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ADC - Bit readout explained


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Readout from sCMOS (Scientific CMOS) sensors used in XIMEA cameras is a bit more complicated which is why this simple explanation can help understand the final speed (frames per second) and output:

For PCI Express cameras - CB042 models

Rolling shutter

Sensor readout 2 x 12 bit means one High gain (HG) and one Low gain (LG) channel.
That is how these sensors work.

Transport depends on the output_bit_depth parameter.
It can be set to 8, 10, 12, 16 which means 2x8, 2x10 and so on.

Image formats:
RAW8, RAW16 - data are merged from HG and LG.
RAW8x2, RAW16x2 - RAW data from the sensor. Both HG and LG data for each pixel are present.

Global shutter

Sensor readout 4 x 10 bit means again High gain and Low gain data, but for Signal image and Reset image each.

Transport depends on output_bit_depth parameter.
It can be set to 8, 10, 12, 16 which means 4x8, 4x10 and so on.

Image formats:
RAW8, RAW16 - data are merged from HG and LG, and both Signal and Reset images.
RAW8x4, RAW16x4 - RAW data from the sensor. Both HG and LG data for a pixel for Signal image and Reset image are present.

Calculator

To calculate the frame rate and other parameters you can use this helpful TOOL.
Fill inputs (the orange values) and see the expected results.
Each input is connected to a xiApi parameter - e.g. Transport bit depth field is XI_PRM_OUTPUT_DATA_BIT_DEPTH xiapi parameter
Because of the processes mentioned above, in this case, if you change the Transport Bit Depth from 8 to 10 the Frame rate is still the same for example.

Note: 1300 MB limit was introduced into the Calculator because not all computer configurations were able to support higher speed, but now certain PCs could reach stable 1750 MB.

For USB3 + sCMOS cameras - xiJ models

Gpixel sensors (GSENSE400, GSENSE2020, GSENSE5130 and others) offer 3 operation modes:
HDR, STD and CMS
These sensors have two on-chip 12bit ADCs.
First, high gain ADC (HG) is optimized for low readout noise.
Second, low gain ADC (LG) is optimized for full well capacity.

1. HDR.

Merger:

O16 = (DL12 * k + DH12 * (1 – k) * g) * 16 = DL12 * k * 16 + DH12 * (1 – k) * g * 16


there:


O16 – 16bit output signal.
DL12 - 12bit data of LG channel.
DH12 - 12bit data of HG channel.
k = DH12 / 4095.
g = gL / gH.
gL – gain of LG channel.
gH – gain of HG channel.


In HDR mode the analog gains are fixed (gL and gH) and can’t be changed by API.

2. CMS mode.

SUM:


O16 = (DL12 + DH12) * 8 = D13 * 8


gL = gH
Both ADCs readout the same pixel at one time.

3. STD mode.

Packer:


O12 (pixel 0) = DL12 (pixel 0, row 0)
O12 (pixel 1) = DH12 (pixel 0, row 1)
O12 (pixel 2) = DL12 (pixel 1, row 0)
O12 (pixel 3) = DH12 (pixel 1, row 1)


gL = gH
In CMS and STD modes gain on both channels can be changed in API.

For large sensor and pixel cameras - MX377 models

Here you can see a table with the preliminary image quality parameters for different modes:


Modes gain [dB] dark noise [e] FWC [ke] Dyn.Range [dB]
HDR_LG -- 9.8 130 82.45
CMS 0 21.5 115 74.57
CMS 25 2.65 7.6 69.15
STD 0 26 105 72


Full HDR mode is also planned which will have lower dark noise (as compared to HDR_LG mode) while leaving the FWC same.