Sample-and-hold circuit device

Abstract: A sample -and-hold circuit device characterised by comprising: first and second sample-and-hold means (209, 210) for receiving an input signal; first and second buffer means (211, 212) for receiving outputs from said first and second sample-and-hold means, respectively; first and second switch means (213, 214) which are alternately opened or closed to alternately connect output terminals of said first and second buffer means to an internal terminal so that one of said first and second buffer means samples and holds the input signal and the other thereof reads out an signal held therein; and third buffer means (215) including said internal terminal as an input terminal.

Description: The present invention relates to a high-precision sample-and-hold circuit device for driving a liquid crystal display or the like. According to a conventional sample-and-hold circuit device, a p-channel MOS transistor and an n-channel MOS transistor are connected in parallel with each other to constitute a MOS analog switch with one connection node being connected to an input terminal, and the other connection node being connected to a capacitor and an output terminal. In the conventional sample-and-hold circuit device, control signals 1 and 2 having opposite phases are supplied to the gates of the parallel-connected MOS transistors to control an opening/closing operation of the MOS analog switch. When the MOS analog switch is set in a closed state by the control signals 1 and 2, charges are accumulated in the capacitor until the terminal potential of the capacitor reaches an input terminal potential. When the MOS analog switch is opened by the control signals 1 and 2, the charges accumulated in the capacitor is held. Such an operation is called a sample-and-hold operation. In the above sample-and-hold operation, if the sampling rate is increased, it is difficult that the conventional sample-and-hold circuit performs an ideal sample-and-hold operation. The reasons for this will be described below. Stray capacitance called overlap capacitances are present between the gate electrode and the drain electrode and between the gate electrode and the source electrode of the MOS transistors. When the MOS switch is changed from a closed state to an open state by the control signals 1 and 2, the control signals leak from the gate electrodes into the capacitor through the overlap capacitor. The leaking signals serve as error charges and are superposed on the charges held in the capacitor. In addition, according to an operation principle of a MOS transistor, channel charges are generated in the channels of the two MOS transistors during an ON state. When each MOS transistor goes to an OFF state at a very low speed, the channel charge flows in one of the source and drain electrodes which has a lower impedance. In contrast to this, if each MOS transistor goes to an OFF state at a high speed, the channel charge branches and flows in both the source and drain electrodes. Therefore, if sampling rate is increased, a given percent of channel charges are superposed, as error charges, on the charges held in the capacitor. A sample-and-hold circuit is applied to a driving circuit used for a liquid crystal display or the like. In this case, the above-described sample-and-hold circuit is connected to an input terminal, and an output buffer having an output enable terminal is connected to the sample-and-hold circuit. The output terminal of the output buffer is connected to a liquid crystal cell through a switch using a TFT (Thin Film Transistor). Note that a wiring capacitance on the liquid crystal display is formed between the nodes of the output buffer and the TFT. According to the above-mentioned liquid crystal display driving circuit, the sample-and-hold circuit samples/holds a video signal by using the control signals 1 and 2 in a horizontal period of the video signal in which an effective video signal is sent (to be referred to an effective period hereinafter). At this time, the output buffer is set in an OFF state by a control signal OE, and its output terminal is set in an high-impedance state. In a horizontal blanking period after a sample-and-hold operation is performed, the wiring capacitance is charged to a sampled/held potential by a control signal OE. In addition, the liquid crystal cell is driven through the TFT which is set in ON state by a control signal Yc. Since the characteristics of the TFT are inferior to those of a normal MOS transistor, the TFT must be further kept in the ON state for a given interval A after the horizontal blanking period by using the control signal. When the output terminal of the buffer is set in an high-impedance state by the control signal OE, and the TFT is set in an ON state, the potential held in the wiring capacitance drives the liquid crystal cell through the TFT even in the interval A. The conventional liquid crystal display driving circuit has the following drawback as well as errors and difference in characteristics among the sample-and-hold circuits. Since the wiring capacitance is energized by nothing in the liquid crystal display driving interval A, the driving circuit is susceptible to the influences of external noise, and deterioration in picture quality tends to occur. In order to eliminate such a drawback, a conventional sample-and-hold circuit may have a multistage arrangement. In a multistage sample-and-hold circuit, however, errors and differences in characteristics among the respective stages may be superposed on each other. Furthermore, since all the sample-and-hold circuits corresponding to the number of pixels of one horizontal scanning line cannot be integrated into one IC, a large number of ICs incorporating a large number of sample-and-hold circuits are used to drive the display. In this case, considerable differences in brightness or color tone occur in the form of a strip at portions on the screen at which the respective ICs are driven, thus deteriorating the picture quality. As described above, in the conventional technique, the first problem to be solved is associated with an error caused by leakage of control signals, supplied to the sample-and-hold circuit, to the output terminal, and an error caused by channel charges generated when the analog switch constituted by the MOS transistors is in an OFF state. Furthermore, in the IC incorporating a large number of sample-and-hold circuits applied to a driving circuit for driving a liquid crystal display or the like, difference in errors among the sample-and-hold circuits makes the picture quality further deteriorate. The second problem to be solved is associated with the drawback that with a sample-and-hold circuit having a single-stage arrangement, a TFT or a liquid crystal cell of a liquid crystal display can be driven in only a horizontal blanking period of one horizontal period, and hence external noise appears on the screen to deteriorate the picture quality. If a sample-and-hold circuit having a two-stage arrangement is used to prevent such a problem, output attenuation or variation is undesirably increased. In addition to the problems in ICs, the third problem to be solved must be considered, which is associated with the drawback that differences in brightness or color tone occur in the form of a strip at portions on the screen driven by the respective ICs, due to difference in characteristics among the ICs, so as to considerably deteriorate the picture quality. EP-A-0200402 describes a sample and hold circuit wherein two sample-and-hold circuits are connected in parallel. The output voltages held by the capacitors of the sample-and-hold circuits are selected directly by a pair of switches and supplied to a voltage follower circuit. EP-A-0157607 discloses an analog data storage system in which the outputs of a plurality of memory cells are selected by switch means via a buffer in the form of an inverter. EP-A-0 240 682 discloses a sample and hold circuit in which the sampled signal is read from the storage capacitor through a source follower circuit. EP-A-0398778 which falls under Article 54(3) EPC discloses a sample-and-hold unit having two identical parallel channels, each of which has a first follower switch, a capacitor and a second holding switch. The channels are controlled by a single clock, the complimentary or phase-shifted signal of which addresses the samples sequentially towards the output. It is an object of the present invention to provide a sample-and-hold circuit device which decreases error and variation of an output signal. It is another object of the present invention to provide a sample-and-hold circuit device which can drive a liquid crystal display for an arbitrary period, within one horizontal period, which is longer than a horizontal blanking period without causing an increase in error and difference in characteristics among sample-and-hold circuits. It is still another object of the present invention to provide a sample-and-hold circuit device which compensates for difference in characteristics among ICs. According to the present invention, there is provided a sample-and-hold circuit device as defined in claim 1. Preferred features are specified in dependent claims 2 to 5. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 44 is a circuit diagram showing a sample-and-hold circuit device for outputting a sampled/held value for one horizontal period according to an embodiment of the present invention; Fig. 45 is a timing chart of control signals used for the sample-and-hold circuit device in Fig. 44; Fig. 46 is a circuit diagram showing a sample-and-hold circuit device for outputting a sampled/held value for one horizontal period according to another object of the present invention; Fig. 47 is a timing chart of control signals used for the sample-and-hold circuit device in Fig. 46; Fig. 48 is a circuit diagram showing a sample-and-hold circuit device for outputting sampled/held value for one horizontal period according to still another embodiment of the present invention; Fig. 49 is a timing chart of control signals used for the sample-and-hold circuit device in Fig. 48. A sample-and-hold circuit device according to an emodiment of the invention comprises first and second sample-and-hold circuits for receiving an input signal, a first buffer for receiving an output from the first sample-and-hold circuit, a second buffer for receiving an output from a second sample-and-hold circuit, a first switch having an internal terminal connected to the output terminal of the first buffer and ON/OFF-operated by a first control signal, a second switch having an internal terminal connected to the output terminal of the second buffer and ON/OFF-operated by a second control signal, and a third buffer having an internal terminal as an input terminal. According to the above-described sample-and-hold circuit device, the first and second sample-and-hold circuits are alternately operated in units of horizontal intervals to repeat sampling and outputting operations in the following manner. A potential sampled by the first sample-and-hold circuit in an effective time interval within an Nth horizontal period of an image is output through the first and third buffers by turning on the first switch using the first control signal and turning off the second switch using the second control signal in an (N+1)th horizontal period. Subsequently, a potential sampled in an (N+1)th horizontal period is output through the second and third buffers by turning on the second switch using the second control signal and turning off the first switch using the first control signal. The above-described sample-and-hold circuit device will be described in detail below with reference to Fig. 44. In the circuit device shown in Fig. 44, an input terminal 208 is connected to the input terminals of the first and second sample-and-hold circuits 209 and 210. The output terminals of the first and second sample-and-hold circuits 209 and 210 are respectively connected to first and second switches 213 and 214 through first and second buffers 211 and 212. The output contacts of the first and second switches 213 and 214 are connected to an output terminal 216 through a third buffer 215. An operation of the circuit shown in Fig. 44 will be described below with reference to a timing chart in Fig. 45. A signal input from the input terminal 208 is sampled in the Nth horizontal interval by the first sample-and-hold circuit 209 in response to a control signal 36. At this time, the first switch 213 is set in an OFF state by control signals 38 and 39. In addition, in response to a control signal 37, the second sample-and-hold circuit 210 holds a potential sampled in an (N-1)th horizontal period. This potential is output through the second buffer 212, the second switch 214, and the third buffer 215 in the Nth horizontal period. In an (N+1)th horizontal period, the first switch is turned on by the control signal 38, and the second switch is OFF by the control signal 39. At this time, in response to the control signal 36, the first sample-and-hold circuit holds the potential sampled in the Nth horizontal period. This potential is output in the (N+1)th horizontal period through the first buffer 211, the first switch 213, and the third buffer 215. In response to the control signal 37, the second sample-and-hold circuit samples an input signal. Therefore, the first and second sample-and-hold circuits are alternately operated in units of horizontal periods to repeat sampling and outputting operations. Even if the switches 213 and 214 are realized by analog switches constituted by MOS transistors, since either of the switches 213 and 214 is turned on and is driven by an output from the buffer 211 or 212, error charges such as channel charges generated upon an ON/OFF operation of the switches 213 and 214 are absorbed by the buffers to prevent the error charges from appearing at the output terminal, thus driving the output terminal 216 in one horizontal interval without increasing errors or difference in characteristics. The embodiment shown in Fig. 44 will be described in more detail below with reference to a circuit in Fig. 46 and a timing chart in Fig. 47. Referring to Fig. 46, the output terminal of a source follower 238 constituted by a MOS transistor 218 and a current source 219 is connected to sample-and-hold circuits 239 and 240. The sample-and-hold circuits 239 and 240 are respectively connected to switches constituted by MOS transistors 232 and 233. These switches are connected to a voltage follower 241 including a source follower consisting of transistors 235 and 236 and a current source 237 in a negative feedback path. The output terminal of the voltage follower 241 is connected to an output terminal 242. The sample-and-hold circuits 239 and 240 are designed to include buffers formed of source followers constituted by a MOS transistor 223 and a current source 224, and a MOS transistor 229 and a current source 230 respectively, and hence also serve as the buffers 211 and 212 shown in Fig. 44. In such a circuit, errors can be reduced by setting the output impedance of the source follower 238 to be equal to that of the source follower included in each of the sample-and-hold circuits 239 and 240. By additionally arranging a level shift circuit constituted by MOS transistors 235 and 236 and a current source 237 in the negative feedback path in the voltage follower used as the output buffer 241, the voltage follower also serves to perform DC offset compensation of potential differences between the gate-source paths of the source followers 238, and the sample-and-hold circuits 239 and 240. Therefore, difference in output characteristics can be reduced to the above-mentioned matching precision by performing matching between the transistors 218, 223, 229, 235, and 236 constituting the source followers and the level shift circuits and the current sources 219, 224, 230 and 237 in Fig. 46. The overall circuit is operated in the same manner as described with reference to Fig. 44. Error charges such as channel charges generated upon ON/OFF operations of the switches 232 and 233 are absorbed by the source followers included in the sample-and-hold circuits 239 and 240 to prevent the error charges from appearing at the output terminal, thereby driving the output terminal in one horizontal period without increasing errors and difference in characteristics. Fig. 48 shows a circuit wherein the first and second sample-and-hold circuits 209 and 210 in Fig. 44 are realized by the circuit described with reference to Fig. 15, and the first and second buffers 211 and 212 are respectively realized by source followers 266 and 267. The source followers 266 and 267 are connected to an output buffer 268 through switches 258 and 259. In this circuit, circuit elements 244 to 248 respectively correspond to the circuit elements 55, 91, 56, and 58 in Fig. 15. Similarly, circuit elements 249 to 253 respectively correspond to the circuit elements 55, 91, 56, and 58 in Fig. 15. In addition, circuit elements 260 to 263 respectively correspond to the circuit elements 102 and 105 in Fig. 30. Fig. 49 shows control signals 46 to 51 to be input to the circuit in Fig. 48. The output buffer 268 of the circuit in Fig. 48 also serves as a level shift circuit and hence compensates for a DC offset due to a forward voltage drop of a diode, generated in each of the first and second sample-and-hold circuits 264 and 265, and a DC offset due to a voltage drop in the gate-source path of each of the source followers 266 and 267, as in the embodiment of Fig. 30, for compensating for a DC offset. Output variations can be reduced to the above-mentioned matching precision by performing matching between diodes 246, 251, and 262 and between transistors 254, 256, and 261 in Fig. 48.

Claims: A sample-and-hold circuit device comprising: first and second sample-and-hold means (209, 210) for receiving an input signal; first and second buffer means (211, 212) for receiving outputs from said first and second sample-and-hold means, respectively; first and second switch means (213, 214) which are alternately opened or closed; and third buffer means (215) connected to an internal terminal as an input terminal; said first buffer means including first source follower circuit means (223, 224, 266) connected to said first sample-and-hold means (220, 221, 264) and an output terminal, for receiving an output from said first sample-and-hold means; said second buffer means including second source follower circuit means (229, 230, 267) connected to said second sample-and-hold means (226, 227, 265) and an output terminal, for receiving an output from said second sample-and-hold means; said first and second switch means (232, 233, 258, 259) being alternately opened or closed to alternately connect the output terminals of said first and second buffer means to an internal terminal; and said third buffer means (241, 268) including an input terminal connected to said internal terminal and voltage follower circuit means having a feedback loop containing a level shift circuit (235, 236, 237, 261, 262, 263). A sample-and-hold circuit device according to claim 1, characterized in that said third buffer means comprises an operational amplifier (234, 260) having a first input terminal (+) connected to said internal terminal and a second input terminal (-) connected to the feedback loop of said voltage follower circuit means, and an output terminal (242, 269) connected to an output terminal of said operational amplifier and to an input terminal of said voltage follower circuit means. A sample-and-hold circuit according to claim 2, characterized in that said voltage follower circuit means comprises first and second transistors (235, 236) and a current source (237), all of which are connected in series, with a node between said second transistor and said current source being connected to the second input terminal of said operational amplifier to form the feedback loop. A sample-and-hold circuit device according to claim 2, characterized in that each of said first and second sample-and-hold means comprises a first MOS transistor (244 or 249) connected to an input terminal receiving an input signal, a first diode (246 or 251) connected between said first MOS transistor (244 or 249) and the corresponding one of said first and second buffer means(266 or 267), a first capacitor (247 or 252) corresponding to one of said first and second buffer means, a second capacitor (245 or 250) connected to a node between said first MOS transistor and said first diode, and a second MOS transistor (248 or 253) for initializing said first capacitor; and said voltage follower circuit means comprises a third MOS transistor (261), a second diode (262), and a current source (263), all of which are connected in series, with a node of said second diode and said current source being connected to the second input terminal of said operational amplifier (260) to form the feedback loop. A sample-and-hold circuit device according to claim 4, characterized in that each of said first and second buffer means (266, 267) comprises a fourth MOS transistor (254 or 256) having a gate connected to an output terminal of the corresponding one of said first and second sample-and-hold means and a second current source (255 or 257) connected in series to said fourth MOS transistor, with a node between said fourth MOS transistor and said second current source being connected to the corresponding one of said first and second switching means (258, 259).

Inventors: ITAKURA TETSURO, SHIMA TAKESHI, ITAKURA, TETSURO, SHIMA, TAKESHI, Itakura, Tetsuro, c/o Intell. Prop. Div., Shima, Takeshi, c/o Intell. Prop. Div.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: G11C27
IPC codes: G11C27, G09G3