A low noise CMOS image sensor read-out circuit based on VCO and frequency counter

A novel low noise read-out circuit for CMOS image sensor is proposed, based on VCO and frequency counter. By such a technique, the output voltage of the selected pixel is as a voltage bias to control the frequency of the oscillator. As a result, the oscillation frequency is a function of the photocurrent. A n-bit counter is adopted to count the output frequency signal and converts it into the digital domain, by which the conventional array-ADC structure is not required and a lot of chip area is saved due to removing ADC. Furthermore, this counter can also function as a CDS circuit. The noise of the proposed architecture is low due to a equivalent bandpass filter is achieved by using the counter and the CDS technique. Additionally, the operation speed of the counter can be extremely fast because the frequency of the VCO could be pushed up to several hundreds MHz or even higher.