Patent Issued for Analog-To-Digital Conversion in Image Sensors

By a News Reporter-Staff News Editor at Journal of Engineering -- According to news reporting originating from Alexandria, Virginia, by VerticalNews journalists, a patent by the inventors Purcell, Matthew (Edinburgh, GB); Storm, Graeme (Forres, GB); Tolmie, Derek (Edinburgh, GB); El Hachimi, Mhamed (Edinburgh, GB); Simony, Laurent (Grenoble, FR); Qu, Min (Shanghai, CN), filed on November 19, 2009, was cleared and issued on November 6, 2012.

The assignee for this patent, patent number 8305474, is STMicroelectronics (R&D) Ltd. (Marlow, Buckinghamshire, GB).

Reporters obtained the following quote from the background information supplied by the inventors: "The present invention relates to analog-to-digital conversion (ADC) in an image sensor.

"In common forms of image sensors, a two-dimensional array of pixels is read out row by row, the pixels of each column sharing column readout circuitry which commonly comprises correlated double sampling and ADC."

In addition to obtaining background information on this patent, VerticalNews editors also obtained the inventors' summary information for this patent: "In an embodiment, an image sensor has a pixel array and comprises: an input circuit comprising a first input for receiving an analog signal from a pixel of the pixel array, the signal varying during a pixel readout period and having a first level during a first reset period and a second level during a second read period, a second input for receiving a time variant reference signal, and two coupling capacitors; and a comparator circuit that compares the time variant reference signal and the analog signal; wherein the analog signal from the pixel and the time variant reference signal are constantly read onto one of the two coupling capacitors respectively of the input circuit during both the first reset period and the second read period; and wherein the comparator circuit includes a differential input for said combination of the analog pixel signal and the time variant reference signal, said differential input being biased by a fixed reference signal.

"The use of a differential input as first stage of the comparator circuit reduces the effect of ground bounce and hence reduces x-droop. The differential input may comprise a differential amplifier or a differential front end circuit comprising first and second transistor devices, wherein the control electrode of the first transistor device provides said input receiving the combination of the analog pixel signal and the time variant reference signal, and the control electrode of the second transistor device provides said second input receiving a fixed reference signal, said devices being connected in series between a current source and ground.

"The differential amplifier is typically a long tail pair.

"Preferably the output of the differential amplifier is connected to an inverter via a further capacitor.

"The fixed reference signal is preferably provided by sampling and holding a reference voltage, said sampling being performed during autozeroing of the pixel and comparator. This eliminates the effect of any noise present in the reference voltage.

"The image sensor is particularly suitable to be incorporated in a mobile device; for example a mobile phone having a camera, a personal digital assistant having a camera, a digital camera, an optical pointing device, (such as an optical mouse), an endoscope apparatus, or an ambient light sensor.

"In another embodiment, a method is provided for converting an analog signal from an image sensor pixel, during a pixel readout period to a digital signal using an analog to digital converter circuit. The analog to digital converter comprises an input circuit having first and second coupling capacitors, a time varying reference circuit enabled to provide a time varying reference signal, the first coupling capacitor being continuously connected to the time varying reference circuit and the second coupling capacitor being continuously connected to the pixel during the pixel readout period. The method comprises: (i) resetting the pixel, thereby sampling an analog signal equivalent to a pixel reset voltage onto the second capacitor; (ii) removing the pixel from reset, wherein the analog signal is corrected for pixel reset noise on the second capacitor; (iii) integrating light received on the pixel, wherein the analog signal is reduced according to the amount of light received; (iv) providing the time varying reference signal; (v) comparing the analog signal with the time varying reference signal; and (vi) outputting an output signal when the time variant reference signal corresponds to the analog signal. Further; said comparing is effected by applying the combination of the analog pixel signal and the time variant reference signal to an input of a differential amplifier, or other differential front end circuit, and biasing the differential amplifier with a fixed reference signal."