Power saving a/d converter

Abstract: An A/D conversion system comprises an A/D converter, a mode selection section (10) for periodically selecting a conversion mode and a standby mode, and a control section (12A) for controlling the A/D converter (12) based on the respective modes. The control section (12A) breaks the current path for the A/D converter during a standby mode. The duration of the standby mode can be specified from outside the system for power saving.

Description: BACKGROUND OF THE INVENTION (a) Field of the Invention The present invention relates to a power saving A/D converter and, more particularly, to an A/D converter suited for use in a portable data communication system such as a personal data assistance (PDA). (b) Description of the Related Art An A/D converter is used in a portable data communication system for converting an analog input signal, such as an audio signal or image signal, to a digital output signal for data transmission. JP-A-3(1991)-185928 proposes an A/D conversion system which repeats A/D conversion in a cyclic operation. Fig. 1 shows the proposed A/D conversion system, which comprises an A/D converter 12 for effecting A/D conversion and a control section 36 having therein an interval counter (not shown) for counting the clock pulses in the interval clock input 41. Referring additionally to Fig. 2 showing the timing chart for the A/D converter of Fig. 1, after the interval counter in the control section 36 counts up to the setting of number of counts specified by the interval setting signal 19, the interval counter supplies a conversion start signal 38 to the A/D converter 12. The A/D converter 12 effects A/D conversion of the input analog signal 14, sets the output digital signal 20 after the completion of the conversion and supplies an end of conversion (EOC) signal 40 to the control section 36. The control section 36 then supplies a conversion stop signal 39 and activates the interval counter at the same time. Subsequently, the interval counter again counts the clock pulses in the interval clock input 41 and supplies a conversion start signal 38 after the count up of the specified setting. These steps are repeated for a cyclic A/D conversion. In a portable data communication system, power saving is increasingly requested for a longer operational period of a battery. However, the conventional A/D converter as described above does not sufficiently meet this request. From US 5,294,928 a micro controller device having an associated analog digital converter is known, wherein the micro controller can be switched into an inactive or sleep mode in order to eliminate noise arising from switching activities during A/D conversion, which may be performed with the micro controller processor in the sleep mode. From the JP 01/265722 an analog digital converter is known which allows a reduction of current consumption by turning off a switch transistor to shut the power to the digital analog conversion section at the end of the analog digital conversion operation. JP 08/102675 describes an A/D converter comprising an element for switching the supplies of power supply voltage for an inverter latched converter and ground voltage. When A/D conversion operation is not executed electric power to be consumed by a ladder resistor can be cut off independently of the A/D conversion operation. JP 03/185928 describes an A/D converter circuit comprising a circuit element sending a conversion starting signal and a conversion stopping signal to the conversion circuit as necessary in order to shorten the processing time of the result of A/D conversion. It is therefore an object of the present invention to provide an A/D conversion system suited for use in a portable data communication system by reducing the power consumption of the A/D converter. The present invention provides an A/D conversion system as defined in claim 1. In accordance with the A/D conversion system according to the present invention, since the A/D converter is stopped for operation during the standby mode, power consumption of the A/D conversion system can be reduced. The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a conventional A/D conversion system; Fig. 2 is a timing chart for the A/D conversion system of Fig. 1; Fig. 3 is a block diagram of an A/D conversion system according to an embodiment of the present invention; Fig. 4 is a block diagram of an example of the A/D converter shown in Fig. 3; Fig. 5 is a block diagram of an example of the standby signal generator shown in Fig. 3; Fig. 6 is a timing chart for the A/D conversion system of Fig. 3; Fig. 7 is a block diagram of an example of the resistor string circuit shown in Fig. 4; and Fig. 8 is a block diagram of an example of the comparator shown in Fig. 4. PREFERRED EMBODIMENTS OF THE INVENTION Now, the present invention is more specifically described with reference to accompanying drawings, wherein similar constituent elements are designated by the same reference numerals throughout the drawings. Referring to Fig. 3, an A/D conversion system according to an embodiment of the present invention comprises an A/D converter 12 for effecting an A/D conversion of an input analog signal 14, a mode selection section including a timing signal generator 11 for generating timing signals 15 and 18 and a standby signal generator 13 for generating a standby signal 17, and a control section 12A for breaking a current path for the A/D converter 12 in response to the standby signal. The A/D converter 12 receives analog input signal 14, converts the same to a digital data in response to a clock signal not shown, and supplies the digital output data 20. The timing signal generator 11 receives a clock input 16, and supplies timing signal 15 to the A/D converter 12 for starting and stopping of the A/D conversion and timing signal 18 to the standby signal generator 13 for starting a standby mode. The standby signal generator 13 receives an interval setting signal 19, clock input 16 and timing signal 18, and generates standby signal 17, which is supplied to both the A/D converter 12 and the timing generator 11, for effecting a standby mode of the A/D conversion for a duration based on the sampling rate of the A/D conversion. In the standby mode wherein standby signal 17 is active, the current path to the main circuit portion of the A/D converter 12 is cut or broken for power saving by the control section 12A, and the timing signal generator stops for counting of the clock pulses. Referring to Fig. 4, the A/D converter 12 is of a successive approximation type and comprises a resistor string circuit 21 acting as a D/A converter, a comparator 22 for comparison of analog inputs, and a successive approximation register (SAR) 23 for supplying output digital data 20 after completion of the conversion. The resistor string circuit 21 receives a digital feed-back data 25 from the successive approximation register 23 and supplies an internal analog data 24 to the comparator 22 based on digital feed-back data 25. The comparator 22 compares internal analog data 24 from the resistor string circuit 21 against analog input signal 14 input to the A/D conversion system, to thereby output a result signal 26 assuming a high level or low level based on the result of the comparison. The successive approximation register 23 first stores a provisional data, then increases or decreases the provisional data based on the output of the comparator 22, supplies the increased or decreased data as digital feed-back data 25 to the resistor string circuit 21, repeats the increase or decrease of the stored provisional data to the least significant bit, and holds and outputs a final data obtained by the successive approximation. Timing signal 15 from the timing signal generator 11 is supplied to the successive approximation register 23, comparator 22 and resistor string circuit 21. Standby signal 17 is supplied to the resistor string circuit 21 and comparator 22. Referring to Fig. 5, the standby signal generator 13 comprises a standby counter 27 and a RS-flipflop (RS-FF) 28. The standby counter 27 responds to timing signal 18 to start for counting the clock pulses in the clock signal 16, and counts up to the setting by the interval setting signal 19 to output a reset signal 29 for the RS-FF 28. The RS-FF 28 is set by timing signal 18 and reset by the output 29 of the standby counter 27, and outputs standby signal 17. Referring to Fig. 6, in addition to Figs. 3, 4 and 5, the timing of the A/D conversion by the A/D conversion system of the present embodiment is controlled mainly by timing signal 15 including a conversion start signal 15a and a conversion stop signal 15b, as follows. When conversion start signal 15a turns to an active level or high level, the comparator 22 shown in Fig. 4 starts for comparing analog input signal 14 against analog internal data 24 decoded by the resistor string circuit 21, to thereby output a high level or low level for allowing the successive approximation register to effect successive approximation. When conversion stop signal 15b turns to an active level or high level, the successive approximation register 20 holds the data stored therein as an output digital data 20 for the duration of the next standby mode. Just after conversion stop signal 15b turns active, timing signal 18 turns active to thereby set the RS-FF 28, which outputs an active level of standby signal 17. The standby counter 27 starts for counting the clock pulses in clock signal 16 in response to the active level of timing signal 18 and counts up to the setting specified by interval setting signal 19 to thereby output the reset signal for the RS-FF 28, which turns the standby signal 17 to an inactive level. The setting specified by the interval setting signal 19 is determined by the sampling period of the A/D conversion system which is a sum of the time length for the A/D conversion and the time length for the standby mode. During the time interval when standby signal 17 remains at an active level or high level, the A/D converter 12 stops its operation. After standby signal 17 is reset by the reset signal 29 from the standby counter 27, the timing signal generator 11 starts for operation to turn conversion start signal 15a to an active level or high level, thereby allowing the A/D converter 12 to effect A/D conversion. In this way, the A/D conversion system operates at a specified sampling rate periodically or intermittently. Referring to Fig. 7, an example of the resistor string circuit 21 in the A/D converter 12 comprises a resistor block or resistor string 30 including a plurality of resistors R 1 to R n connected in series between Vdd line and GND, and a decoder 26 for decoding the voltages at the respective taps p 1 to p n-1 of the resistor string 30. A pMOSFET 32 acting as an element of the control section 12A is connected between Vdd line and the resistor string 3. The pMOSFET 32 receives standby signal 17 and turns off in response to the active level of standby signal 17 to break the current path for the resistor string 30 for power saving during the standby mode. During an A/D conversion mode wherein standby signal 17 is inactive, the decoder 31 receives the feed-back data 25 from the successive approximation register 23, and outputs an analog voltage supplied from one of the taps p 1 to p n-1 of the resistor string 30 as a decoded feed-back data. In this operation, the resistor string circuit 21 functions as a D/A converter for decoding the feed-back data 25 from the successive approximation register 26. Referring to Fig. 8, an example of the comparator 22 in the A/D converter of Fig. 4 comprises a pair of differential transistors formed of pMOSFETs MP1 and MP2, a first current mirror formed of nMOSFETs MN1 and MN2 and connected between the pair of differential transistors MP1 and MP2 and GND for acting as active loads for the pair of differential transistors, a second current mirror formed of pMOSFETs MP3 and MP4 and acting as a constant current source for the pair of differential transistors MP1 and MP2, and a constant current source I0 for providing the constant current for pMOSFET MP3 of the second current mirror. The common gate of the second current mirror MP3 and MP4 is connected to Vdd line through a switching transistor acting as another element of the control section 12A and formed of pMOSFET MP5 having a gate receiving an inverted signal of standby signal 17. The pMOSFET MP1 receives decoded feed-back data 24 or internal analog data whereas the pMOSFET MP2 receives analog input signal 14. Thus, during an A/D conversion mode, the comparator 22 compares input analog signal 14 against the decoded feed-back data 24 and outputs a high or low level in accordance with the low or high level of the decoded feed-back data 24 with respect to analog input signal 14. On the other hand, during a standby mode, the current path for the comparator 22 is cut by the pMOSFET MP5 for power saving. In accordance with the A/D conversion system according to the present embodiment, the duration for the standby mode can be specified from outside the system to obtain an optimum periodic or intermittent operation of the system while saving the electric power by cutting the current path for the A/D conversion and by stopping the counting operation of the timing generator. The variable setting for the standby counter 27 enables a single clock signal to control different timings in the system, thereby avoiding an increase of the size of the system. Further, a single clock signal may be used in common for the A/D converter, timing signal generator and the standby signal generator, if desired. Since the above embodiments are described only for examples, the present invention is not limited to the above embodiments and various modifications or alterations can be easily made therefrom by those skilled in the art without departing from the scope of the present invention.

Claims: An A/D conversion system comprising: an A/D converter (12) for converting an analog input signal (14) to a digital output signal (20); a timing signal generator (11) for receiving a clock signal (16) to generate a timing signal (15, 18), said timing signal controlling said A/D converter (12) to operate for periodical conversion; a standby signal generator (13) for receiving said clock signal (16) to activate a standby signal (17) for stopping operation of said A/D converter (12) and said timing signal generator (11) during periodical conversions by said A/D converter (12); and a control section (12A) for stopping operation of said A/D converter (12) during activation of said standby signal (17), wherein said standby signal generator (13) comprises a standby counter (27) for counting clock pulses in said clock signal (16) during activation of said standby signal (17), and inactivates said standby signal (17) after said standby counter (27) counts up to a setting input from outside said A/D conversion system. An A/D conversion system as defined in claim 1 wherein said control section (12A) comprises a switching transistor (32, MP5) for breaking a current path of at least a part of said A/D converter (12) during said standby mode. An A/D conversion system as defined in claim 1, wherein said A/D converter (12) is controlled by clock pulses in said clock signal for A/D conversion. An A/D conversion system as defined in claim 1 wherein said A/D converter (12) is of a successive approximation type.

Inventors: SUZUKI KENICHI, SUZUKI, KENICHI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1997
Main classification: H03M1
IPC codes: G06F3, H03M1