Method and system for digitizing ccd data

Abstract: The invention relates to a method for digitizing ccd-data, in which a A/D-converter (14) is controlled such that the analog data are evaluated at the correct timings. Instead of delaying one of the clock signals (PHI; CONV), it is proposed to delay the analog data (V ccd ) in a filter (20) which serves as an analog delay circuit. This makes it possible to adjust the delay time more easily and more accurately and reliably.

Description: Method and system for digitizing ccd data The invention relates to a method and a system for digitizing analog data. More particularly, the invention relates to a method and system for digitizing data that have been generated by an image sensor including a charge coupled device (ccd), a MOS-imager which is also known as a "charge injection device", and the like. Charge coupled devices usually comprise a large number of light sensitive cells which generate and hold electric charges in response to light incident thereupon. The charges accumulated in each cell during a certain integration time are then transferred to one or more outputs of the ccd and are converted into electric voltages under the control of and in synchronism with a clock signal. The analog data obtained in this way at the output of the ccd are supplied to an A/D-converter (ADC) via a conditioning circuit in which the offset and the gain factor are adapted to the input characteristics of the ADC. A block diagram of a conventional ccd system of this type is illustrated in Fig. 3A. A ccd-array 10 supplies CCD-data V ccd to the conditioning circuit 12 the output of which is connected to the data input of ADC 14. In this example, the ccd-array is of a type which is controlled by a clock signal PHI, the inverted clock signal PHIn and a reset signal RS. The waveforms of the signals V ccd , PHI and RS are shown in Fig. 3B. The signals PHI and PHIn serve as transport clock pulses by which the charges accumulated in the ccd cells are shifted through analog shift registers to the output port of the ccd-array which operates as a charge/voltage converter. The signal RS provides reset pulses for this charge/voltage converter. When the converter has been reset in the "low" period of the signal RS, the output port assumes a voltage V os which corresponds to a ccd cell which has not been exposed to light. At the subsequent transition of the signals PHI and PHIn the next charge quantity is transferred to the output port and converted into a voltage V out which represents the amount of light that has been received by the corresponding ccd cell. This voltage V out has to be converted into digital data by the ADC 14. Accordingly, the ADC has to be controlled by an appropriate converter clock signal CONV in such a manner that the voltage of the signal V ccd is sampled and converted at a time t 0 within the period in which this signal is valid, i.e. has the value V out . As is shown in Fig. 3B, the converter clock signal CONV is a pulse signal which is obtained by delaying the clock signal PHI by a predetermined delay time T. The voltage values are digitized at the H/L-transitions of the signal CONV. The converter clock signal CONV with the appropriate delay can be obtained in various ways, two of which are illustrated in Figs. 4 and 5. In Fig. 4, the signal CONV is derived from the clock signal PHI by means of a delay circuit 16. The delay circuit 16 may be formed for example by a series connection of a certain number of buffers each of which has a certain delay time, so that the total delay time is determined by the number of buffers. However, this implementation has the drawback that the delay times of commercially available buffers are known only within certain tolerances and the tolerances of the individual buffers in the series connection are accumulated, so that the total tolerance may become too large. Alternatively, a delay circuit may be used which is specifically designed for delaying a logic signal with a well defined delay time. However, such delay circuits are comparatively expensive and may not be commercially available for the specific delay time which is needed in a given application. In such cases, a user specified device would have to be developed which would increase the costs even more. In another known implementation which is shown in Fig. 5 (A) and (B), all the signals PHI, PHIn, RS and CONV are derived from a common basic clock signal CLK by means of a circuit 18 which is made up of a number of digital counters and may be considered as a state machine. In this case, the durations of the H and L periods of all signals output by the circuit 18 can only be integral multiples of the period of the basic clock signal CLK. Accordingly, in order to achieve a sufficient time resolution, the basic clock signal must have a comparatively high frequency which may be difficult to generate and may be unfavorable because it leads to the generation of electromagnetic noise and to problems of electromagnetic compatibility (EMC). In addition, these known techniques have the drawback that the digital output signal of the ADC 14 is delayed relative to the clock signal PHI, so that an additional step is necessary for synchronizing the digital output again with the system clock. This specific problem is overcome by a system according to the preamble of claim 1 which has been disclosed in DE-A-36 24 252. In this system, as is shown in Fig. 6, the clock signal is directly applied to the control input of the ADC and serves as the converter clock signal CONV. The correct time relationship between the operations of the ccd-array and the ADC is established by appropriately delaying the clock signal supplied to the clock input of the ccd-array. Thus, the output of the ADC remains synchronized with the original clock signal. However, the above-mentionend problems relating to the provision of a delay device which can delay a logic clock signal with sufficient accuracy and reliability are still encountered in this prior art. EP-A-0 755 194 which belongs to the prior art pursuant to Article 54(3) EPC discloses a method and system for digitizing analog data that have been generated as a pulse sequence in accordance with a clock signal and are supplied to an A/D-converter such that they will be sampled and digitised at appropriate timings, wherein the analog data are supplied to the A/D-converter via a correlator circuit for removing noise from the analog signal. Since the correlator circuit includes a low pass filter, it will also cause a certain delay in the analog signal. It is not disclosed in this document how the clock signal for the A/D-converter is obtained and how it is related to the clock signal for generating the analog data. It is an object of the invention to provide a method and system for digitizing analog data in which the desired time relationship between the operation of the ADC and that of the analog data generating means (ccd) can be established with high accuracy and with simple, robust and low-cost hardware. This object is achieved with the features indicated in claims 1 and 2. According to the invention, instead of delaying a clock signal, the analog output signal of the data generating means is delayed in an analog delay circuit. An analog delay circuit that is suitable for this purpose can be made up of rather inexpensive components and permits to set the delay time with high accuracy and low tolerances, so that an efficient mass production of the ccd systems becomes possible. The reason is that the analog delay circuit is rather insensitive to tolerances in the parameters of its constituents, such as inductance elements, capacitance elements and the like. It is admissible that the analog delay circuit distorts the waveform of the analog signal to some extent, as long as the portion of the output signal V ccd which represents the valid data V out remains unaffected for a time interval long enough for the ADC to sample the voltage value. Thus, the construction of the analog delay circuit may be simpler than that of a dedicated delay circuit for logic signals which is normally equipped with a pulse-shaping output stage. By appropriately adjusting the parameters of the delay circuit, the delay time may be set as desired (steplessly) without any need for a high frequency basic clock signal. Thus, the highest clock frequency occurring in the system will be that of the clock signal PHI supplied to the ccd-array. As a consequence, a high frequency may be selected for the clock signal PHI in order to obtain a fast image sensor without causing EMC problems. In addition, the clock signal PHI (or the inverted clock signal PHIn or else the signal RS) can be used directly for controlling the ADC, thereby achieving a high robustness of the system as a whole, since the digital output signal obtained from the ADC will intrinsicly be synchronized with the system clock. Useful details of the invention are indicated in the dependent claims. The analog delay circuit may simply be formed by an all-pass filter or a low-pass filter, such as an LC-filter. Since the delay time of such a filter is dependent on its order (i.e., the number of complex impedance elements involved) and on its cut-off frequency (in case of a low-pass filter), the delay time can easily be adjusted by appropriately selecting the order and the cut-off frequency, respectively. In order to avoid a significant distortion of the analog signal, the filter should provide the same delay time for all frequency components passing therethrough. A low-pass filter has the additional advantage that undesirable high-frequency noise is filtered-out. The cut-off frequency should be significantly higher than the frequency of the clock signal PHI in order to make sure that the waveform of the output signal of the ccd-array is not distorted too much. It has been found that a Bessel filter, for example a fifth order Bessel filter, is particularly useful. A preferred embodiment of the invention will now be described in conjunction with the accompanying drawings in which Fig. 1(A) is a block diagram of a ccd system according to the invention; Fig. (B) illustrates waveforms of signals occurring in the system shown in Fig. 1(A); Fig. 2 shows an embodiment of an analog delay circuit; Fig. 3(A) is a block diagram showing the main components of a conventional ccd system; Fig. 3(B) illustrates waveforms occurring in the system shown in Fig. 3(A); Fig. 4 is a block diagram of another conventional ccd system; Fig. 5(A) is a block diagram of yet another conventional ccd system; Fig. 5(B) illustrates waveforms occurring in the system shown in Fig. 5(A); and Fig. 6 is a block diagram of still another conventional ccd system. In the following description, those parts of Fig. 1 which have already been referred to in the description of the prior art in conjunction with Figs. 3 to 6 will be designated with the same reference numerals. As can be seen in Fig. 1(A), the clock inputs of the ccd-array 10 and the ADC 14 are directly connected with each other and, accordingly, the converter clock signal CONV is identical with one of the clock signals (PHIn) supplied to the ccd-array. A low-pass filter 20 (LPF) has been inserted between the output of the conditioning circuit 12 and the ADC 14. The LPF 20 serves as an analog delay circuit which delays the output signal of the conditioning circuit 12 by a predetermined delay time T'. In the present embodiment, the LPF 20 is a fifth order Bessel filter which has the construction shown in Fig. 2. A resistor 22 and two inductance elements 24, 26 are connected in series between the output of the conditioning circuit 12 and the data input of ADC 14. Capacitors 28, 30 and 32 are connected between ground and the junction point of resistor 22 and inductance element 24, the junction point between the inductance elements 24 and 26 and the junction point between the inductance element 26 and the ADC 14, respectively. Another resistor 34 is connected in parallel with the capacitor 32. The resistance, capacitance and inductance values are so selected that the cut-off frequency of the LPF 20 is approximately four times the frequency of the clock signal PHI. Under these conditions, the LPF will preserve the voltage level V out of the CCD signal V ccd with sufficient accuracy, so that the analog-to-digital conversion will not be affected by the presence of the LPF. In Fig. 1(B) the original (undelayed) signal V ccd is shown in faint lines whereas the delayed signal is shown in bold lines. The ADC 14 is controlled by the converter clock signal CONV (=PHIn) to read and convert the delayed signal V ccd at the timings t 0 when the signal CONV drops from a high level to a low level. At these instances the read voltage has the value V out which represents the amount of light received by the corresponding ccd cell. While a specific embodiment has been described, it should be noted that the invention is not limited to this embodiment but encompasses all possible modifications that will occur to a person skilled in the art and are covered by the appended claims. For example, since the H/L-transitions of the signal RS coincide with those of PHIn, it is possible to use RS as the converter control signal. Accordingly, the signal RS should be considered as a "clock signal" in the meaning of the claims. In addition, the invention is also applicable to a system in which the analog data source (ccd or equivalent) operates with only one clock signal and/or the ADC is active at the L/H-transitions of the control signal CONV applied thereto.

Claims: Method for digitizing analog data (V ccd ) that have been generated as a pulse sequence in accordance with a clock signal (PHI, PHIn, RS), wherein an A/D-converter (14) receives a clock signal (CONV) which is identical with said clock signal for generating the pulse sequence of the analog data, and the analog data (V ccd ) are supplied to the A/D-converter (12) via an analog delay circuit (20) such that they will be sampled and digitized at appropriate timings. System for digitizing analog data, comprising analog data generating means (10) for generating said analog data as a pulse sequence in accordance with a clock signal (PHI, PHIn, RS), an A/D-converter (14) for digitizing the analog data (V ccd ) output from the data generating means, clock means supplying an identical clock signal (PHI, PHIn, RS; CONV) to the data generating means and the A/D-converter, and an analog delay circuit (20) provided between the output of the data generating means (10) and the data input of the A/D-converter (14). System as claimed in claim 2, wherein the analog delay circuit is formed by a low-pass filter (20). System as claimed in claim 3, wherein said low-pass filter is a Bessel filter. System as claimed in any of the claims 2 to 4, wherein said data generating means is a ccd-array. System as claimed in any of the claims 2 to 4, wherein said data generating means is a MOS-imager.

Inventors: VAN ACQUOIJ CATHARINUS, VAN ACQUOIJ, CATHARINUS
Applicants: OCE TECH BV, OCE-TECHNOLOGIES B.V.
Application year: 1995
Main classification: H04N3
IPC codes: H04N5, H03H17, G06F3, H03M1