Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-03-08
2001-04-24
Mack, Ricky (Department: 2873)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
Reexamination Certificate
active
06222175
ABSTRACT:
BACKGROUND
The present disclosure relates, in general, to image sensors and, in particular, to charge-domain analog readout circuits for such sensors.
Image sensors find applications in a wide variety of fields, including machine vision, robotics, guidance and navigation, automotive applications, and consumer products. In many smart image sensors, it is desirable to integrate on-chip circuitry to control the image sensor and to perform signal and image processing on the output image. Unfortunately, charge-coupled device (CCD), which have been one of the dominant technologies used for image sensors, do not easily lend themselves to large scale signal processing and are not easily integrated with CMOS circuits. Moreover, a CCD is read out by sequentially transferring the signal charge through the semiconductor, and the readout rate is limited by the need for nearly perfect charge transfer.
Active pixel sensors (APS), which have one or more active transistors within the pixel unit cell, can be made compatible with CMOS technologies and promise higher readout rates compared to passive pixel sensors. Active pixel sensors are often arranged as arrays of elements, which can be read out, for example, a column at a time. Each column can be read out at one time, driven and buffered for sensing by a readout circuit.
An exemplary voltage mode circuit is shown in FIG.
1
. Each column, such as the column
10
, includes a source-follower
12
. The column
10
is enabled by a switch
14
to drive its output onto a common bus line
18
. Other columns such as
16
can alternately be driven onto the bus line
18
. The bus line includes an inherent stray capacitance shown as
20
. Typically, a single constant current
22
is used in common for all the source-followers.
The source-follower
12
is formed with a drain, source, and gate. The speed of such a source-follower can be increased by increasing the channel length which also requires increasing the current on the source
22
. However, increasing the channel length has the undesirable effect of increasing the stray capacitance
20
, thereby decreasing the speed. Such a trade-off results in a little improvement in speed because the increase in capacitance tends to offset the increase in power.
Accordingly, it is desirable to provide a circuit which has improved capacity for reading out signals from an array of active pixel sensors.
SUMMARY
In general, according to one aspect, a charge-domain readout circuit includes multiple column readout circuits each of which can sample and store signal and reset values of an active pixel sensor. Each of the column readout circuits is associated with a respective column of sensors in an active pixel sensor array. The charge-domain readout circuit includes a first bus for receiving the signal value stored by a selected one of the column readout circuits and a second bus for receiving a reset value stored by the selected one of the column readout circuits. An operational amplifier-based charge sensing circuit maintains a substantially constant voltage on the first and second buses and provides a differential output based on the values stored by the selected one of the column readout circuits.
According to another aspect, a CMOS imager includes an array of active pixel sensors and a charge-domain readout circuit similar to that just described.
Various implementations include one or more of the following features. Each column readout circuit can include multiple sample and hold circuits. Each of the sample-and-hold circuits can include a charge storage element and a first switch which selectively can be enabled to sample a value from a sensor in the array to be stored by the charge storage element. For example, each column readout circuit can include multiple capacitive elements for storing correlated double sampled signal and reset values from a sensor in the array. Each column readout circuit can include second switches which selectively can be enabled to hold one side of the charge storage elements at a reference voltage when a corresponding one of the first switches is enabled to sample a value from a sensor. In some implementations, each column readout circuit includes a switch, such as a crow-bar switch, which selectively can be enabled to short together one side of each charge storing element.
The charge sensing circuit can includes, for example, a first switched integrator coupled to the first bus and a second switched integrator coupled to the second bus. Each of the switched integrators can include an operational amplifier, a feedback capacitive element coupled between an output and a first input of the operational amplifier, and a switch coupled between the output and the first input of the operational amplifier to selectively reset the switched integrator.
Each operational amplifier can have a reference voltage coupled to its second input. The switches in the switched integrators selectively can be enabled to hold one side of a corresponding one of the charge storage elements in a sample-and-hold circuit at the reference voltage when a corresponding one of the first switches in the sample-and-hold circuit is enabled to sample a value from the sensor.
According to another aspect, a method of reading out values from active pixel sensors in an array of sensors includes selecting a row of sensors whose values are to be read out and storing correlated double sampled values for multiple sensors in the selected row. The values for each sensor are stored by a respective readout circuit associated with a column in the array in which the sensor is located. The method also includes sensing the stored values associated with the sensors in the selected row using an operational amplifier-based charge sensing circuit that is common to the readout circuits. A differential output is sequentially provided from the sensing circuit for each of the sensors in the selected row.
In various implementations, the act of storing correlated double sampled values can include sampling and storing a signal value of a sensor and sampling and storing a reset value of the sensor. The method can further include setting a reference voltage on first sides of respective capacitive elements and subsequently coupling the signal and reset values to second sides of the respective capacitive elements. The reference voltage can be provided from the common operational amplifier-based charge sensing circuit. Furthermore, sensing the stored values can include using a crowbar switch to force charge stored in each respective readout circuit onto feedback capacitive elements in the operational amplifier-based charge sensing circuit.
In the present description, the functions performed with respect to columns and rows of pixels in an array can be reversed. Accordingly, a reference to a column in a two-dimensional pixel sensor array should be understood as referring to one or more pixel sensors along one axis of the array, and a reference to a row in the array should be understood as referring to one or more pixel sensors along a second axis of the array, where the second axis is orthoganol to the first axis.
Various implementations include one or more of the following advantages. Sensing charge injected onto a bus line using a charge-sensitive operational amplifier-based circuit allows the voltage on the bus to remain substantially constant. That, in turn, permits the stored pixel values to be read out at a high rate. In addition, the column readout circuits can be simplified by coupling the sampling capacitors directly to the bus. The column drivers and the column readout circuits can, therefore, be relatively small. In addition, the ability of the charge-domain readout circuit to operate in a double sampling differential mode with a crowbar circuit can provide very sensitive performance. For example, sampling both the reset and signal levels allows correlated double sampling (CDS) to be performed which can reduce various types of noise. Use of the crowbar switch can help reduce fixed pattern noise (FPN) which is dominated by column-to-column vari
Fish & Richardson P.C.
Mack Ricky
Photobit Corporation
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