Help understanding input referred read noise?

Started May 9, 2018 | Questions thread
dosdan Contributing Member • Posts: 536
Re: Help understanding input referred read noise?
3

The Source Follower (SF) MOSFET is a significant source of noise. In a sensor there is a design compromise:

  • Low Floating Diffusion (FD) Capacitance = High Conversion Gain (CG). This high gain helps to overwhelm the RN contribution from the SF. But it also reduces the FWC. So HCG is bad at low ISO where big FWC is desirable. Low FWC/Low RN = Reduced DR. But HCG is good at high ISO where the analogue-amplified pixel RN becomes the dominant noise source. So LCG means less amplified RN.
  • High FD Capacitance = LCG. This low gain increases FWC because more photoelectrons can be stored before the SF output voltage swing  clips (reaches its max. voltage level). So LCG is good at low ISO for a high DR. But LCG is bad at high ISO because pixel RN is larger. So LCG means more amplified RN.

So the value of the CG chosen by the sensor designer is a compromise. But with Dual CG sensors, this compromise is reduced.

The step in the RN/ISO curve at low-mid ISO is the CG switch from LCG to HCG. In LCG-mode extra capacitance in switched in parallel, increasing the FD capacitance. HCG switches out this extra capacitance.

Any step at high ISO is raw-level NR being switched on.

Currently, Dual CG is the best we have. But Triple CG is possible. See "An 87dB Single Exposure Dynamic Range CMOS Image Sensor with a 3.0μm Triple Conversion Gain Pixel" :

http://www.imagesensors.org/Past%20Workshops/2017%20Workshop/2017%20Papers/R29.pdf

Dan.

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