Abstract
This article presents in-pixel (of a CMOS image sensor (CIS)) temperature sensors with improved accuracy in the spatial and the temporal domain. The goal of the temperature sensors is to be used to compensate for dark (current) fixed pattern noise (FPN) during the exposure of the CIS. The temperature sensors are based on substrate parasitic bipolar junction transistor (BJT) and on the nMOS source follower of the pixel. The accuracy of these temperature sensors has been improved in the analog domain by using dynamic element matching (DEM), a temperature independent bias current based on a bandgap reference (BGR) with a temperature independent resistor, correlated double sampling (CDS), and a full BGR bias of the gain amplifier. The accuracy of the bipolar based temperature sensor has been improved to a level of ±0.25 °C, a 3σ variation of ±0.7 °C in the spatial domain, and a 3σ variation of ±1 °C in the temporal domain. In the case of the nMOS based temperature sensor, an accuracy of ±0.45 °C, 3σ variation of ±0.95 °C in the spatial domain, and ±1.4 °C in the temporal domain have been acquired. The temperature range is between −40 °C and 100 °C.
Highlights
Nowadays, CMOS image sensors are widely used in different applications like astronomy, medicine, and especially in mobile phones [1,2,3]
The mismatch is cancelled by using dynamic element matching which averages the total current provided to the nMOS SF) measure the temperature via the PTAT differential voltage, they need to be biased by two different currents in a ratio N : 1. The current mirror composed of currents I1 to IN provides the current ratio for the temperature sensors
Improvements on the accuracy of the in-pixel temperature sensors have been reached by using a bandgap reference circuit with temperature compensated resistors
Summary
CMOS image sensors are widely used in different applications like astronomy, medicine, and especially in mobile phones [1,2,3]. The external VBE of the BJT including the series resistance is shown in Equation (4): VBE = VB E. where VB E corresponds to the intrinsic base-emitter voltage, IE is the emitter current, RE is the emitter series resistance, IB is the base current, RB is the base series resistance, and βF corresponds to the current gain. The effect of the temperature dependent βF on RS can be reduced by selecting the right emitter-current (bias current) range where the βF is current independent [23]. An accurate PTAT differential base-emitter voltage can be generated by a well-defined emitter-current ratio (resulting in a well-defined collector-current ratio as well) where βF is independent of the Ibias in the selected temperature range. From Equations (6) and (7), it is clear that the bias current and the series resistance affect the linearity of the VBE and ∆VBE. Equation (9) shows that the series resistance has been cancelled by using sequential compensation
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