Successive-approximation analog-digital converter and related operating method

Abstract: Herein described is a successive-approximation analog-digital converter comprising a logic control circuit (1) timed by means of an external clock signal (clock). Said logic control circuit (1) comprises a register (11), which contains a first digital signal (D1) formed of N bits and obtained from a first analog-digital conversion. Said control circuit (1) is suitable for producing a second digital signal (D) formed of N bits through a second analog-digital conversion in N clock cycles. Said analog-digital converter comprises a digital-analog converter (2) which converts the second digital signal (D) sent by the logic circuit (1) to an analog signal (A), a comparator (3) which compares the analog signal (A) with an analog signal (B) which is in input to the analog-digital converter. The converter comprises a device (20, 4) which enables the increase of the analog signal (A) in output from the digital-analog converter (2) and in input to the comparator (3) of a preset value (Voffs) when the bit of the first digital signal (D1) which corresponds in position to the bit of the second digital signal (D) which must be decided in a clock cycle is zero.

Description: The present invention relates to a successive-approximation analog-digital converter. The use of analog-digital converters that use a successive approximation technique, that is in which an analog signal is converted into a digital signal at N bits in N clock cycles, is generally known in the technical field. The converter basically consists of a comparator, a digital-analog converter (DAC) and a digital control logic. The function of the digital control logic is to determine the value of each bit by a sequential mode based on the output of the comparator. The cycle of conversion starts by sampling the analog input signal that has to be converted; however if the input signal is slowly variable that is, basically a signal that has a variation less than the least significant bit (LSB) for the duration of a conversion it cannot be sampled. Then the logic control circuit assumes that the most significant bit (MSB) is 1 and all the other bits 0. The digital signal is sent to the DAC which generates an analog signal, for example of 0.5 Vref, where Vref is a reference voltage, which is compared with the analog input signal already sampled. If the output of the comparator is high the digital control logic makes the MSB 1, otherwise if the output of the comparator is low the digital control logic makes the MSB 0. This completes the first step in the approximation sequence. At this point, the value of the most significant bit is known. The approximation sequence continues another time sending a digital signal to the DAC, with the most significant bit at its indicated value, the second bit placed at 1 and all the other bits which have a value 0. The comparison between the input signal sampled and the output of the DAC is again made; if the output of the comparator is high, the second bit is placed at 1, otherwise the second bit is placed at 0. The process continues in this manner until all the bit of the digital signal have been determined by the successive approximation process and therefore the number of cycles required for the conversion of an analog signal in a word of N bits is N. In the case in which it is necessary to convert slowly variable signals, that is such that they do not significantly vary between one conversion and the next one (basically a signal with a variation that is less than one LSB (the least significant bit) for the duration of one conversion), given that said signals can be affected by disturbances (for example, noise) which cause undesired rapid variations in given periods of time, the digital output signal from an analog-digital converter like the above mentioned one will present anomalies. In particular, if the slowly variable analog signals that present oscillation in correspondence with the transition threshold between the value of the digital signal and the value of the continuous signal are considered, said oscillations may cause anomalies in the systems that have to be controlled by the converted digital signal. US 5119097 discloses a parallel comparison type A/D converter having an hysterisis in which a reference voltage level for comparing with an input analog signal is switched according to a digital converted signal so that the output digital value has a hysterisis characteristic with respect to the input analog signal. US 4196420 discloses an A/D converter including a first D/A converter for converting the most significant bits of a digital output signal having a given number of bits to a first analog reference signal having a value that is proportional to the value of the most significant bits minus one-half the least significant bit of the digital output signal whenever the value of the most significant bits is other than zero. The A/D converter comprises a second digital signal converter for converting the least significant bits of the digital output signal to a second analog reference signal and a comparator for comparing an analog input signal with the sum of the analog reference signals. The A/D converter comprises also a successive approximation register connected to be driven by the comparator for successively providing the bits of the digital output signal beginning with the most significant bit in accordance with the comparison of the analog input signal with the sum of the analog reference signals, for providing the most significant bits of the digital output signal to the first D/A converter to drive it and for providing the least significant bits of the digital output signal to the second D/A converter to drive it. The A/D converter comprises a logic circuit coupled with the successive approximation register for providing an indication of whether the value most significant bit is other than zero and the second D/A converter is responsive to the indication by the logic circuit for causing the second analog reference signal to have a value proportional to the value of the least significant bits whenever the value of the most significant bits is other than zero, and a value proportional to the value of the least significant bits minus one-half the least significant bit of the digital output signal whenever the value of the most significant bits is zero. In view of the state of the technique described, the object of the present invention is to present a successive-approximation analog-digital converter which overcomes the above mentioned drawback. According to the present invention, such object is reached by means of an analog-digital converter as defined in claim 1. Moreover, according to the present invention, a method for the conversion of an analog signal to a digital signal as defined in claim 5 can be made. Thanks to the present invention it is possible to make a successive-approximation analog-digital converter which, by the addition of a hysteresis at the DAC output, permits the elimination of the anomalies presented by known successive-approximation analog-digital converters in case of conversion of a slowly variable signal which presents oscillations at the transition threshold between the value of the digital signal and the value of the continuous signal. The characteristics and the advantages of the present invention will appear evident from the following detailed description of an embodiment thereof, illustrated as non-limiting example in the enclosed drawings, in which: Figure 1 is a principle diagram of a circuit according to the invention; Figure 2 is a more detailed diagram of block 1 of Figure 1; Figures 3 and 4 are time diagrams of the forms of voltage in output from the DAC in two different cases; Figure 5 shows more in detail the implementation of a DAC for the circuit in Figure 1. With reference to Figure 1 a successive-approximation analog-digital converter is shown in which a logic control circuit 1 generates the digital signal D in input to a digital-analog converter (DAC) 2. The latter generates an analog signal A in input to a comparator 3 at its negative terminal; the comparator 3 compares said analog signal A with an analog signal B which represents the analog signal to convert that has already been sampled. The control circuit 1 is a timed logic control circuit as an external clock timing signal acts on it. Between the DAC 2 and the comparator 3 a switch 4 is provided which switches between the two possible states 5 and 6 so that the analog voltage signal A in input to the comparator 3 is or the signal in output from the DAC 2 (in said case the switch 4 is in state 5), or the signal in output from the DAC 2 increased by a low value of voltage Voffs (with the switch 4 in the state 6). The switch 4 is controlled by a signal C coming from a control circuit 20 being part of the logic control circuit 1. Generally a logic control circuit 1 for a successive-approximation analog-digital converter comprises an outputs register 1 and a digital circuit 12 on which the external clock signals act, start (start of conversion) and end (end of conversion), as can be seen in Figure 2. The digital circuit 12 controls the register 11 and also the switch 4 given that it comprises also the control circuit 20. The latter, which has in input a digital signal D1 previously converted and contained in the register 11, makes switch 4 to switch to position 6 only in the case in which in a clock cycle the bit of the digital signal D1 which corresponds in position to the bit of the digital signal D to convert is a zero. The operation of such a kind of converter is the following. If we presume that the number of the bits is N=3 we have a 3 bit successive-approximation converter with therefore 3 clock cycles. The digital signal D is formed of bits b0, b1, b2 and the register 11 contains the result of the previous conversion, that is the digital signal D1 formed of bits b0', b1', b2'. The logic control circuit 1 assumes that the b0 of the digital signal D is 1 and all the other bits (bits b1 and b2) 0. The digital signal D is sent to DAC 2 which generates the analog signal A, for example by 0.5Vref, where Vref is a reference voltage in input at DAC 2. At this point if bit b0' of the digital signal D1 is 0 the control circuit 20, through the signal C commands the increase of the analog voltage signal A in output from the DAC 2 of the low Voffs voltage, by means of the switch 4 placed in position 6, otherwise if the bit b0' is 1 the analog signal A is left unaltered. The analog signal A is compared with the analog signal in input B. If the output of the comparator is high the logic control circuit 1 makes the MSB equal to 1; otherwise, if the output of the comparator is low, the logic control circuit 1 makes the MSB equal to 0. This completes the first stage of the approximation sequence that is a clock cycle. At this point the value of the most significant bit is known. The approximation sequence continues once again sending a digital signal D to the DAC, with the most significant bit b0 at its indicated value, the second bit b1 placed at 1 and the bit b2 placed at 0. At this point again if the bit b1' of the digital signal D1 is 0 the analog voltage signal A in output from the DAC 2 is increased by the low voltage Voffs; otherwise if the bit b1' is 1 the analog signal A is left unaltered. There is again the comparison between the signal in input B and the signal A; if the output of the comparator is high, the second bit is placed at 1, otherwise the second bit is placed at 0. At this point, the value of the second bit is also known. The approximation sequence continues once again sending a digital signal D to the DAC, with the bits b0 and b1 at their indicated values, and the bit b2 placed at 1. Again if the bit b2' of the digital word D1 is 0 the analog voltage signal A in output from the DAC 2 is increased by the low Voffs voltage otherwise if the bit b2' is 1 the analog signal A is left unaltered. Again there is the comparison between the signal in input B and signal A; if the output of the comparator is high, the third bit is placed at 1 otherwise the third bit is placed at 0. At the end of the process the signal D determines the digital signal out which is the output of the analog-digital converter. In the graphs of Figures 3 and 4, the examples are shown of the signals of voltage A in input to the comparator 3 in the case of digital signal D 101 and digital signal D 011 with the corresponding digital signals D1 101 and 011 which are placed on the x-coordinate in the graphs of Figures 3 and 4. In Figures 3 and 4 the variation of the voltage signal A is shown in relation to time t/T with T clock period and it can be seen how the voltage signal A is increased by the low voltage Voffs each time the bit of the digital signal D1 which corresponds in position to the bit of the digital signal D is 0. The increase in the voltage signal A by the low voltage Voffs creates a hysteresis of voltage 200 which enables the elimination of the anomalies presented by the analog digital successive approximation converters known in the case of conversion of a slowly variable signal which presents oscillations at the transition threshold between the value of the digital signal and the value of the contiguous signal. Generally said value of the voltage Voffs is less than the value of voltage of conversion of one least significant bit (LSB). A possible implementation of the DAC 2 and the switch 4, in the case of a three bit analog-digital converter, is shown in Figure 5. The circuit 100 of Figure 5 consists of an n channel transistor MOS M1 and of a p channel transistor MOS M2 both controlled by the signal C and of a common resistive string DAC with resistances R and R/2 and switches controlled by signals b0-b2 and b0neg-b2neg which represent the value and the negation of the bit which compose the digital signal D in input to the DAC 2. The signal C, which is generally high when closing M1 and opening M2, in the case in which the voltage signal A in output to the circuit 100 must be increased by a Voffs voltage, becomes low causing the opening of M1 and the closing of M2. In this latter case, in comparison to the case of a similar and common resistive string DAC, the voltage signal A in output will be increased by Vref/16 which therefore represents the voltage Voffs.

Claims: Analog-digital converter comprising a logic control circuit (1) timed by means of an external clock signal (clock), said control circuit (1) being suitable for producing a digital signal (D), corresponding to a first analog signal (A), formed of N bits by means of an analog-digital conversion in N clock cycles, a comparator (3) which compares said first analog signal (A) with a second analog signal (B) which is in input to said analog-digital converter, said logic control circuit (1) comprising a register (11) and the analog-digital converter comprising a device (4, 20), characterised by comprising a digital-analog converter (2) which converts said digital signal (D) sent by said logic circuit (1) to said first analog signal (A), and in that said analog-digital converter is of the successive approximation type, said register (11) contains a previous digital signal (D1) formed ofN bits and obtained from a previous analog-digital conversion and said device (4, 20) enables the increase of said first analog signal (A) in output from the digital-analog converter (2) and in input to said comparator (3) by a preset value (Voffs) when the bit of said previous digital signal (D1) which corresponds in position to the bit of said digital signal (D) which must be decided in a clock cycle is zero. Converter according to claim 1, characterised in that said device (4, 20) comprises a control circuit (20) and a switch (4) which is switchable to two possible states (5, 6) for the addition or not of said preset value (Voffs) to said first analog signal (A) by means of said control circuit (20). Converter according to claim 1, characterised in that said first analog signal (A) in output from the digital analog converter and said second analog signal (B) in input to said analog digital converter are voltage signals. Converter according to claim 1, characterised in that said preset value (Voffs) is less than the conversion value of a least significant bit (LSB). Method for converting an analog signal (B) into a digital signal at N bits in N clock cycles by means of an analog-digital converter, said converter comprising a logic control circuit (1) timed by means of an external clock signal (clock), said control circuit (1) being suitable for producing a digital signal (D), corresponding to a first analog signal (A), formed ofN bits through a second analog-digital conversion in N clock cycles, a comparator which compares said first analog signal (A) with a second analog signal (B) which is in input to said analog digital converter, said logic control circuit (1) comprising a register (11) and the analog-digital converter comprising a device (4, 20), characterised in that said analog-digital converter is of the successive approximation type and comprises a digital-analog converter (2) which converts said digital signal (D) sent by said logic circuit (1) to said first analog signal (A) and said register (11) contains a previous digital signal (D1) formed of N bits and obtained from a previous analog-digital conversion and that each of said clock cycles comprises a step in which the first analog signal (A) in output from said digital-analog converter (2) and in input to said comparator (3) is increased by a preset value (Voffs) by said device (4, 20) if the bit of the previous digital signal (D1) corresponds in position to the bit of the digital signal (D) which must be decided in said clock cycle is zero. Method according to claim 5, characterised in that said increase of a preset value (Voffs) of said first analog signal (A) in output from said digital-analog converter (2) and in input to said comparator (3) is carried out through a device comprising a control circuit (20) and a switch (4) switchable to two possible states (5, 6) for the addition or not of said preset value (Voffs) to said first analog signal (A) through said control circuit. Method according to claim 5, characterised in that said first analog signal (A) in output from the digital-analog converter and said second analog signal (B) in input to said analog-digital converter are voltage signals. Method according to claim 5, characterised in that said preset value (Voffs) is less than the conversion value of a least significant bit (LSB).

Inventors: BARDELLI ROBERTO, TARANTOLA MARIO, BARDELLI, ROBERTO, TARANTOLA, MARIO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 2000
Main classification: H03M1
IPC codes: H03M1