Apparatus and method for sigma-delta analog to digital conversion

An analog-to-digital converter (ADC) comprising:

• a comparator;
• a first ADC portion supplying the comparator with a constant reference voltage for providing at least one high-order bit;
• and
• a second ADC portion supplying the comparator with a variable reference voltage for providing at least one low-order bit.

An analog-to-digital converting method comprising:

• receiving an analog signal;
• sampling the received signal;
• comparing the sampled signal with a constant reference voltage;
• providing at least one high-order bit responsive to the constant reference comparison;
• comparing the sampled signal with a variable reference voltage;
• providing at least one low-order bit responsive to the variable reference comparison;
• and
• combining the at least one high-order bit with the at least one low-order bit.

An image sensing method comprising:

• sensing an analog voltage signal of a sensor pixel;
• sampling the sensed signal;
• comparing the sampled signal with a constant reference voltage;
• providing at least one high-order bit responsive to the constant reference comparison;
• comparing the sampled signal with a variable reference voltage;
• providing at least one low-order bit responsive to the variable reference comparison;
• and
• combining the at least one high-order bit with the at least one low-order bit.

A CMOS image sensor comprising:

• a pixel array;
• a driver connected to the pixel array;
• and
• an analog-to-digital converter (ADC) unit connected to the driver, the ADC unit having a sigma-delta ADC for sampling an analog input signal at a first sampling rate and providing at least one upper bit of a digital output signal and an error signal, and a single-slope ADC connected to the sigma-delta ADC for receiving the error signal and providing at least one lower bit of the digital output signal.