Minimized SAR-type column-wide ADC for image sensors
View Patent ↗An improved analog-to-digital converter wherein a minimal amount of circuitry is provided for conversion of an analog signal to a series of digital bits. A comparator is provided for generating digital values to which the digital bits correspond. A digital-to-analog converter is provided for generating, via successive approximation, a feedback analog signal based on bits previously generated by the comparator. The analog-to-digital converter compares the feedback analog signal to the input analog signal, and based on the comparison generates a digital value that corresponds to a digital bit. In the method of the invention, an analog signal is converted to a digital value using the analog-to-digital converter, and a digital-to-analog converter comprising two capacitors and a reference selector generates, via successive approximation, a feedback analog signal that is applied to a comparator for comparison to the input analog signal being digitized.
1. A method of operating an image sensor, said method comprising:
holding a first analog input signal at the output of a digital-to-analog converter, said first analog input signal including a floating diffusion reset voltage;
applying said first analog input signal from the output of said digital-to-analog converter to an operational amplifier having a first input line and a second input line, the floating diffusion reset voltage being applied to the second input line to generate an operational amplifier output;
applying the operational amplifier output to a first side of a sample and hold capacitor;
applying a second analog input signal to a second side of the sample and hold capacitor to generate an adjusted input voltage across said sample and hold capacitor, wherein the adjusted input voltage is the difference between the voltage at the first side of said sample and hold capacitor and the voltage at the second side of said sample and hold capacitor; and
applying the adjusted input voltage to the first input line of said operational amplifier.
2. The method of operating an image sensor of claim 1 wherein:
said voltage at said second side of said sample and hold capacitor further includes a reference voltage.
3. A method of generating an adjusted pixel voltage signal in an imager, said method comprising:
generating a floating diffusion reset voltage;
applying the floating diffusion reset voltage to a side one of a sample and hold capacitor of an analog-to-digital converter;
generating a pixel signal voltage;
applying the pixel signal voltage to a side two of the sample and hold capacitor;
generating the adjusted pixel voltage signal based on the floating diffusion reset voltage at side one of the sample and hold capacitor and the pixel signal voltage at side two of said sample and hold capacitor.
4. The method of generating an adjusted pixel voltage signal of claim 3 , further comprising digitizing the adjusted pixel voltage signal.
5. The method of generating the adjusted pixel voltage signal of claim 3 wherein the step of applying the floating diffusion reset voltage further comprises first applying the floating diffusion reset voltage to an operational amplifier of the analog-to-digital converter, then applying an amplifier output signal corresponding to said floating diffusion reset voltage to side one of the sample and hold capacitor.