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2.3 Understanding Read Noise in sCMOS
sCMOS technology boasts an ultra-low read noise floor that significantly exceeds that of even the best CCDs,
and at several orders of magnitude faster pixel readout speeds. For those more accustomed to dealing with
CCDs, it is useful to gain an understanding of the nature of read noise distribution in CMOS imaging sensors.
CCD architecture is such that the charge from each pixel is transferred through a common readout structure, at
least in single output port CCDs, where charge is converted to voltage and amplified prior to digitization in the
Analog to Digital Converter (ADC) of the camera. This results in each pixel being subject to the same readout
noise. However, CMOS technology differs in that each individual pixel possesses its own readout structure for
converting charge to voltage. In the sCMOS sensor, each column possesses dual amplifiers and ADCs at both
top and bottom (facilitating the split sensor readout). During readout, voltage information from each pixel is fed
directly to the appropriate amplifier/ADC, a row of pixels at a time (see Technical Note on Rolling and Global
Shutter modes).
As a consequence of each pixel having its own individual readout structure, the overall readout noise in
CMOS sensors is described as a distribution, as exemplified in
Figure 10
below, which is a representative
noise histogram from a Zyla 5.5 camera at the fastest readout speed of 560 MHz (or 280 MHz x 2 halves). It
is standard to describe noise in CMOS technology by citing the median value of the distribution. In the data
presented, the median value is 1.38 electron RMS. This means that 50% of pixels have a noise less than 1.38
electrons, and 50% have noise greater than 1.38 electrons. While there will be a small percentage of pixels
with noise greater than 2 or 3 electrons, observable as the low level tail towards the higher noise side of the
histogram, it must be remembered that a CCD Interline camera reading out at 20 MHz would have 100% of its
pixels reading out with read noise typically ranging between 6 and 10 electrons RMS (depending on camera
design).
Figure 10: Representative histogram showing read noise distribution at fastest readout speed of Zyla 5.5, 560 MHz (280 MHz x2).
The median value of 1.38 e
-
means 50% pixels have read noise less than 1.38 e- and 50% have greater than 1.38 e-. The line at 6 e-
represents a typical read noise value from a well optimized Interline CCD – all pixels in a CCD essentially share the same noise value
Optimized Interline CCD
(at 20MHz readout rate)
Median = 1.38e
-
Summary of Contents for Zyla sCMOS 4.2 PLUS
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