# Patent

## Title
Reference voltage stabilization in CMOS sensors

## Abstract

A reference voltage generator for use in an image sensor provides a reference voltage to an S/H block during a pixel read-out operation and another reference voltage to an analog-to-digital converter (ADC) during a digitization operation. The reference voltage generator includes a variable voltage generator, a sample-and-hold circuit to sample a reference voltage prior to the pixel read-out operation or the digitization operation, and a buffer amplifier to drive the appropriate reference voltage to the relatively high impedance load presented by the S/H block and the variable impedance load provided by the ADC.

## Background
This application is a continuation of U.S. patent application Ser. No. 09/976,843, filed Oct. 12, 2001 is now as U.S. Pat. No. 6,567,028, the entire disclosure being incorporated herein by reference.


BACKGROUND
Active pixel sensor (APS) imaging devices are described in U.S. Pat. No. 5,471,515. These imaging devices include an array of pixel cells, arranged in rows and columns, that convert light energy into electric signals. Each pixel includes a photodetector and one or more active transistors. The transistors typically provide amplification, read-out control and reset control, in addition to producing the electric signal output from the cell. Providing amplification at each pixel may help to reduce noise and distortion levels.
Main sources of image sensor noise include fixed pattern noise (FPN) or temporal noise. FPN may manifest as a stationary background pattern in the image which is caused by mismatches in device parameters. Temporal noise is the temporal variation in pixel output values under uniform illumination due to device noise. Row-wise temporal noise (RTN) may manifest as stripes of different intensity in an image produced by an object with uniform intensity. Both FPN and RTN may be caused by voltage fluctuations in the sensor.
SUMMARY
A sensor includes a pixel array with pixels arranged in rows and columns. Analog signals produced by the pixels during an exposure are passed to a read-out chain. The read-out chain includes a sample-and-hold (S/H) block and an analog-to-digital converter (ADC). A reference voltage generator provides a reference voltage to capacitors in each of the S/H units in the S/H block (one per column) during a pixel read-out operation. The reference voltage generator provides another reference voltage to capacitors in the ADC during a digitization operation.
The reference voltage generator includes a variable voltage generator, e.g., a resistor ladder with a current source and multiple switches which may be selected in different numbers to generate different voltages. The reference voltage generator also includes a sample-and-hold circuit to sample a reference voltage prior to the pixel read-out operation or the digitization operation and a buffer amplifier to drive the appropriate reference voltage to the relatively high impedance load presented by the S/H block and the variable impedance load provided by the ADC.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a reference voltage generator according to an embodiment.
FIG. 2 is a block diagram of a sensor according to an embodiment.
FIG. 3A is a schematic diagram of a sample-and-hold unit according to an embodiment.
FIG. 3B is a timing signal diagram for signals in the sample-and-hold unit during a sampling operation according to an embodiment.
FIG. 4 is a schematic diagram of an analog-to-digital converter (ADC) according to an embodiment.



DETAILED DESCRIPTION
FIG. 1 is a reference voltage generator 100 for use in a CMOS sensor according to an embodiment. The reference voltage generator may be used to generate stable reference voltages for different stages in a read-out chain of the CMOS sensor.
The reference voltage generator 100 includes a resistor ladder 102 with a current source 104, which provides, for example, a nominal current of about 100 μA. The resistor ladder 102 may be configured as a voltage divider and include a set of programmable switches 106 to select one of a number of available reference voltage values Vref0–Vref15. The selected reference voltage is passed to a sample-and-hold (“S/H”) circuit 110. When selected, a sampling switch 112 samples the selected referenced voltage onto a holding capacitor 114. The sampled reference voltage is passed to a buffer amplifier 120 that amplifies the signal and passes the reference voltage to the appropriate stage of the read-out chain. The buffer amplifier 120 may be a voltage amplifier with unity gain. The buffer amplifier 120 acts as a voltage driver, and enables the reference voltage generator to provide a stable reference voltage with enough current to drive both high and low impedance loads.
FIG. 2 illustrates a CMOS sensor 200 including the reference voltage generator 100. The sensor 200 may be an active pixel sensor (APS), in which the pixel array 202 includes a grid of individually addressable pixels 204 arranged in rows and columns. Each pixel 204 includes a photodetector, such as a photogate, photodiode, or pinned photodiode. The photodetector converts light energy received in the form of photons into an electric charge. This electric charge corresponds to an amount of light that the pixel 204 receives during an exposure to an image. The amount of light received by each pixel in the array during exposure to the image is used by the sensor 100 to produce a corresponding digital image.
Analog signals generated by the pixels 204 are read out row-by-row to a read-out chain 206. The read-out chain includes a S/H block 210, a multiplexing block 212, a gain block 214, and an analog-to-digital converter (“ADC”) block 216. The reference voltage generator 100 provides a reference voltage Vcl—col to the S/H block 210 and a reference voltage Vref the ADC block 216.
The S/H block 210 includes a number of S/H units 300, one for each column in the pixel array 202. The S/H units 300 may have a column parallel architecture, as shown in FIG. 3A. FIG. 3B illustrates the timing signals for switches in the S/H unit 300 during a row read-out operation. When the row is selected, the enable switch 302 and clamp switches 304 are closed substantially simultaneously. This clamps the back plates 307, 309 of a signal capacitor 306 and a reset capacitor 308 to a reference voltage Vcl—col supplied by the reference voltage generator 100. The reference voltage Vcl—col is selected from the resistor ladder 102 using the programmable switches 106 and is sampled onto the capacitor 114 in the S/H circuit 100 prior to the row read-out operation. The buffer amplifier 120 drives the sampled reference voltage Vcl—col to the load presented by the S/H units 300 in the S/H block 210.
The signal on the pixel in the column is sampled through Sh_sig switch 310 onto the signal capacitor 306 in the first portion of a row sampling period. After the capacitor is charged to the proper voltage, the photosensitive element in the pixel is reset. The reset level of the pixel is sampled through Sh_rst switch 312 during a second portion of the row sampling period.
During column read-out of the S/H block 210, the S/H units are read out sequentially. When the S/H unit 300 is selected, the column select switches 320 and crowbar switch 330 are closed. This shorts the front plates 311, 313 of the signal capacitor 306 and the reset capacitor 308, respectively, driving the respective charges on these capacitors out to the multiplexer block 212.
Each of the signal and reset capacitors may be relatively small, e.g., about 1 pF. However, during the row sampling operation, the reference voltage generator 100 must provide the clamping voltage Vcl—col to all of the S/H units 300 in the S/H block 210 simultaneously. This presents a relatively large load. For example, in a Common Image Format (CIF)-size sensor with 352H×288V pixel array with 1 pF capacitors, the load exceeds 350 pF. The buffer amplifier 120 provides enough current to drive each of the signal and reset capacitors in the S/H block to the appropriate reset voltage.
Sampling the reference voltage Vcl—col prior to each row read-out operation and reliably providing the sampled reference voltage to each S/H unit 300 ensures that the sampled signal and reset values for each pixel in the row are clamped to the same voltage value. This may substantially reduce fixed pattern noise (FPN), which may be caused by mismatched reference voltages in the S/H block 210.
The reference voltage generator 100 also provides a reference voltage to the ADC block 216 prior to row digitization. In a row digitization operation, the sampled analog signal values read out from pixels in a row are converted into digital values. The ADC block 216 may include an 8-bit successive approximation ADC 400, as shown in FIG. 4. The ADC 400 includes a comparator 402 that compares the analog signal ASC+ and a digital signal from a digital-to-analog converter (DAC) 404. The DAC 404 includes an array of binary weighted capacitors C0–C7 410–417 and two inputs including an analog signal ASC- and the reference voltage Vref, respectively. The ADC 400 generates an 8-bit digital signal in a sequence of successive approximations.
The reference voltage Vref is selected from the resistor ladder 102 using the programmable switches 106 and is sampled onto the capacitor 114 in the S/H circuit 100 prior to the row digitization operation. The buffer amplifier 120 drives the sampled reference voltage Vref to the load presented by the ADC 400 in the ADC block 216.
The number of capacitors 410–417 coupled to the Vref node 405 changes depending on the size of the analog signal sampled from a pixel. Thus, the load on the reference voltage generator 100 varies during the analog-to-digital conversion in the row digitization process. To reduce noise, all pixel digitization should have the same Vref. The voltage buffer amplifier 120 provides the same sampled reference voltage Vref to the ADC despite the variations in the loads for different analog pixel signals.
Sampling the reference voltage Vref prior to each row digitization operation and driving the sampled reference voltage Vref with the buffer amplifier 120 may enhance the stability of the reference voltage provided to the ADC 400 for each pixel digitization. This may reduce any noise disturbances coupled into the ADC 400 due to variations in the reference voltage Vref during digitization of different signals since all pixel digitizations within each row are reliably provided with the same reference voltage. This may be particularly useful when the ADC 216 includes multiple ADCs for digitization of the pixel signals from the same row.
A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.

## Claims

1. An imager comprising:
an image sensor including an array of pixels, said image sensor further comprising:
a first operative device for receiving signals and coupled to receive a first reference voltage;
a reference voltage generator operative to generate said first reference voltage, said reference voltage generator including a sample-and-hold circuit coupled to a voltage source and operative to sample-and-hold said voltage source to produce said first reference voltage; and
a second operative device for receiving signals from said first operative device, said second operative device coupled to a second reference voltage, said sample-and-hold circuit of said reference voltage generator operative to sample-and-hold a second voltage of said voltage source to produce said second reference voltage.

2. The imager according to claim 1, wherein said first operative device is a pixel signal sample-and-hold block.

3. The imager according to claim 1, wherein said second operative device is an analog-to-digital converter (ADC).

4. The imager according to claim 1, wherein said reference voltage generator further comprises a buffer amplifier coupled to said sample-and-hold circuit and operative to amplify an output of said sample-and-hold circuit and produce said second reference voltage.

5. The imager according to claim 1, further comprising a buffer amplifier coupled to said sample-and-hold circuit and operative to amplify an output of said sample-and-hold circuit to produce said first reference voltage prior to use of said first reference voltage by said first operative device and said second reference voltage prior to use of said second reference voltage by said second operative device.

6. The imager according to claim 5, wherein said buffer amplifier comprises a voltage amplifier.

7. The imager according to claim 6, wherein said voltage amplifier has unity gain.

8. An apparatus comprising:
a voltage generator including a voltage tree operative to generate a plurality of reference voltages from a voltage source, said voltage generator including a sample-and-hold circuit coupled to said voltage generator and operative to sample-and-hold said plurality of reference voltages;
a pixel signal sample-and-hold block for receiving a first one of said plurality of reference voltages; and
an analog-to-digital conversion circuit for converting a sampled pixel signal into a corresponding digital value, said analog-to-digital conversion circuit receiving a second one of said plurality of reference voltages.

9. The apparatus according to claim 8, further comprising a buffer amplifier coupled to said sample-and-hold circuit and operative to amplify an output of said sample-and-hold circuit to produce said plurality of reference voltages.

10. The apparatus according to claim 9, wherein said buffer amplifier comprises a voltage amplifier.

11. The apparatus according to claim 10, wherein said voltage amplifier has unity gain.

12. The apparatus according to claim 8, wherein said voltage generator comprises a resistance ladder coupled to a current source, said resistance ladder including a plurality of resistors coupled to a plurality of selection switches.