LESSI – Faculté des Sciences – Fès – Morocco

The design of active pixel sensor (APS) imagers fabricated in traditional CMOS foundries has been a topic of renewed interest in the last several years. The performances of the APS image sensors are a function of temporal and spatial noise sources associated to the elements of the unit pixel and the components of the readout circuit.

White noise and Low-frequency noise sets a fundamental limit on APS performance, especially for low-flux applications. In this paper, a detailed theoretical analysis of the in-pixel amplifier and the readout circuit response to white noise and low-frequency noise is given. An original method based on power spectral density (PSD) characteristics of the input noise source is used. Furthermore, creating a random noise input requires writing a program to generate a set of random numbers which can then be incorporated in a sinusoid (SIN) source declaration within a PSPICE net-list. The formulae obtained allow to compute output rms noise value in time domain with the help of PSPICE.

The objective of this paper is to develop a methodology be able to make simulations of noise and treat the types of the noise sources. The approach is to adapt the simulator PSPICE to simulating temporal noise sources in time domain. To obtain the effect of these sources on the output signals, we introduce temporal noise sources into the noisy devices then we make transient simulation of the circuit. The introduced sources should represent the physical noise generated by devices in real time, with which the characteristics are supplied by the models of power spectral densities. In transient analysis, the polarization of non-linear device varies sometimes strongly and changing the operating point of the device. It is so interesting that the noise is correctly modeled according to the polarization.

An accurate analysis of noise in CMOS active pixel sensor is provided, considering both the white noise and low-frequency noise sources. The noise for each stage of the sensor operation is analyzed, identifying the noise contribution from each source. In this study, the capacitance nonlinearity of the sense node is taken into account. PSPICE simulations are used to compare analytical results for noise during integration, reset and readout. A good agreement between both results is observed. In order to determine the effect of the bias of the in-pixel buffer, the total noise is computed for several bias values, and confirms that this noise depends strongly on the bias current of the first source-follower. In the same way, analysis of the influence of scaling transistor of the in-pixel amplifier, shows that at low channel width values the output referred noise decrease. Thus, there exist an optimum channel width, which gives an optimum noise. Finally, for reducing output noise, the sampling capacitance’s CR and CS must be chosen large.

Abbreviations

APS – Active Pixel Sensor

PSPICE – Simulation Program with Integrated Circuit Emphasis available on the IBM-PC

CMOS – Complementary Metal-Oxide-Semiconductor

PSD – Power Spectral Density

RMS – Root Mean Square

RTS – Random Telegraph Signal