Binning of High Energy CCD Detectors
Understand binning in high-energy CCDs
CCD's are very versatile devices and their readout pattern can be
manipulated to achieve various effects. One of the most commonly
used is binning. Binning allows charges from adjacent pixels to
be combined on the sensor before the charge is readout through
the amplifier, the dominant noise source on a CCD. This can offer
benefits in faster readout speeds and improved signal to noise ratios
with reduced spatial resolution.
To illustrate the difference between the two readout modes we have
used a step by step readout diagram.
The diagram is of a CCD made up of 4 pixel imaging array,in grey,
with a readout register below, in blue. The charge held in the pixel is
indicated in the bottom right of the pixel.
It is important to highlight the main differences in the two readout
schemes. In the first we achieve the full spatial resolution of the
sensors array. In the binned example we have reduced the 4 pixel
pattern to a single pixel 'Superpixel' and hence have lost spatial
resolution. However, the binned operation takes fewer steps to
readout the sensor and hence is faster. Typically binning 2 x 2 is
twice as fast; this is achieved by having to shift the readout register
only every two vertical shifts. This relationship holds if we were
binning 3 x 3 or 4 x 4 on a CCD with the readout being three and
four times faster respectively.
1. The light falls evenly on the four pixels and creates a charge of
20 e- in each of the four pixels.
2. The first operation is to shift the charge down one row. The charge
from the lowest pixels gets shifted into the readout register.
3. (A) For single pixel readout, the charge in the readout register
is shifted to the right and into the readout amplifier. (B) In
the binning operation the charge is shifted down again and the
charge from the first row is added, or summed, to the first row in
the readout register.
4. (A) For single pixel readout, the first pixel is readout while the
readout register is shifted again to shift the charge in the second
pixel into the readout amplifier. (B) In the binning operation the
summed charge from two right two pixels is shifted into the
5. (A) In the single pixel readout, the next row is shifted vertically
into the readout register. (B) In the binning operation the readout
register is shifted again to sum the charge from the five pixels in
the readout amplifier before being readout.
6. (A) In the single pixel readout mode, the readout register is shifted
to the right again to readout the next pixel. Binned operation is
7. (A) In the single pixel readout the readout register is shifted to the
right again to readout the final pixel.
The binned example also highlights how binning improves the
signal to noise ratio. If we assume our CCD has a readout noise of
10 e-, then in the single pixel example each pixel is readout with a
noise of 10 e- hence we achieve a signal to noise ratio of 2:1
(20 e-/10 e-). Even if we subsequently sum the four pixels in a
computer after readout the signal to noise ratio becomes 4:1. In
summing the charge of the four pixels, we sum the signal (4 x 20 ei.
e. 80 e-) and the noise is added in quadrature i.e. square root of the
sum of the noises squared ( √4 X (10e-)2 i.e. 20e-).
In the binned example there is no noise until the signal is readout by
the amplifier, so the signal to noise ratio is 8:1(80 e- / 10 e-) i.e. twice
as good as the single pixel readout mode.
One of the most common applications of custom binning in X-ray
is in energy-dispersive spectroscopy where low flux conditions with
incident higher energies X-ray photons can be directly detected with
the larger superpixel gaining the advantage of the lower noise and
faster readout to achieve a user definable optimal performance.