Why doesn’t full well capacity match the ADC range on my FLIR Blackfly sensor?
Asked 9/12/2020
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On a FLIR Blackfly BFS-U3-200S6M, the EMVA 1288 data lists a system gain of 0.24 e-/ADU and a 12-bit ADC. That suggests a maximum digital range of about 4096 ADU × 0.24 e-/ADU = 17,062.5 electrons. But the listed saturation capacity is only 14,837 electrons. Why is the sensor saturating before the ADC’s full numeric range is reached? Am I misunderstanding the relationship between gain, full well capacity, saturation, and ADC bit depth?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
1
The datasheet states that the sensor has a DR capability of 11.93bit which is less than the 12bit accuracy/capability of the ADC. I.e. it is not using all of that 12bit conversion, but 12bit is required. And it will be stuck into a file format that can accept 16bit because that is also required; but it will still be 11.93bit (max).
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
5y ago
0
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You’re not necessarily missing anything. The key point is that the sensor’s usable dynamic range can be slightly less than the ADC’s nominal bit depth.
A 12-bit ADC can encode 4096 levels, but that does not guarantee the sensor itself will fill all of them before reaching saturation. In your case, the sensor’s saturation capacity is about 14,837 electrons, while 0.24 e-/ADU across the full 12-bit range would imply about 17,062 electrons. That means the analog signal reaches its limit before the ADC uses every possible code value.
Put another way: the ADC has 12-bit capability, but the sensor/electronics chain only delivers about 11.93 bits of dynamic range, as noted in the datasheet. So some top-end ADC codes may simply never be used in normal operation.
This is normal in real camera systems: ADC resolution, sensor full well, and effective dynamic range are related, but they do not have to match exactly.
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