CN112217519B - Oblique wave nonlinear distortion correction method for oblique wave generator - Google Patents
Oblique wave nonlinear distortion correction method for oblique wave generator Download PDFInfo
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- H03M1/38—Analogue value compared with reference values sequentially only, e.g. successive approximation type
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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 wave generator. The nonlinear distortion of the ramp wave is derived from the RC delay of the output point of the digital-to-analog converter, and can be compensated by superimposing a step signal on the output of the digital-to-analog converter, and the step signal can be superimposed on 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, which can be determined by correction; the invention can eliminate nonlinearity near the inflection point in the initial stage of the oblique wave and the slope control process, so that the error area of the corrected oblique wave 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 oblique wave nonlinear distortion of an oblique wave generator.
Background
A large number of array analog-to-digital converters are required in an image sensor system. For example, a sensor with a lateral pixel count of 4K typically requires 2000 analog-to-digital converters above and below the pixel array. A single analog-to-digital converter requires a very small area, typically with a center-to-center distance of less than ten microns. 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. Thus, the ramp generator is well suited for this application. In recent years, the resolution and frame rate of image sensors have been higher and higher, so that higher and higher speeds have been demanded for analog-to-digital converters therein. The performance of the single-oblique wave analog-to-digital converter is mainly determined by the quality of the generated oblique wave, and the higher the slope and the higher the linearity of the oblique wave, the faster the speed and the higher the accuracy of the analog-to-digital converter are. However, in the image sensor, since the ramp generator needs to load thousands of analog-to-digital converters, a curve is formed by relatively large RC delay distortion in the initial stage of the ramp. Another feature of image sensors is that the analog-to-digital converter is required to have high accuracy at low light levels and the accuracy at high light levels is reduced. The low light corresponds to the initial stage of the ramp, and the distortion of the ramp can greatly limit the accuracy of the analog-to-digital converter. The high light corresponds to the second half of the ramp, and the ramp slope is increased in some image sensors to increase speed because of reduced accuracy requirements. The slope is increased by a factor of 1 for every 1 bit reduction in accuracy requirement. However, distortion due to RC delay at the transition slope forms a curve after the transition point, which greatly limits the application of slope increasing techniques.
Based on the above-mentioned problems, the present invention provides a nonlinear distortion correction technique for a ramp wave, which is used for solving nonlinear distortion after the initial stage and slope transition of the ramp wave.
Disclosure of Invention
The invention aims at providing a method for correcting nonlinear distortion of a ramp wave for 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 operations include;
1) When a digital step signal is input to the digital-to-analog converter, the sampling period of the digital-to-analog converter is set as T S Gain is 1; the slope of the input signal is k, and the time constant of the output point is tau; the output of the digital-to-analog converter is at a point in time nT S The voltage at can be regarded as n step signals kT with a slope k S Superposition after different period delays; the creation and superposition of step signals can be expressed by the following formula,
when T is S Approaching 0, the final output can be integrated, thus seeing that the DAC output is formed by kt and kτ (1-e -t/τ ) Composition; the first part kt is a linear term determined by the input data, and the second part kτ (1-e -t/τ ) Is a typical step response term; thus superimposing the digital-to-analog converter output with an appropriate oneA step signal of a magnitude that compensates for this nonlinear distortion; 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 is reflected to the analog output, so that the nonlinear distortion of the ramp wave is compensated; the magnitude of the step signal can be obtained by the following calibration: because the ramp slope of the output of the digital-to-analog converter is only determined by the digital input frequency and the voltage range of the digital-to-analog converter, the analog voltage difference corresponding to the two digital input codes D1 and D2 is known in advance, the codes D1 and D2 are respectively corresponding to ideal analog outputs V1-ideal and V2-ideal similar to the high-to-low scanning common in the image sensor, the initial step signal size delta D=0 is set, when the actual output of the digital-to-analog converter crosses V1-ideal and V2-ideal, the current digital inputs D1 real and D2 real are recorded, if (D1 real-D2 real) D1-D2D 1 is delta D=delta D+1, if (D1 real-D2 real) D1-D2 is delta D=delta D-1, the delta D value required to be added at the digital input can be obtained; in actual scanning, the input code jumps from the maximum Dmax to Dmax-DeltaD, and then is successively reduced by 1;
3) When the slope of the ramp wave needs to be increased during scanning, the step signal is overlapped again, and the step signal is multiplied by delta D by the multiple of the slope increasing part, so that recalibration is not needed.
In the step 2), the digital input codes D1 and D2 of the scan digital-to-analog converter are input in the following order when the scan is actually performed, that is, when the digital-to-analog converter starts to operate or starts to calibrate:
Dmax→Dmax-ΔD→Dmax-ΔD-1→Dmax-ΔD-2→……→Dmin。
the invention has the beneficial effect of eliminating nonlinearity near the inflection point in the initial stage of the ramp wave and the slope control. The error area of the corrected ramp wave is obviously reduced
Drawings
FIG. 1 is a block diagram of a nonlinear distortion calibration circuit for a ramp
FIG. 2 is a logic block diagram of nonlinear distortion calibration of a ramp
FIG. 3 (a) shows the overall waveform of the ramp before calibration and FIG. 3 (b) shows the overall waveform of the ramp after calibration
FIG. 4 (a) shows a partial waveform at the initial point and the first inflection point of the ramp before correction
FIG. 4 (b) shows a partial waveform at the initial point and the first inflection point of the corrected ramp
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 wave
Detailed Description
The invention provides a method for correcting nonlinear distortion of a ramp wave, which is used for a ramp wave generator, wherein 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; when a digital step signal is input to the digital-to-analog converter, the sampling period of the digital-to-analog converter is set as T S Gain is 1; the slope of the input signal is k, and the time constant of the output point is tau; the output of the digital-to-analog converter is at a point in time nT S The voltage at can be regarded as n step signals kT with a slope k S Superposition after different period delays; the creation and superposition of step signals can be expressed by the following formula,
when T is S Approaching 0, the final output can be integrated, thus seeing that the DAC output is formed by kt and kτ (1-e -t/τ ) Composition; the first part kt is a linear term determined by the input data, and the second part kτ (1-e -t/τ ) Is a typical step response term; thus, the nonlinear distortion can be compensated by superposing a step signal with proper size on the output of the digital-to-analog converter; the magnitude of this compensated step signal is related to the time constant and the input slope.
Embodiments of the present invention will be described in detail below with reference to the drawings and examples,
a nonlinear distortion calibration circuit block diagram of a ramp wave as shown in fig. 1. In the figure, the calibration technology only needs one comparator 3 and the correction logic 1 at the front end of the digital-to-analog converter except the original digital-to-analog converter 2, and after the calibration is finished, the connection between the comparator 3 and the digital-to-analog converter 2 can be disconnected without any modification to the digital-to-analog converter 2. The reference voltage for the comparator 3 can be taken from the resistor string 4, switched by the S1 switch 6 and the S2 switch 5; v1_ideal and v2_ideal correspond to the static values of the digital-to-analog converter 2 when the inputs are D1 and D2, respectively. Since the same comparator is used to make the difference between the two results, the delay and the error will not affect the results.
A nonlinear distortion calibration logic block diagram of a ramp wave as shown in fig. 2; the nonlinear distortion calibration includes the steps of:
step 1, in an initialization stage, setting the input Din of a digital-to-analog converter as Dmax and setting the overlapped step signal delta D as 0;
step 2, when the digital-to-analog converter starts to work or starts to calibrate, the input codes of the scanning digital-to-analog converter are input in the following sequence: dmax-DeltaD-1-Dmax-DeltaD-2- … … -Dmin;
step 3, when calibration is started, only the S1 switch is turned on, and the output of the comparator is waited to rise;
step 4, recording the digital-to-analog converter input as D1_real when the comparator is from low to high in the stage of waiting for the rising of the comparator output;
step 5, when the digital-to-analog converter input is reset to Dmax, the S1 switch is closed, and the S2 switch is turned on; after the ramp voltage is recovered to the maximum value, the output of the comparator naturally becomes low; recording the digital-to-analog converter input as d2_real when the comparator goes from low to high again in the stage of S2 switch on;
step 6, comparing the sizes of (D1_real-D2_real) and (D1-D2) to determine the increase and decrease of the delta D; when the input of the digital-to-analog converter is reset again, the S1 switch conduction stage is entered again, and the operation is repeated; in addition, dmin during calibration may not be the code corresponding to the minimum output voltage of the digital-to-analog converter, and a smaller range may be scanned repeatedly to reduce time consumption.
After the corrected Δd of the ramp wave is obtained, a step signal is added again in the process of increasing the slope, wherein the added amount is the multiple of the slope increasing part multiplied by Δd, and the ramp wave is increased to 4 times of the slope near the middle point, for 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 curves before and after the ramp calibration; a very straight ramp can be obtained after calibration at the initial point and at the inflection point of the increasing slope. Fig. 3 (a) and (b) are oblique wave integral waveforms before and after calibration. Fig. 4 (a) and (b) are partial waveforms at the initial point and the first inflection point of the ramp before and after correction. Fig. 5 (a) and (b) are partial waveforms at the second inflection points of the ramp wave before and after correction. The calibrated ramp can see a significant reduction in error area at both the initial point and at both inflection points. The linearity before correction generally only supports about 10 bits of accuracy, and can reach 14 bits after correction.
Claims (3)
1. A method for correcting nonlinear distortion of a ramp wave for a ramp wave generator, wherein the nonlinear distortion of the ramp wave is derived from an RC delay at an output point of a digital-to-analog converter, the nonlinear distortion being compensated by superimposing a step signal, the step signal being superimposed at a digital input of the digital-to-analog converter; the specific operation comprises the following steps:
1) When a digital step signal is input to the digital-to-analog converter, setting the sampling period of the digital-to-analog converter as Ts and the gain as 1; the slope of the input signal is k, and the time constant of the output point is tau; the voltage of the output of the digital-to-analog converter at the time point nTs is regarded as the superposition of n sampling periods kTs with the slope of k after different period delays; the creation and superposition of step signals is expressed by the following formula:
when Ts approaches 0, the final output is integrated, thus seeing that the DAC output is formed by kt and kτ (1-e -t/τ ) Composition; the first part kt is a linear term determined by the input data, and the second part kτ (1-e -t/τ ) Is a typical step response term; thus superimposing a step signal V of suitable magnitude to the digital-to-analog converter output o (t) compensating for nonlinear distortion; compensated step signal V o The magnitude of (t) is related to the time constant τ and the input signal slope k;
2) The step signal of the compensation ramp wave is compensated in the input digital domain of the digital-to-analog converter, and the step signal input in the digital domain is reflected to the analog output, so that the nonlinear distortion of the ramp wave is compensated; the magnitude of the step signal is obtained by the following calibration: because the ramp slope of the digital-to-analog converter output is only determined by the digital input frequency and the voltage range of the digital-to-analog converter, the analog voltage difference corresponding to the two digital input codes D1 and D2 is known in advance, the codes D1 and D2 are corresponding to ideal analog outputs V1_ideal and V2_ideal respectively, the initial step signal size AD=0 is set, when the actual output of the digital-to-analog converter crosses the V1_ideal and the V2_ideal, the current digital inputs are recorded as D1_real and D2_real respectively, and if (D1_real-D2_real) > (D1-D2), AD=AD+1; if (d1_real-d2_real) < (D1-D2), ad=ad-1, then the value of AD to be added at the digital input is obtained; in actual scanning, the input code is successively reduced by 1 after starting to jump from the maximum Dmax to Dmax-AD;
3) When the slope of the ramp wave needs to be increased during scanning, the step signal V is superimposed again o (t) the magnitude of which is a multiple of the slope increasing portion multiplied by AD without requiring recalibration.
2. The method for correcting nonlinear distortion of ramp wave according to claim 1, wherein in said step 2), at the time of actual scanning, i.e., when the digital-to-analog converter starts to operate or starts to calibrate, digital input codes D1, D2 of the digital-to-analog converter are input in the following order:
Dmax→Dmax-AD→Dmax-AD-1→Dmax-AD-2→……→Dmin。
3. the method for correcting nonlinear distortion of ramp wave for ramp wave generator according to claim 1, wherein said step 3) is to superimpose step signal again when slope increase of ramp wave is required during scanning, the magnitude of which is a multiple of slope increase portion multiplied by AD, scanning in the following code order:
Dmax→Dmax-AD→Dmax-AD-1→Dmax-AD-2→……→Dmax-AD-Dmax/2
→Dmax-Dmax/2-4AD→Dmax-Dmax/2-4AD-4→Dmax-Dmax/2-4AD-8
→……→Dmin。
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| CN101953073A (en) * | 2008-01-18 | 2011-01-19 | 松下电器产业株式会社 | Ramp wave output circuit, analog-to-digital conversion circuit, and camera |
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