AMPLITUDE MODULATED PIXEL SETUP FOR HIGH-SPEED READOUT OF CMOS IMAGE SENSORS

Abstract

An apparatus for increasing readout speed for Complimentary Metal Oxide Semiconductor (CMOS) image sensors. The apparatus is useful with CMOS image sensors in all high-tech industries and used to capture images digitally. Specifically, the apparatus provides a CMOS image sensor which employs an analog network-on-chip for increasing readout speed. The apparatus includes an array of carrier signal generators which are used to modulate the pixel exposure to allow all pixels to be read and discerned simultaneously.

Claims

1. An apparatus for sensing an image, said apparatus comprising: an array of pixels with an intensity-based readout; and an analog network including at least one carrier generator operably coupled to said array of pixels.

2. The apparatus as claimed in claim 1, wherein said array of pixels and said analog network are configured on an integrated circuit chip.

3. The apparatus as claimed in claim 1 wherein each pixel is fed with a carrier signal.

4. The apparatus as claimed in claim 1 wherein, for a specific row of said array, all pixel outputs are output onto a row bus simultaneously.

5. The apparatus as claimed in claim 1 wherein each pixel is fed with a unique carrier wave.

6. The apparatus as claimed in claim 1 wherein, for a specific column of said array, all pixel outputs are output onto a common bus simultaneously.

7. The apparatus as claimed in claim 3 wherein said carrier signal and said intensity-based readout are combined such that pixel signals are superimposed on a bus and recoverable.

8. The apparatus according to claim 1 wherein said pixels output a current, I.sub.Pixel, in the form of
I.sub.Pixel=A.sub.Photodiode*sin.sub.Carrier(2πft+τ)+I.sub.Offset, where f is the frequency of the carrier, τ is the phase of the carrier, I.sub.Offset is a DC offset current, and t is time.

9. A method for sensing an image, said method comprising: providing an array of pixels having an intensity-based readout; and coupling said array with an analog network comprising at least one carrier generator.

10. The method as claimed in claim 9, wherein said pixels output a current, I.sub.Pixel, in the form of
I.sub.Pixel=A.sub.Photodiode*sin.sub.Carrier(2πft+τ)+I.sub.Offset, where f is the frequency of the carrier, τ is the phase of the carrier, I.sub.Offset is a DC offset current, and t is time.

11. The method as claimed in claim 10, wherein said pixels output said current, I.sub.Pixel, onto a column bus without interfering with one another and total current output from said array forms a superposition of all pixels.

12. A system for sensing an image, the system comprising: an array of pixels, each pixel having an intensity-based output; a plurality of mixer circuit elements; and at least one bus line to which mixer circuit elements are coupled, each of said at least one bus line carrying a bus signal; wherein, for a plurality of said pixels, each pixel's output is mixed by one of said mixer circuit elements with a carrier signal such that said bus signal on said at least one bus line is a superposition of multiple pixel outputs modulated by said carrier signal.

13. The system according to claim 12, wherein each pixel has a corresponding mixer circuit element.

14. The system according to claim 12, wherein said array of pixels comprises at least two rows of pixels.

15. The system according to claim 14, wherein for each row of pixels, pixel outputs are modulated by a different carrier signal such that outputs of pixels in different rows of pixels are modulated by different carrier signals.

16. The system according to claim 12, wherein said multiple pixel outputs are demodulated to recover pixel exposure values for said multiple pixels.

17. The system according to claim 15, wherein said different carrier signals are non-overlapping in a frequency domain.

18. The system according to claim 12 wherein pixel outputs of pixels in a specific column in said array are simultaneously retrieved by retrieving said bus signal from a bus line coupled to said specific column.

19. The method of claim 9, wherein said at least one carrier generator generates at least two different carrier signals such that outputs of pixels in different rows of pixels in said array of pixels are modulated by different carrier signals.

20. The method of claim 9, wherein said at least one carrier generator generates at least one carrier signal such that outputs of pixels in different rows of pixels in said array of pixels are modulated by carrier signals that do not interfere with one another.

21. The method of claim 20, wherein said at least one carrier generator generates at least one carrier signal such that outputs of pixels in different rows of pixels in said array of pixels are modulated by carrier signals with different frequencies.

22. The method of claim 20, wherein said at least one carrier generator generates at least one carrier signal such that outputs of pixels in different rows of pixels in said array of pixels are modulated by carrier signals with different phases.

23. The apparatus according to claim 1, wherein said at least one carrier generator generates at least two different carrier signals such that outputs of pixels in different rows of pixels in said array of pixels are modulated by different carrier signals.

24. The apparatus according to claim 1, wherein said at least one carrier generator generates at least one carrier signal such that outputs of pixels in different rows of pixels in said array of pixels are modulated by carrier signals that do not interfere with one another.

25. The apparatus according to claim 24, wherein said at least one carrier generator generates at least one carrier signal such that outputs of pixels in different rows of pixels in said array of pixels are modulated by carrier signals with different frequencies.

26. The apparatus according to claim 24, wherein said at least one carrier generator generates at least one carrier signal such that outputs of pixels in different rows of pixels in said array of pixels are modulated by carrier signals with different phases.

27. The system according to claim 18, wherein, for said system, each column output is differently modulated from other column outputs.

28. The system according to claim 27, wherein all pixel outputs are simultaneously retrieved by simultaneously reading all column outputs.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The present invention will now be described by reference to the following figures, in which identical reference numerals refer to identical elements and in which:

[0031] FIG. 1 is a diagram of a 2×2 pixel grid illustrating the basic information flow in terms of on-chip and off-chip components for an embodiment of the present invention;

[0032] FIG. 2 is a signal flow graph illustrating the flow of information from the carrier signal generators to the low pass filter to the ADC to demodulation in accordance with the present invention;

[0033] FIG. 3 shows an Amplitude Modulated (AM) pixel setup as a logarithmic pixel with continuous output to the column bus in accordance with the present invention;

[0034] FIG. 4 shows a graphical representation of a Fast Fourier Transform (FFT) of an expected column bus output given ideal carriers and signals in accordance with the present invention;

[0035] FIG. 5 illustrates a known schematic representing rolling and global shutting schemes in accordance with the prior art; and

[0036] FIG. 6 illustrates a standard sinusoidal phase shift oscillator.

DETAILED DESCRIPTION

[0037] While the present description provides for specific details, it should be understood that such details are presented for purposes of illustration and are not to be taken as limiting any particular implementation of the present invention whereby other implementations may be practiced without these specific details. As well, it should be understood that the specifics of standard circuit architectures have been removed in order to ease understanding of the description.

[0038] Referring now to FIG. 1, one embodiment of the present invention is illustrated. Here, a diagram of a 2×2 pixel grid showing the basic information flow in terms of on-chip and off-chip components for an embodiment of the present invention. As shown, each pixel 10 includes a photodiode 11 which outputs to a mixer 12 into which a reference oscillator 13 feeds a generated carrier signal. As shown, each set (row-wise) of pixels on the 2×2 grid is fed by a different reference oscillator (frequency generator A and B). The 2×2 grid are an on-chip analog network. The off-chip components as shown may include a low pass filter and an ADC (analog to digital converter) for further processing of the output of the pixel grid. Such off-chip components may be embodied in an integrated circuit such as a field-programmable gate array (FPGA). The frequency generators may also be moved off chip without changing the function of the sensor.

[0039] The present invention serves to increase readout speed in CMOS image sensors in a manner similar to the principle in radio where a carrier wave modulates a low-frequency data signal for transmission. In this way, different carrier frequencies allow for signals to be transmitted simultaneously without interference. One preferred embodiment implements sinusoidal carrier waves using standard circuit architectures. A standard sinusoidal phase shift oscillator is shown in FIG. 6. It should, however, be clear that other possible oscillators that could be used include but are not limited to the Wien bridge oscillator, the Bubba oscillator, and the quadrature oscillator. The carrier waves may take any form so long as they do not overlap in the frequency domain in a way that cannot be reversed. For example, generated sinusoids should be of different frequencies and phases such that they may be demodulated later in the signal chain. These non-overlapping carriers are observable in FIG. 4 as the series of spikes representing the carriers, and the surrounding taper showing the pixel exposure. Of course, alternate implementations may include other carrier shapes. These carrier signals are then modulated with a value representing the pixel exposure.

[0040] Regarding the frequencies used for the carrier waves, these should be different. The difference in frequency primarily determines the obtained frame rate. As an example, if the carrier frequencies are 1 MHz apart, then the maximum theoretical frame rate is 500,000 fps. However, if the carrier frequencies are 2 MHz apart, then the maximum obtainable frame rate is 1,000,000 fps.

[0041] Modulation of the carrier signal with the pixel exposure voltage as illustrated in the figures is accomplished as follows.

[0042] With reference to FIG. 2, there is shown a signal flow of information from the carrier signal generators 20 through the pixel array 21 to the off-chip elements comprising the low pass filter 22, ADC 23, and module 24 for demodulation and image construction in accordance with the present invention.

[0043] FIG. 3 shows an Amplitude Modulated (AM) pixel setup as a logarithmic pixel with continuous output to the column bus in accordance with the present invention.

[0044] In operation and with regard to FIG. 3, the voltage across the given photodiode in the pixel array is amplified through a conventional drain amplifier of gain close to 1. The buffered voltage VDD is on the source of a second transistor. The base of the transistor is fed the carrier signal VSINE and which signal is established such that the transistor remains exclusively in the linear region. The drain of the second transistor is held to a virtual source such that current flowing through the transistor takes an approximate form of:

I.sub.Pixel=A.sub.Photodiode*sin.sub.Carrier(2πft+τ)+I.sub.Offset,

[0045] where f is the frequency of the carrier,

[0046] τ is the phase of the carrier,

[0047] I.sub.Offset is a DC offset current,

[0048] and t is time.

[0049] This way, multiple arrayed pixels may output the current, I.sub.Pixel, onto the column bus without interfering with each other and the total current on the line becomes a superposition of all pixels. Alternate implementations are of course within the scope of the intended invention which may replace this modulation scheme with a Gilbert cell or other multiplication circuit to create a signal on the column bus that represents a superposition of all pixels and their carrier signals.

[0050] The output current from the combination of pixels is then converted into a voltage using an operational amplifier configured as a transimpedance amplifier. The offset currents are removed to maximize output swing. The final voltage is then passed through a low pass filter and sampled using an analog to digital converter. This analog to digital converter should have a sampling frequency that fulfills the Nyquist requirement. In the digital domain, the individual pixels may be recovered using standard demodulation techniques.

[0051] For the preferred implementation of the present invention, demodulation should include passing the signal through a bandpass filter to remove other pixels, removing the negative values, and passing the signal through a low pass filter to recover the pixel exposure value. This is a standard method of demodulating AM signals. Alternate implementations may be provided with the following variations: filtering the signal before converting the signal from current to voltage; performing the bandpass filtering before sampling the signal; performing the low pass filtering before sampling; or using an intermediate frequency conversion to lower the Nyquist frequency.

[0052] After the signal has been sampled and demodulated, all the pixel values may therefore be obtained. In this manner, the maximum recoverable framerate would be half of the separation of the carriers in the frequency domain.

[0053] It should be clear that, for an array of pixels, the pixel outputs for a single column can be read/retrieved simultaneously by simply reading and demodulating the bus signal for the bus line serving that single column. Similarly, the pixel outputs for other columns can be read/retrieved by reading and demodulating the bus signal for the bus line serving those other columns. If desired, this concept can be applied to pixels being read in a row-wise manner. The output of pixels in a specific row can be read/retrieved by reading the bus signal for the row bus line serving that specific row.

[0054] From the above, it should be clear that, for an array of pixels, the output of all the pixels can be read by serially reading the bus signal on each bus line serving each column. Or, of course, alternatively, the output of all the pixels can be read serially by reading the bus signal on each bus line serving each row.

[0055] The above concept can be expanded by having an array of pixels such that each column output and each row is modulated separately from the other rows and columns. This arrangement would allow for simultaneously reading/retrieving the output of all the pixels. The output of all the pixels can then be amplified by, preferably, a single amplifier.

[0056] It should also be clear that the system may be configured such that the modulating carrier signal for columns is different from the modulating carrier signal for rows. The column modulating carrier signal may be completely different from the row modulating carrier signal. These modulating carrier signals may be generated by different carrier generators. Or, alternatively, a single carrier signal generator may be used to generate the modulating carrier signals. In the variant with a single carrier signal generator, the column modulating carrier signals would have different phases and/or frequencies from the row modulating carrier signals.

[0057] The above allows for simultaneous output of all the pixels. With each row being modulated differently from other rows and with each column output being modulated differently from other column outputs, all outputs can thus be read simultaneously. All row outputs can be placed on their respective column buses and are superimposed over one another. All column outputs, since they are all differently modulated as well, can thus be read/retrieved simultaneously as well. This allows for simultaneous or near-simultaneous retrieval of all pixel outputs for the array. The modulated column outputs can then be demodulated to retrieve each separate column output. Each demodulated column output can then be, in turn, demodulated to retrieve the various pixel outputs.

[0058] A person understanding this invention may now conceive of alternative structures and embodiments or variations of the above all of which are intended to fall within the scope of the invention as defined in the claims that follow.