weighted capacitor. Further, this array comprises a voltage comparator, switches for interconnecting the capacitors with certain predetermined voltage levels and the comparator, and a sequencing circuit. One side of all of the capacitors is connected to one input terminal of the comparator, and the other side has applied thereto a signal to be quantized. A switch sequencing combines divided portions of a reference voltage with the signal to be quantized for presentation to the input of the comparator which thereby provides a serial digit output connected to the sequencing circuit. In this fashion, a linear conversion between an analog and a digital signal is made by the sequencing circuit. Further, prior art document ELECTRONICS AND COMMUNICATIONS IN JAPAN, vol. 67, no. 1, January 1984, pages 77-86, TSUKADA et al.: "Converter with Inherent Self-Compensation" describes A/D converters which have an inherent "self-compensation" circuit which enables high-accuracy analog-to-digital conversion without trimming. This circuit is a successive-approximation A/D converter in which an input charge stored on a capacitor array is redistributed over the capacitors. Errors in the capacitors are evaluated prior to the conversion step using the capacitor array itself as a reference. The present invention has been made to fundamentally solve the problems associates with a great change in potential at the input terminal node of the voltage comparing amplifier caused by the influences of a D/A-converting operation for generating a local analog signal when the sample mode is switched to the approximation mode, as described above. It is an object of the present invention to provide a successive approximation type A/D converter, which can prevent a great change in potential at the input terminal node when the sample mode is switched to the approximation mode, can prevent charge leakage of this node, and can properly perform high-precision A/D conversion even if the amplitude of an analog input voltage is equal to the power source voltage amplitude. To solve this object the present invention provides a successive approximation type A/D converter as specified in claim 1. A successive approximation type A/D converter according to the present invention comprises a capacitor having a predetermined capacitance value between the input terminal of a voltage comparator and a fixed potential terminal having a predetermined potential. According to the successive approximation type A/D converter of the present invention, with the capacitor arranged between the input terminal of the voltage comparator and the fixed voltage terminal with the predetermined voltage, the potential at the input node of the voltage comparator can be prevented from being greatly changed to exceed a power source voltage range because of the influences of the operation of the local D/A converter when the sample mode is switched to the approximation mode, thereby preventing leakage of the charges stored in the input side of the voltage comparator. Therefore, high-precision A/D conversion can be properly performed even when the amplitude of an analog input voltage is equal to the power source voltage amplitude. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a circuit diagram of a conventional successive approximation type A/D converter; Fig. 2A is a circuit diagram showing a detailed arrangement of a switch SW in Fig. 1; Fig. 2B is an equivalent circuit of Fig. 2A; Fig. 3 is a circuit diagram of a successive approximation type A/D converter according to an embodiment of the present invention; Fig. 4 is a circuit diagram showing a detailed arrangement of the input terminal of inverting amplifier in Fig. 3; Fig. 5 is a circuit diagram showing a modification of the inverting amplifier in Fig. 3; and Fig. 6 is a circuit diagram of a successive approximation type A/D converter according to another embodiment of the present invention. Embodiments of the present invention will now be described below with reference to the accompanying drawings. Fig. 3 shows a 3-bit successive approximation type A/D converter formed into an MOS (insulated-gate type) integrated circuit. Referring to Fig. 3, reference symbols C1 and C2 denote capacitors each having predetermined capacitance value C; C3, a capacitor having capacitance value 2C; and C4, a capacitor having capacitance value 4C. The other terminal of each of capacitors C1 to C4 is commonly connected, and is also connected to the input terminal (node N) of inverting amplifier A. Switch SW is connected between the input and output terminals of inverting amplifier A. Inverting amplifier A and switch SW constitute a voltage comparator. Reference symbols SL1 to SL4 denote selectors for selectively connecting to reference voltage 2 or ground terminal 3, one terminal of each of capacitors C1 to C4. The selectors are controlled by successive approximation control digital signal outputs from successive approximation type control circuit B, which will be described later. Capacitors C1 to C4 and selectors SL1 to SL4 constitute a circuit for sampling analog voltage input Vain supplied to analog input terminal 1 and storing the resultant value, and a local D/A converter of a capacitor array charge redistribution type for performing D/A (digital-to-analog) conversion using successive approximation control digital signal outputs. Successive approximation type control circuit B described above has functions of successively determining the value of each bit of an A/D conversion output on the basis of an output from inverting amplifier A described above, and of outputting successive approximation control signals for successively generating local analog voltages required for successive approximation. In addition, according to the embodiment, capacitor C F having predetermined capacitance value C F (e.g., set to be 4C, which is 1 / 2 capacitance value Ci = 8C of the capacitor array) is additionally connected between input node N of inverting amplifier A and a fixed potential terminal having a predetermined potential, e.g., ground terminal 3. In this case, the A/D-converting operation of the above-described successive approximation type A/D converter is substantially the same as that of the conventional successive approximation type A/D converter described with reference to Fig. 1. However, they differ in the following points. According to the embodiment, in the sample mode, capacitance Q8 = (Vop - Vain)·8C is stored in capacitors C2 to C4 of the capacitor array, and capacitance Q F8 = Vop·C F is stored in capacitor C F . Thus, for example, when selector SL4 is set in a state for selecting the reference voltage before selectors SL1 to SL3 are set in a state for selecting the ground voltage terminal because of the influences of a switching operation in the capacity array when the sample mode is switched to the approximation mode, potential V1 at node N becomes: V = {1/(4C + C F )}{(Vop + V R - Vain)·4C + Vop·C F } In this case, when power source voltage V DD , reference voltage V R , and threshold voltage Vop of the above-described A/D converter are respectively set at 5 V, 5 V, and 2.5 V, potential V1 becomes 5 V because C F = 4C. Therefore, even when the CMOS analog switch shown in Figs. 2A and 2B is used as switch SW, the charges stored in capacitors C1 to C4 will not leak therefrom because diodes D P are not turned on. In addition, the input impedance of inverting amplifier A is set to be very high using, e.g., the gate input of a MOS transistor as the input stage, thereby preventing the charges stored in capacitors C1 to C4 from leaking. As is apparent from equation (6), potential V1 at node N decreases with an increase in analog input voltage Vain as compared with potential 0 V when the sample mode is switched to the approximation mode. Therefore, the charge leakage due to diodes D P can further and more reliably prevented. In contrast to the above case, for example, when selectors SL1 to SL3 are set in a state for selecting the ground terminal before selector SL4 is set in a state for selecting the reference voltage terminal because of the influences of a switching operation in the capacitor array when the sample mode is switched to the approximation mode, even if Vain = 5, diodes D N of the CMOS analog switch shown in Figs. 2A and 2B are not turned on because potential Vl becomes 0 V at V R = 0 V according to equation (6). It is for this reason, the charges at node N do not leak. Furthermore, in this case, as Vain becomes lower than 5 V, potential V at node N becomes higher than the 0 V, as is apparent from equation (6). Therefore, the charge leakage due to diodes D N can be further reliably prevented. As has been described above, according to the A/D converter of the present invention, since the presence of capacitor C F prevents the potential at the input terminal node of the voltage comparing inverting amplifier from being greatly changed to become higher than power source voltage V DD or lower than the ground voltage due the influence of a switching operation of the capacitor array for generating a local analog signal, leakage of charges at the node can be prevented. Therefore, even if the amplitude of an analog input voltage is equal to the power source voltage amplitude, high-precision A/D conversion can be properly performed. Note that a high impedance can be obtained by using, e.g., a CMOS inverter comprising p- and n-channel transistors TP2 and TN2 as the input stage of inverting amplifier A in the above-described embodiment. In addition, the voltage comparator is not limited to the combination of inverting amplifier A and switch SW as described above. Any circuit can be used as the voltage comparator as long as it can compare an input terminal voltage in the sample mode with that in the approximation mode. Accordingly, inverting amplifier A is not limited to the 1-input type as described in the above embodiment, but a 2-input type (differential type) amplifier as shown in Fig. 5 may be used. Although in the above embodiment, a capacitor array charge redistribution type is used as the local D/A converter, the present invention is not limited to this. For example, as shown in a successive approximation type A/D converter in Fig. 6, local D/A converter D of, e.g., a resistor segment type may be used, switch SW1, which is turned on in the sample mode ( = "1"), may be connected between analog input terminal 1 and one terminal of capacitor C0, and switch SW2, which is turned on in the approximation mode (  = "1"), may be connected between the analog output terminal of local D/A converter D and one terminal of capacitor C0. In this case, voltage comparing inverting amplifier A and switch SW are connected to the other terminal of capacitor C0, successive approximation type control circuit B is connected to the output side of inverting amplifier A, and successive approximation control signals from successive approximation type control circuit B are supplied to local D/A converter D. In the above-described A/D converter, by setting the capacitance value of additional capacitor C F to be substantially equal to that of capacitor C0, a great change in potential at node N when the sample mode is switched to the approximation mode can be prevented.

Claims: A successive approximation type A/D converter comprising: a circuit (SL1, SL2, SL3, SL4; SW1) connected to one terminal of a first capacitor (C1, C2, C3, C4; C0), for sampling an analog signal input (Vain) in a sample mode and supplying the sampled analog signal input to said one terminal of said first capacitor (C1, C2,C3, C4; C0); a voltage comparator (A, SW), connected to the other terminal of said first capacitor (C1, C2, C3, C4; C0), for comparing an input terminal voltage in the sample mode with an input terminal voltage in an approximation mode; a local D/A converter (D) for generating a local analog signal in the approximation mode in accordance with a successive approximation control digital signal and supplying the local analog signal to said voltage comparator (A, SW) through said first capacitor (C1, C2, C3, C4; CO); a successive approximation type control circuit (B) for successively outputting the successive approximation control digital signal in the approximation mode while determining values of bits of an A/D conversion output on the basis of a logic level of an output from said voltage comparator (A, SW); and a second capacitor (CF) connected between the input terminal of said voltage comparator (A, SW) and a predetermined voltage, said second capacitor being adapted to prevent leakage of charge at the input terminal of said voltage comparator (A, SW) to a degree that the input terminal is held at a voltage, characterized in that the capacitance value of said second capacitor (CF) is substantially equal to that of said first capacitor (C1, C2, C3, C4; C0), and said input terminal of said voltage comparator (A, SW) is held at a voltage between and not approximating either of a power supply voltage and said predetermined voltage. A converter according to claim 1, characterized in that said voltage comparator (A, SW) comprises an inverting amplifier (A), and a switch (SW) connected between input and output terminals of said inverting amplifier (A). A converter according to claim 1 or 2, characterized in that said switch (SW) is a CMOS analog switch. A converter according to claim 2, characterized in that a MOS transistor having a high input impedance is used as an input stage of said inverting amplifier (A). A converter according to claim 1, characterized in that said local D/A converter (D) and said first capacitor (C1, C2, C3, C4; C0) constitute a capacitor array charge redistribution type D/A converter. A converter according to claim 1, characterized in that said local D/A converter (D) is of a resistor segment type.

Inventors: GOTO JUNKEI, IIDA TETSUYA, GOTO, JUNKEI, IIDA, TETSUYA, IIDA, TETSUYA, C/O PATENT DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1988
Main classification: H03M1
IPC codes: H03M1