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.

5 Responses to “Difference between binning and averaging (1)”

  1. Joe says:

    Hi Albert,
    what’s the value “?n” in the digital domain binning section?
    As per my understanding it should be sqrt(n)

    in the averaging…
    – the signal will not change in value (photon shot noise negligible in low light)
    – the noise will be sqrt(n) but is averaged over n capacitors (supposed to be identical)
    therefore the SNR will be sqrt(n) times original signal-to-noise ratio. the same as digital binning.
    right?

  2. albert says:

    Joe, that depends on the defintion of “n”. In my case the binning takes place in a nxn matrix of pixels. So the total amount of pixels is n^2. After binning in the digital domain the increase in S/N is sqrt(n^2) = n.
    Regards, Albert.

  3. Joe says:

    Good morning,
    in this case I can’t understand the conclusion of this post because following your comments all binnings will result with same increasing of n and would same as full light condition too (next post on this topic)

    I would ask: if total pixels binned is nxn,
    – in case of charge domain binning (like with CCD) S/N increases by in^2
    – in case of voltage or digital binning S/N increases by n

    is the this interpretation correct?
    what I’m missing?
    Thanks, Joe

  4. albert says:

    Joe,
    1) in the case the read noise (= noise of all electronic components) is the dominant noise source and if the binning of nxn pixels is done in the charge domain, you gain a factor n^2 in S/N,
    2) in the case the read noise (= noise of all electronic components) is the dominant noise source and if the binning of nxn pixels is done in the voltage or digital domain, you gain a factor n in S/N,
    3) in the case the photon shot noise is the dominant noise source, you gain a factor n in S/N irrespective whether the binning is done in the charge, voltage or digital domain.
    Albert.

  5. Joe says:

    Dear Albert,

    Thank you, everything is clear now.
    definitely charge domain binning only helps in low light conditions.

    I would also add that, in bright conditions (when the shot noise is dominant), in case of charge binning, the charge can easily overflow and the saturation effects should be considered.

    it looks averaging is the most practical approach since you will always have gain factor of n.

    Thanks again

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