New technique reduces CMOS noise

17 Jun 2002

Researchers at Pixel Devices International (PDI) have reported a new technique for reducing noise that could improve the appeal of CMOS image sensors for a range of applications.

Noise in CMOS image sensors is typically much larger than in CCDs and this restricts their potential applications. The noise is caused by the residual charge that remains on the photodiode after it has been reset. For a low cost CMOS image sensor this is typically 30 to 40 electron RMS. Correlated double sampling eliminates much of the noise on CCDs so low cost CCDs typically have 15 to 20 electron RMS of noise.

In a paper at Photonics West last week, Boyd Fowler of PDI described a technique called active reset where the voltage of a photodiode is reset to a fixed level with minimum noise. This is achieved through bandlimiting and capacitive feedback and the researchers say that this technique will reduce the read noise on a photodiode CMOS sensor to about 5 to 10 electron RMS.

Fowler says that active reset does require additional pixel area and more time than traditional reset techniques but he is confident about its value. "PDI believes that active reset can and will be used in both low cost and scientific image sensor applications where read noise is a concern," said Fowler.

SH