Test and Measurement
A
random telegraph signal (RTS) is a noise data, an off-the-shelf, self-contained RTS noise solution
random process that switches between with guaranteed specifications is highly desirable.
+/-1 and that has the number of zero The B1530A Waveform Generator / Fast Measurement
crossings it makes in any time interval Unit (WGFMU) is an available module for the B1500A
described by a Poisson process. RTS Semiconductor Device Analyzer. It possesses the ability to
noise (sometimes also called burst or popcorn noise) exhibits make RTS measurements without the need for any additional
RTS behavior and typically appears as a low-level signal measurement equipment. The WGFMU module has a noise
superimposed on a much larger signal. In CMOS image floor of less than 0.1 mV (rms), with current measurement
sensors (also known as active pixel sensors) RTS noise can sampling rates from 1 S/sec to 200 MS/sec and a bandwidth
generate erroneous white spots in what should otherwise be extending from dc to 16 MHz.
dark areas. As feature sizes continue to decrease, the impact A deep measurement memory of up to 4 million points
of RTS noise on the MOSFETs used to read out pixel data has per channel combines with these measurement capabilities to
become more serious. enable the B1500A’s WGFMU module to measure RTS noise
Until now, RTS noise measurement solutions have over a wide frequency range. In addition, sample RTS noise
consisted of user-configured instrument setups, usually analysis software is supplied with the WGFMU module so that
consisting of components such as a low noise power supply, the user can start RTS noise analysis immediately.
current to voltage convertor and oscilloscope (or voltage
sampler). However, these measurement solutions have Random noise in CMOS image sensors
difficulty producing stable and consistent measurement In contemporary CMOS image sensors, RTS noise is generally
results. the dominant noise source affecting the pixels. Each CMOS
This is mostly due to poorly calibrated components or to image sensor pixel has a readout amplifier that amplifies the
the lack of calibration of the entire system. In addition, RTS photoelectric current generated by a photo diode (Figure 1).
noise measurement solutions constructed from multiple In order to increase the pixel density the amplifier size has to
instruments can easily generate measurement errors due to be reduced in order to maximize the size of the photo diode
their complicated cabling and the overall error arising from and in-turn improve the signal-to-noise ratio. This reduces the
26
the cumulative errors of the individual instrument size of the MOSFET used in the amplifier, which makes it
components. Therefore, in order to acquire consistent RTS more susceptible to RTS noise.
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Issue III 2009
Characterizing Random Noise
in CMOS Image Sensors
The continued scaling of CMOS image sensors using MOSFET devices has seen
a corresponding increase in RTS single electron noise behaviour that can change the
appearance of some pixel sensor information. Awareness of this phenomena and it
impact is vital for designers to design optimal circuits. Awareness of the noise is not
enough and manufacturers and designers need tools that enable them to
characterize and respond to random noise in CMOS image sensors. In this article
Agilent discuss a potential solution that ensures correct characterizing.
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