Design of Low Power CMOS Read-Out with TDI Function for Infrared Linear Photodiode Array Detectors

A new low voltage CMOS infrared readout circuit using the buffer-direct injection method is presented. It uses a single supply voltage of 1.8 volts and a bias current of luA. The time-delay integration technique is used to increase the signal to noise ratio. A current memory circuit with faulty diode detection is used to remove dark current for background compensation and to disable a photodiode in a cell if detected as faulty. Simulations are shown that verify the circuit that is currently in fabrication in 0.5ym CMOS technology. Readout circuits for infrared imagers have many commercial, scientific and military applications which are continually expanding [I]-[5]. The main function of an infrared readout circuit is to transform a very small (nAs) diode incremental current, generated by ineared radiation, into a relatively large measurable output voltage. This is commonly done by integrating the photocurrent in a small capacitor during a fixed period of time. The capacitor's voltage at the end of the integration period should be proportional to the current and as such to the incident infrared radiation at a pixel corresponding to the location of the infrared diode photo sensor. Infrared imagers consist of linear or two dimensional arrays including a very large number of infrared photo sensors. These arrays are denoted linear or focal plane arrays (FPAs). Given that in the most general case each pixel of an image requires an individual readout circuit, the electronics associated to an infrared imager consists of a very large number (thousands) of readout circuits. The basic readout circuit is denoted "unit cell". Readout electronics is implemented as very large scale custom integrated circuits or ASICs in CMOS technology. Due to the fact that infrared imagers can have several thousand unit cells, the unit cell is required to be very compact, to have very low power dissipation and at the same time to have high performance characteristics. A versatile buffer direct injection (BDI) circuit has been conceived with several innovative features: 1) low single supply voltage 2 Umesh D. Pate1 National Aeronautics and Space Administration Goddard Space Flight Center, Code 564 Greenbelt, MD 2077 1 (VDD=l .8V) and uses an amplifier with a very low quiescent current (1 PA); 2) low input resistance (<lkQ) this in spite of the very small input currents achieved by the injection amplifier which has a high GB; 3) incorporates a compact current memory cell for background compensation; 4) utilizes a shared buffer amplifier to minimize area for the purposes of time-delay integration (TDI). The TDI technique integrates the current from each pixel over several periods. This allows the integration to take place over a longer time giving a more reliable output. The TDI technique reduces the noise by a factor .IN where N is the number of integration periods per pixel [6]. The current memory cell is used for background compensation. This is required to prevent the dark current from being integrated along with the photocurrent signal. This allows the utilization of a smaller integrating capacitor and improves dynamic range. A. Bzflfered Direct Injection Circuit Design The scheme of a buffered direct injection readout unit cell is shown in Figure 1. The IR diode is modeled as a current source in parallel with a shunt resistance Ro and a shunt capacitance Co. Transistor M1 is used as a current buffer to pass the diode photocurrent to the integrating capacitor. In order to achieve high current efficiency from the diode it is required that the input impedance, Rx, be much less than Ro. A practical problem in direct injection readout circuits is that with extremely small input currents, the input resistance of MI in the common gate configuration (given by Rin = llgm) can take very high values (Rin -10MQ). This can lead to very poor current efficiency and low dynamic range. Amplifier A is used to accurately set the voltage Vx used as the photodiode bias voltage. With very little fluctuations in Vx, amplifier A establishes a virtual ground in which the photodiode can efficiently inject current. Buffer BA serves all the amplifiers in a large array of unit cells. Other approaches attempt to generate a low impedance The project was supported by the 2006 ESMD -NASA Goddard Summer Program. https://ntrs.nasa.gov/search.jsp?R=20070018847 2018-02-10T08:30:10+00:00Z