Difference between binning and averaging (1)

Especially in the CMOS world there seems to be some confusing about the definition of binning and averaging.

Binning is a technique that allows to add up two (or more) pixel output signals to increase the signal-to-noise ratio of the image sensor at the expense of resolution.  The original binning method was done by means of adding of the output signals in the charge domain, but with the introduction of CMOS imagers, binning is also applied in the voltage domain or digital domain.  The charge domain binning is always done on-chip, voltage binning or digital binning can be done on-chip as well as off-chip.

Charge domain binning : this is the only binning method that can be completely done noiseless.  In the case n pixels are binned, the signal after binning will be n times the signal of each individual pixel.  Readout of the signal after binning will only once add the noise of all readout circuitry (= readout noise), so the signal-to-noise ratio AFTER binning is equal to n times the signal-to-noise ratio of the un-binned signal.

Charge domain binning is very easy to implement in monochrome CCDs by means of an adapted timing, colour CCDs may need a more complicated clocking scheme and/or a dedicated design to perform binning because charge domain binning needs to be done in each colour plane.  Charge domain binning in CMOS image sensors is limited to pixels that share a floating diffusion.

Voltage or digital domain binning : both binning methods can only be applied AFTER the pixels are being readout, and thus after the readout noise is included in the output signal.  In the case n pixels are binned, the signal after binning will be n times the signal of each individual pixel, but the noise will be added in quadrature, and will be equal to ?n times the noise of a single pixel.  So the signal-to-noise ratio after binning in the voltage or digital domain will be ?n times the original signal-to-noise ratio.

Averaging of signals takes place when two (or more) capacitors holding pixel output signals in the voltage domain are short-circuited.  The charges on each capacitor are summed, but so are the capacitances.  In the simple case of averaging n signals (present on n capacitors of equal value), the averaged signal will not change in value.  But the noise on the other hand will be added in quadrature and will be stored on the summed capacitors.  Any idea what will happen with the final signal-to-noise ratio ?

Conclusion : charge domain binning is more efficient in increasing the signal-to-noise ratio compared to binning/averaging in the voltage domain or binning in the digital domain.  The explanation of binning and averaging as well as the discussion about signal-to-noise ratio in this blog takes into account that the noise content of the pixel output signals is dominated by readout noise.  The story becomes slightly different is the signals are shot-noise limited.  This will be explained next time.

Albert, 21-05-2016.

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