CN112217519A - Ramp nonlinear distortion correction method for ramp generator - Google Patents
Ramp nonlinear distortion correction method for ramp generator Download PDFInfo
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Abstract
The invention belongs to the technical field of analog-to-digital conversion and image sensing, and particularly relates to a method for correcting nonlinear distortion of a ramp wave for a ramp generator. The nonlinear distortion of the ramp is derived from the RC delay of the output point of the digital-to-analog converter, and the nonlinear distortion can be compensated by superposing a step signal on the output of the digital-to-analog converter, wherein the superposition of the step signal can be realized at the input digital signal of the digital-to-analog converter; the magnitude of this compensated step signal is related to the RC constant and the input slope, and can be determined by calibration; the invention can eliminate the nonlinearity near the inflection point when the slope is controlled and the initial stage of the ramp, so that the error area of the corrected ramp is obviously reduced.
Description
Technical Field
The invention belongs to the technical field of analog-to-digital conversion and image sensing, and particularly relates to a method for correcting nonlinear distortion of a ramp wave for a ramp generator.
Background
A large number of array analog-to-digital converters are required in an image sensor system. For example, a sensor with a 4K pixel count in the horizontal direction typically requires 2000 analog-to-digital converters located above and below the pixel array. A single analog to digital converter requires a very small area, typically less than ten microns on center to center. The plurality of single-ramp analog-to-digital converters have the characteristic of sharing one ramp generator, and each analog-to-digital converter only needs to comprise one comparator and one counter. Therefore, a ramp generator is well suited for this application. In recent years, the resolution and frame rate of image sensors have been higher, so that higher speed is required for the analog-to-digital converters therein. The performance of the single-ramp analog-to-digital converter is mainly determined by the quality of the generated ramp, and the higher the slope and the higher the linearity of the ramp, the faster the speed and the higher the precision of the analog-to-digital converter. However, in the image sensor, since the ramp generator needs to load thousands of analog-to-digital converters, the RC delay distortion of the ramp generator at the initial stage of the ramp is large and forms a curve. Another feature of the image sensor is that it requires a high accuracy of the analog-to-digital converter at low light, and the accuracy is reduced at high light. The low light corresponds to the initial stage of the ramp, and the distortion of the ramp greatly limits the accuracy of the analog-to-digital converter. High illumination corresponds to the second half of the ramp, and because the accuracy requirements are reduced, the ramp slope is increased in some image sensors to increase speed. If the precision requirement is reduced by 1 bit, the slope is increased by 1 time. However, the problem of the slope increase technique being greatly limited in its application because RC delay distortion forms a curve after a transition point when the slope is transitioned.
Based on the above problems, the present invention provides a ramp nonlinear distortion correction technique for solving the nonlinear distortion at the initial stage of the ramp and after slope transition.
Disclosure of Invention
The invention aims to provide a method for correcting nonlinear distortion of a ramp wave generator, which is characterized in that the nonlinear distortion of the ramp wave is derived from RC delay of an output point of a digital-to-analog converter, the nonlinear distortion is compensated by superposing a step signal, and the step signal can be superposed at a digital input of the digital-to-analog converter; the specific operation comprises the following steps of;
1) when inputting digital step signal to the D/A converter, setting the sampling period of the D/A converter as TSThe gain is 1; the slope of an input signal is k, and the time constant of an output point is tau; the output of the digital-to-analog converter being at a point in time nTSThe voltage at can be regarded as n step signals kT with the slope of kSSuperposition after different period delays; the creation and superposition of the step signal can be expressed by the following formula,
when T isSApproaching 0, the final output can be integrated, thus seeing the digital-to-analog converter output as kt and k τ (1-e)-t/τ) Composition is carried out; the first part kt is a linear term determined by the input data, the second part k τ (1-e)-t/τ) Is a typical step response term; therefore, the step signal with proper size is superposed on the output of the digital-to-analog converter, so that the nonlinear distortion can be compensated; the magnitude of this compensated step signal is related to the time constant and the input slope;
2) the step signal of the compensation ramp wave is compensated in the digital domain, which is the input of the digital-to-analog converter, and the step signal input in the digital domain can reflect to the analog output, so that the nonlinear distortion of the compensation ramp wave is compensated; the magnitude of the step signal can be obtained by the following calibration: because the slope of the ramp wave output by the digital-to-analog converter is determined only by the digital input frequency and the voltage range of the digital-to-analog converter, the corresponding analog voltage difference between two digital input codes D1 and D2 is known in advance, similar to the high-to-low scanning common in image sensors, the codes D1 and D2 correspond to ideal analog outputs V1_ ideal and V2_ ideal respectively, the initial step signal size Δ D is set to 0, when the actual output of the analog-to-analog converter crosses V1_ ideal and V2_ ideal, the digital inputs at that time are recorded as D1_ real and D2_ real respectively, if (D1_ real-D2_ real) > (D1-D2), Δ D is set to Δ D +1, and if (D1_ real-D2_ real) < (D1-D2), Δ D is set to Δ D-1, and the value of the digital input can be obtained at the position; during actual scanning, the input code jumps from the maximum Dmax to Dmax-delta D, and then is gradually reduced by 1;
3) and when the slope of the ramp wave needs to be increased in the scanning process, the step signal is superposed again, and the magnitude of the step signal is multiplied by the increased slope part by delta D, so that recalibration is not needed.
In step 2), when the actual scanning is performed, i.e. when the analog-to-digital converter starts to operate or starts to calibrate, the digital input codes D1 and D2 of the scanning digital-to-analog converter are input in the following order:
Dmax→Dmax-ΔD→Dmax-ΔD-1→Dmax-ΔD-2→……→Dmin。
the method has the beneficial effect of eliminating the nonlinearity near the inflection point in the initial stage of the ramp and the slope control. The error area of the corrected oblique wave is obviously reduced
Drawings
FIG. 1 is a block diagram of a ramp wave non-linear distortion calibration circuit
FIG. 2 is a logic diagram of a ramp nonlinear distortion calibration
FIG. 3(a) is the whole waveform of the ramp wave before calibration, and FIG. 3(b) is the whole waveform of the ramp wave after calibration
FIG. 4(a) is a partial waveform at the initial point and the first inflection point of the ramp wave before correction
FIG. 4(b) is a partial waveform at the initial point and the first inflection point of the corrected ramp wave
FIG. 5(a) is a partial waveform at the second inflection point of the ramp before correction
FIG. 5(b) is a partial waveform at the second inflection point of the corrected ramp
Detailed Description
The invention proposes a method for correcting the nonlinear distortion of a ramp, which originates from the RC delay of the output point of a digital-to-analog converter, for a ramp generator, the nonlinear distortion being compensated by superimposing a step signal, the method comprising the step of generating a step signalThe step signals may be superimposed at the digital input of the digital-to-analog converter; when inputting digital step signal to the D/A converter, setting the sampling period of the D/A converter as TSThe gain is 1; the slope of an input signal is k, and the time constant of an output point is tau; the output of the digital-to-analog converter being at a point in time nTSThe voltage at can be regarded as n step signals kT with the slope of kSSuperposition after different period delays; the creation and superposition of the step signal can be expressed by the following formula,
when T isSApproaching 0, the final output can be integrated, thus seeing the digital-to-analog converter output as kt and k τ (1-e)-t/τ) Composition is carried out; the first part kt is a linear term determined by the input data, the second part k τ (1-e)-t/τ) Is a typical step response term; therefore, the step signal with proper size is superposed on the output of the digital-to-analog converter, so that the nonlinear distortion can be compensated; the magnitude of this compensated step signal is related to the time constant and the input slope.
The embodiments of the present invention will be described in detail below with reference to the accompanying drawings and examples,
fig. 1 shows a block diagram of a circuit for calibrating the nonlinear distortion of a ramp wave. In the figure, the calibration technique only needs one comparator 3 in addition to the original digital-to-analog converter 2, and the calibration logic 1 at the front end of the digital-to-analog converter can disconnect the comparator 3 from the digital-to-analog converter 2 after the calibration is completed, without any modification to the digital-to-analog converter 2. The reference voltage for comparator 3 can be taken from resistor string 4, switched by S1 switch 6 and S2 switch 5; v1_ ideal and V2_ ideal correspond to static values when the input to the digital-to-analog converter 2 is D1 and D2, respectively. Because the same comparator is used for making difference value of two results, its delay and error do not produce influence on result.
A logic block diagram for calibrating the nonlinear distortion of the ramp wave shown in fig. 2; the nonlinear distortion calibration comprises the following steps:
step 4, in the phase of waiting for the output of the comparator to rise, when the comparator is from low to high, the input of the digital-to-analog converter is recorded as D1_ real;
After the ramp wave obtains the corrected Δ D, the step signal is superimposed again in the process of increasing the slope, the superimposed amount is the multiple of the slope increasing part multiplied by Δ D, taking the slope increasing to 4 times near the middle point as an example, the following code sequence is scanned:
Dmax→Dmax-ΔD→Dmax-ΔD-1→Dmax-ΔD-2→……→Dmax-ΔD-Dmax/2→Dmax-Dmax/2-4ΔD→Dmax-Dmax/2-4ΔD-4→Dmax-Dmax/2-4ΔD-8→……→Dmin
as shown in fig. 3 to 5, there are two increasing slopes on the waveform curve before and after the ramp calibration; after calibration a very straight ramp can be obtained at the initial point as well as the inflection point of increasing slope. Fig. 3(a) and (b) are the overall waveforms of the ramp wave before and after calibration. Fig. 4(a) and (b) are partial waveforms at the ramp initial point and the first inflection point before and after correction. Fig. 5(a) and (b) are partial waveforms at the second inflection point of the ramp before and after correction. The calibrated ramp wave can see a significant reduction in the error area at both the initial point and the two inflection points. The linearity before correction generally only supports about 10-bit accuracy, and can reach 14 bits after correction.
Claims (3)
1. A method of correcting for non-linear distortion of a ramp wave for a ramp generator, characterized in that the non-linear distortion of the ramp wave is derived from the RC delay at the output of a digital-to-analog converter, the non-linear distortion being compensated by superimposing a step signal which can be superimposed at the digital input of the digital-to-analog converter; the specific operation comprises the following steps of;
1) when inputting digital step signal to the D/A converter, setting the sampling period of the D/A converter as TSThe gain is 1; the slope of an input signal is k, and the time constant of an output point is tau; the output of the digital-to-analog converter being at a point in time nTSThe voltage at can be regarded as n step signals kT with the slope of kSSuperposition after different period delays; the creation and superposition of the step signal can be expressed by the following formula,
when T isSApproaching 0, the final output can be integrated, thus seeing the digital-to-analog converter output as kt and k tau (1 e)-t/τ) Composition is carried out; the first part ki is a linear term determined by the input data, the second part k τ (1 e)-t/τ) Is a typical step response term; therefore, the step signal with proper size is superposed on the output of the digital-to-analog converter, so that the nonlinear distortion can be compensated; the magnitude of this compensated step signal is related to the time constant and the input slope;
2) the step signal of the compensation ramp wave is compensated in the digital domain, which is the input of the digital-to-analog converter, and the step signal input in the digital domain can reflect to the analog output, so that the nonlinear distortion of the compensation ramp wave is compensated; the magnitude of the step signal can be obtained by the following calibration: because the slope of the ramp wave output by the digital-to-analog converter is determined only by the digital input frequency and the voltage range of the digital-to-analog converter, the corresponding analog voltage difference between some two digital input codes D1 and D2 is known in advance, similar to the high-to-low scanning common in image sensors, the codes D1 and D2 respectively correspond to ideal analog outputs V1_ ideal and V2_ ideal, the initial step signal size Δ D is set to 0, when the actual output of the analog-to-digital converter crosses V1_ ideal and V2_ ideal, the digital inputs at that time are recorded as D1_ real and D2_ real, respectively, if (D1_ real-D2_ real) > (D1-D2), Δ D is set to Δ D |1, if (D1_ real-D2_ real) < (D1-D2), Δ D is set to Δ D1, and the value of the digital input can be obtained at the position; during actual scanning, the input code jumps from the maximum Dmax to Dmax-delta D, and then is gradually reduced by 1;
3) and when the slope of the ramp wave needs to be increased in the scanning process, the step signal is superposed again, and the magnitude of the step signal is multiplied by the increased slope part by delta D, so that recalibration is not needed.
2. The ramp wave nonlinear distortion correction method for a ramp wave generator as claimed in claim 1, characterized in that in said step 2), at the time of actual scanning, i.e. when the analog-to-digital converter starts to operate or starts to calibrate, the digital input codes D1, D2 of the scanning digital-to-analog converter are input in the following order:
Dmax→Dmax-ΔD→Dmax-ΔD-1→Dmax-ΔD-2→……→Dmin。
3. the method according to claim 1, wherein step 3, when the slope of the ramp is required to increase during the scanning, superimposes the step signal again, the magnitude of which is a multiple of the slope increasing part multiplied by Δ D, and scans in the following code sequence:
Dmax→Dmax-ΔD→Dmax-ΔD-1→Dmax-ΔD-2→……→Dmax-ΔD-Dmax/2→Dmax-Dmax/2-4ΔD→Dmax-Dmax/2-4ΔD-4→Dmax-Dmax/2-4ΔD-8→……→Dmin。
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| CN85101241A (en) * | 1985-04-01 | 1987-01-17 | 德国Itt工业有限公司 | The digital deflection bearing calibration and the circuit of television picture tube |
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