One of the last temporal noise sources on the list to implement in the model is the thermal noise of the output (differential) amplifier. The latter represents any noise coming from, for instance, the correlated-double sampler or the on-chip automatic gain control. By definition this type of noise will not influence the fixed-pattern noise, so there is no need to discuss this part of the PTC again (last time was a very long blog, this time it will be a short one).
Our simulation tool generated 100+ dark images at various exposure times (between 0 s and 65 s). The result of this exercise in dark can be seen in the following two figures :
- Figure 1 contains the average dark signal (left axis), and its temporal noise component (right axis) as a function of the integration time (horizontal axis). (See previous blogs to learn how the calculation of the fixed-pattern noise is done.)
- Figure 2 shows the dark current temporal noise versus the dark signal, based on the data shown in Figure 1.
Figure 1 : Dark current and its temporal noisecomponent as a function of the exposure time.
Figure 1 shows the dark signal and the temporal noise as function of the exposure time. Not really that much new information can be extracted from these curves, except the minimum temporal pixel noise being equal to 1.22 DN. The dark current is linearly increasing with the exposure time till the pixels start to saturate. This is the point where the temporal noise reaches its maximum value.
Figure 2 : “PTC” of the sensor.
Figure 2 shows the real Photon Transfer Curve, in which the temporal noise is shown as a function of the signal. The curve shows an indication of the part that is (nearly) independent of the dark signal (with a slope of 0) as well as the part that is directly depending on the dark signal (with a slope of 0.5). The region in the graph showing a collapsing curve indicates the saturation of the pixels.
From the PTC curve the following parameters can be deduced :
- The conversion gain, being equal to 1/100.734 DN/e- = 0.184 DN/e-,
- The total temporal pixel noise (without any influence of the dark current) = 100.085 DN = 1.22 DN, this minimum noise level is representing the temporal noise of the output stage because all other temporal noise sources are set to zero (even the dark current shot noise is zero at an exposure time equal to zero),
- The onset of anti-blooming = 103.32 DN = 2090 DN,
- The saturation level of the pixels = 103.45 DN = 2818 DN,
Conclusion : no new effects can be deduced from the PTC by adding the temporal noise of the output amplifier to the simulation model.
Next time the effect of defect pixels on the PTC will be highlighted.
Albert 2010-02-25
I like these blogs. They are so valuable.