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NOIL1SM0300A Datasheet(PDF) 6 Page - ON Semiconductor |
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NOIL1SM0300A Datasheet(HTML) 6 Page - ON Semiconductor |
6 / 29 page NOIL1SM0300A www.onsemi.com 6 Figure 6. ADC Timing CLK_ADC DUMMY 50ns 3.7ns 5.5 clock cycles ADC_IN D1 D2 D3 D4 D5 D6 D7 D8 D1 D2 D3 D4 ADC_OUT <9:0> Programmable Gain Amplifiers The programmable gain amplifiers have two functions: • Adding an offset to the signal to fit it into the range of the ADC. This is controlled by the VBLACK and VOFFSET SPI settings. • Amplifying the signal after the offset is added. Offset Regulation The purpose of offset regulation is to bring the signal in the input range of the ADC. After the column amplifiers, the signal from the pixels has a range from 0.1V (bright) to 1.3V (black). The input range of the ADC is from 0.75V to 1.75V. The amount of offset added is controlled by two SPI settings: VBLACK<7:0> and VOFFSET<7:0>. The formula to add offset is: Voutput = Vsignal + (Voffset - Vblack) Note that the FPN (fixed pattern noise) of the sensor causes a spread of about 100 mV on the dark level. To allow FPN correction during post processing of the image, this spread on the dark level needs to be covered by the input range of the ADC. This is why the default settings of the SPI are programmed to add an offset of 200 mV. This way the dark level goes from 1.3V to 1.5V and is the FPN information still converted by the ADC. To match the ADC range, it is recommended to program an offset of 340 mV. To program this offset, the Voffset and Vblack registers can be used. Figure 7 illustrates the operation of the offset regulation with an example. The blue histogram is the histogram of the image taken after the column amplifiers. Consider as an example that the device has a black level of 1.45V and a swing of 100 mV. With this swing, it fits in the input range of the ADC, but a large part of the range of the ADC is not used in this case. For this reason an offset is added first, to align the black level with the input range of the ADC. In the first step, an offset of 200 mV is added with the default settings of VBLACK and VOFFSET. This results in the red histogram with a average black level of 1.65V. This means that the spread on the black level falls completely inside the range of the ADC. In a second step, the signal is amplified to use the full range of the ADC. Figure 7. Offset Regulation Volts 1.45V1.65V VADC_HIGH 1.75V |
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