A CMOS Image Sensor Dark Current Compensation Using In-Pixel Temperature Sensors

Abstract

This paper presents a novel technique for dark current compensation of a CMOS image sensor (CIS) by using in-pixel temperature sensors (IPTSs) over a temperature range from -40 °C to 90 °C. The IPTS makes use of the 4T pixel as a temperature sensor. Thus, the 4T pixel has a double functionality, either as a pixel or as a temperature sensor. Therefore, the dark current compensation can be carried out locally by generating an artificial dark reference frame from the temperature measurements of the IPTSs and the temperature behavior of the dark current (previously calibrated). The artificial dark current frame is subtracted from the actual images to reduce/cancel the dark signal level of the pictures. In a temperature range from -40 °C to 90 °C, results show that the temperature sensors have an average temperature coefficient (TC) of 1.15 mV/°C with an inaccuracy of ±0.55 °C. Parameters such as conversion gain, gain of the amplifier, and ADC performance have been analyzed over temperature. The dark signal can be compensated in the order of 80% in its median value, and the nonuniformity is reduced in the order of 55%.

Keywords: CMOS image sensor; dark current compensation; in-pixel temperature sensors.

Figure 1

Block diagram of the CMOS image sensor.

Figure 2

(a) 4T pixel. (b) nMOS source follower temperature sensor.

Figure 3

I-V characteristic of the source follower transistor [33].

Figure 4

(a) Temperature coefficient of the nSFTS for different current ratios [33]. (b) Systematic nonlinearity of the nSFTS after a 1^st-order curve fitting [33].

Figure 5

Circuit of the current sink generator. The circuit is supplied by an external current ($I_{ext}=1 \mathsf{\mu }\mathrm{A}$). Depending on the switches (on/off), the current sink generates a specific current ratio (in this case, 4:1).

Figure 6

Block diagram of the read-out circuit [33].

Figure 7

Temperature frames taken in between image frames.

Figure 8

PCB of the CIS device under test.

Figure 9

Normalized ADC output.

Figure 10

(a) PGA Gain 2 over temperature. (b) Conversion gain of the CIS device over temperature.

Figure 11

(a) Dark signal over exposure time at 0 °C. (b) Dark current over exposure time at 50 °C.

Figure 12

Dark current over temperature.

Figure 13

(a) Differential gate-source voltage over temperature, the TC is 1.15 mV/°C [21]. (b) Systematic nonlinearity.

Figure 14

3σ inaccuracy of the in-pixel temperature sensors [21].

Figure 15

Top: prerecorded dark reference frame; Middle: artificial dark frame; Bottom: subtraction between the dark reference frame and the generated dark frame. (a) At 0 °C and at 10 s. (b) At 50 °C and 1 s.

Figure 16

Compensation of the prerecorded dark reference frame with close mechanical shutter. (a) At 0 °C and 10 s. (b) At 50 °C and 1 s.