Conventionally, a CMOS imager pixel includes a phototransistor or photodiode operating as a light-detecting element. In operation, e.g., the pixel photodiode is first reset to a reset voltage that places an electronic charge across the capacitance associated with the photodiode. Electronic charge produced by the photodiode when exposed to illumination then causes charge of the photodiode capacitance to dissipate in proportion to the incident illumination intensity. At the end of an exposure period, the change in photodiode capacitance charge is detected and the photodiode is reset again. The amount of light detected by the photodiode is computed as the difference between the reset voltage and the voltage corresponding to the final capacitance charge.
Referring to FIG. 1, the operation of a prior art pixel 10 is described. FIG. 1 shows the schematic diagram of a standard active pixel. A photodiode 11 produces a current proportional to the incident light intensity. The resulting photo current is integrated on a charge-sensing capacitor 13. The charge-sensing capacitor 13 is typically reverse-biased PN junction capacitance associated with the photodiode 11 and other parasitic capacitance.
A MOS transistor 15 operates as a source-follower that buffers the voltage on the capacitor 13 nondestructively to a column line 23. A row select MOS switch 17 activates the source-follower transistor 15 when the particular row is selected by connecting the column current source 25 to the source of the source-follower transistor 15.
There are two primary ways to reset an active pixel, using a “soft” reset or using a “hard” reset. When using a “soft” reset, the voltage at the gate 21 of the reset transistor 19 is raised to a voltage that is no higher than the threshold voltage of the reset transistor, VRTTH, above the drain voltage of the reset transistor, typically at VDD. Generally, the voltage at the gate 19 is raised to the same potential as its drain voltage, VDD.
As the capacitor 13 is charged by the current from the reset transistor 19, the voltage at the sense node 27 increases, decreasing the gate-to-source voltage of the reset transistor 19. This in turn decreases the current from the reset transistor 19, and the rate of voltage rise at the sense node 27 decreases. As the gate-to-source voltage of the reset transistor 19 approaches its threshold voltage VRTTH of reset transistor 19, the current through the reset transistor 19 becomes extremely low, and the voltage at the sense node 27 rises very slowly. The voltage at the sense node 27 approaches approximately (VDD−VRTTH) but it never reaches a steady state because the rate of the voltage change becomes ever so slower. Then, the voltage at the gate 21 is lowered typically to ground, completing the reset process. At this time, the sense node 27 is reset to approximately (VDD−VRTTH).
In hard reset, the gate voltage of the reset transistor 19 is raised to a voltage greater than the drain voltage of the reset transistor by at least VRTTH. Typically, the gate voltage of reset transistor 19 is raised to VDD while the drain voltage of the reset transistor is maintained at a reset voltage VRESET that is lower than (VDD−VRTTH). This drives the reset transistor 19 into the triode region, thereby causing it to behave like a resistor.
The reset transistor 19 and the sense capacitor 13 behave like an RC circuit, and the sense node voltage approaches VRESET with an RC time constant, τ=RONC, where RON is the ON resistance of the reset transistor 19 and C is the value of the sense capacitor 13. Since the sense capacitance is on the order of a few femtofarads and the ON resistance is a few tens of kohms, the time constant is on the order of only a nanosecond.
Thus, the sense node typically reaches the full steady-state value VRESET within a few nanoseconds, which is much shorter than typical reset period of many microseconds. Then, the voltage at the gate 21 is lowered typically to ground, completing the reset process. At this time, the sense node 27 is reset to approximately VRESET.
It is well known that by using a “soft” reset, one can realize a lower reset noise, by a factor of √{square root over (2)}, compared to when using a “hard” reset. Thus, it is desirable from signal-to-noise ratio and sensitivity point of view to use a “soft” reset. However, since the sense node never reaches a steady state value, the voltage of the sense node is actually reset to different voltages depending on the initial condition on the sense node. This leads to substantial image lag. Even with popular double sampling method, a significant amount of image lag remains, which gives a blurry picture of moving objects.
Furthermore, it is well known that by using a “hard” reset, one can substantially eliminate the image lag because the voltage to which the pixel is reset is always VRESET. However, the disadvantage of using a “hard” reset is that higher reset noise is realized.
Therefore it is desirable to provide a imaging reset methodology and/or circuitry for an imager that provide a resetting capability, wherein reset noise is significantly reduced, image lag is substantially eliminated, a desirable signal-to-noise ratio is realized, and/or the imager realizes a desirable sensitivity.