Difference between binning and averaging (2)

June 3rd, 2016

In the previous blog the focus was on binning (charge, voltage and digital domain) in the case the readout noise was dominating over the photon-shot noise.  In other words, for the case of small signals or low light levels.  This time, the situation for a shot-noise limited condition is considered.  And actually the story can be very short : it does not matter when or where the binning is done, in all cases the result is exactly the same.

For the charge domain : if n x n charge packets are added together, each with m electrons, then after binning the final charge packet holds :

n x n x m electrons.

The photon-shot noise in each individual charge packet was

sqrt(m) electrons,

so the SNR for every individual charge packet was

SNR = m/sqrt(m) = sqrt(m).

After binning the total photon-shot noise is equal to

sqrt(n x n x m) electrons

and the SNR will be equal to :

SNR = n x n x m/sqrt(n x n x m) = sqrt (n x n x m).

After binning in the charge domain, the increase in SNR will be

sqrt(n x n x m)/sqrt(m) = n !

For the voltage domain or digital domain : if n x n signals are added together, each with m electrons, then before binning the output signal would have been k x m V or DN, with k being the conversion gain from input (charge) to output (Volts or Digital Numbers).  Then after binning the final signal will be

n x n x k x m V or DN.

The photon-shot noise of each individual signal before binning was

k x sqrt(m) V or DN,

so the SNR for the individual signal before binning was

SNR = (k x m)/(k x sqrt(m)) = sqrt(m).

After binning the total photon-shot noise is equal to

k x sqrt(n x n x m)

and the SNR will be equal to :

SNR = (n x n x k x m)/(k x sqrt(n x n x m)) = sqrt (n x n x m).

After binning in the voltage of digital domain, the increase in SNR will be

sqrt(n x n x m)/sqrt(m) = n !

Conclusion : if there is enough light so that the performance of the sensor or the camera is shot-noise limited, it does not matter how the binning is realized, charge domain, voltage domain or digital domain.  The increase of the SNR after binning is always equal to a factor n, being the kernel size in the case of n x n binned pixels.

Albert, 31-05-2016.

HARVEST IMAGING FORUM 2016 “Robustness of CMOS Technology and Circuitry”

May 29th, 2016

I am happy to inform you that the registration for the 2016 Harvest Imaging Forum is open !

Visit the web-pages of the forum at www.harvestimaging.com/forum_introduction_2016.php

Best wishes, Albert.

29-05-2016.

Difference between binning and averaging (1)

May 21st, 2016

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.

Update of the Phase-Detection Auto-Focus Pixel Report

April 19th, 2016

A new update of the PDAF report is available.  Compared to the previous version, extra information is included about a figure-of-merit.  This FoM allows the reader to compare the efficiency of the PDAF pixels coming from different sensors, different technologies and different vendors.  Also a couple of new references are added to list.

If you are interested in buying (unfortunately it is not free of charge) the PDAF report, including the two updates, please contact me through info (at) harvestimaging (dot) com.

Thanks a lot, Albert.

19 April 2016.

Announcement of the fourth Harvest Imaging Forum in December 2016

April 17th, 2016

Mark now already your agenda for the fourth Harvest Imaging Forum, scheduled for December 2016.

After the succesful gatherings in 2013, 2014 and 2015, I am happy to announce a fourth one.  Also this fourth Harvest Imaging Forum will be a high-level, technical, short course focusing on one particular hot topic in the field of solid-state imaging.  The audience will be strictly limited, just to stimulate as much as possible the interaction between the participants and speaker(s).

The subject of the fourth forum will be :

“Durability of CMOS Technology and Circuitry outside the Imaging Core : integrity, variability and reliability”.

More information about the speaker and the agenda of the fourth forum will follow in the coming weeks, but I wanted to share this announcement with you as early as possible to make sure you can keep your agenda free on these days (Dec. 9-10 or Dec. 12-13, 2016).

Albert,

April 17th, 2016.

Imaging Trainings scheduled for Spring 2016

February 27th, 2016

Maybe it is good to remind the visitors of this blog about imaging trainings in the Spring 2016.  There are 6 courses in the pipeline :

– a 2-day class to get an introduction in the world of CMOS image sensors.  This class is intended for people who have almost no background in solid-state imaging.  This course takes place in Taufkirchen (Munich) on June 29-30, 2016, organization through www.framos.com.

– a 5-day class if you want to learn more about imagers than just the working principles.  Also this class is intended for “new-comers” in the field, but also people working already a few years in imaging can revitalize their knowledge.  The course can be considered as the mother of all trainings offered by Harvest Imaging.  Key to this class are the exercise sessions at the end of every day helping the participants to put the theory into practice.   This course takes place on April 4-8, 2016 in Barcelona, and is organized by www.cei.se.

– a 2-day class with hands-on measurements and evaluation of an “unknown” camera.  Because the participants have to perform all characterization work themselves, this course is NOT intended for people fresh in the imaging field.  Preferably the course participants have a few years of experience in the arena of solid-state imaging.  This course takes place in Munich, on March 30-31, 2016, organized by www.framos.com, as well as in Amersfoort, on May 26-27, 2016, organized by www.cei.se.

– a 3-day advanced class focusing on CMOS image sensors.  Because the material presented is on a higher level, this course is intended for people who have a couple of years of experience in the field of digital imaging.  The course is scheduled for May 23-25, 2016 in Amersfoort (Nl), organized by www.cei.se.

– a 3-day course on Digital Camera Systems.  In this training the focus is less on the image sensors, but more on the processing of the signal delivered by the image sensor.   The complete colour processing pipe will be explained and demonstrated by an extensive amount of images and algorithms.  The participants will get a soft copy of all images shown in the course.  Location will be Barcelona, date : June 14-16, organized by www.cei.se.

Looking forward to see you at one of these courses.

Albert, 27 February 2016

Noise Forum at ISSCC 2016

February 8th, 2016

The ISSCC forum, organized on Thursday, was focusing on Noise in Sensors (very general).  A total of 9 presentations were given, of which (only) 3 focused on Imagers.  The undersigned opened the forum with a general overview of Noise in Image Sensors, in the early afternoon Shoji Kawahito give a presentation on Low-Noise Image Sensors, and to conclude the forum, Neale Dutton had a talk about Noise in Single-Photon Detectors. 

On one hand many people appreciated the general overview of noise present in many different type of sensors, on the other hand, not that many imaging engineers attended the forum because of the low number of talks about imagers.  Nevertheless, the ISSCC organization seemed to be pretty happy with the number of registrations. 

One very interesting detail from Neale’s presentation : he showed a very nice graph of published noise data which I include here in this blog (with permission of Neale !).  On the vertical axis the input referred read noise in electrons is shown versus the conversion gain of the pixels.

The three lines shown in the graph are lines of equal read noise, “equinoise” lines, but this time noise expressed in uV.  As can be seen, the lowest noise ever reported was 0.22 electrons, presented in JEDS2015, but the lowest noise ever reported in the voltage domain was 30 uV, presented at ISSCC2012.  I do know that expressing noise in equivalent number of electrons is a very common technique which I support as well, but nevertheless, looking to the noise in the good, old classical way gives a complete other image.  Now the challenge is to keep the 30 uV of noise level alive, while increasing the conversion gain !

Thanks Neale (not Neil) for this “fresh” view on the noise !

Albert, 08-02-2016.

ISSCC 2016 (3)

February 8th, 2016

In this third and last review of the ISSCC, 2 remaining imaging papers are left.

The first one comes from NHK, and deals with a 1.1 um 33 Mpixel device, 3D stacked and 3-stage cyclic-based ADC.  This 3D stacking is realized by means of direct bonding (in the columns).  TSV’s are avoided because they seem to be too expensive, they cost more masks, they consume area and they make a more complicated lay-out needed.  The device is fabricated at TSMC (at least TSMC is mentioned in the acknowledgement), and to my knowledge this is the first CMOS image sensor made in 45 nm 1P4M.  The logic part on the second level of silicon is made in 65 nm 1P5M technology.

The ADC implemented on the chip (by Shizuoka Univ./Brookman Technology) is a three stage design, the first two stages are cyclic ADCs (upper 3 bits and middle 6 bits), the last stage is  a SAR ADC (3 bits).  The sensor can run at full resolution (33 Mpixels !) at a rate of 240 fps, burning 3 Watts.

The last paper from the imaging session is the one that was published by FBK, Trento, with two brothers as authors (does not happen that often).  The device presented is intended for spacecraft navigation and landing.  It contains 64 x 64 pixel digital silicon photomultiplier direct ToF with 100 Mphotons/s/pixel background rejection.  Every pixel (out of the 64 x 64 array) contains 8 SPADs  with extra electronic circuitry.  The pixel is designed such that uncorrelated photons or dark current (which still trigger the SPADs) do not give an output from the pixel.  Only correlated photons give an output.  So the background suppression and dark count suppression is more or less based on the statistics of these signals (compared to the ToF signal), and is implemented in the digital logic within every pixel.  Fabrication technology is 150 nm CMOS with 6 metal layers.  Pixel fill-factor is 26.5 %.

Albert, 08-02-2016.

ISSCC 2016 (2)

February 5th, 2016

At ISSCC several high resolution imagers were presented.  Champion was the device of CMOSIS with 391 Mpixel device for airborne mapping applications.  The device itself is pretty straight forward with 3.9 um pixel pitch, 4T non shared pixels, 31.5 ke full well, 45 uV/e conversion gain, 3.7 e of noise at unity gain, resulting in 78 dB dynamic range.  The 14-bit SS ADCs are placed in the columns and are located at the two sides of the imager.  So the pitch of the ADCs is 7.8 um.  Jan Bogaerts showed impressive images of the device, and after the “show” I had the opportunity to take a look at a real device in its package.  The sensor is using stitching : 6 x 3 blocks are stitched in the active area, with 4352 x 5000  pixels in each stitched block.  Processing was done at ST in a 90 nm FE/65 nm BE process 1P4M.  The device is monochrome, but in the final application, this monochrome sensor is surrounded by CCDs that provide the colour information.  During Q&A it was mentioned that the camera is using a mechanical forward motion compensation technique to compensate for the movement of the camera during exposure.

During the presentation Jan Bogaerts made a comparison with a CCD of the competition.  Amongst several characteristics, he mentioned that his CMOS sensor is free of smear in contradiction to the CCD.  In a private discussion afterwards with Jan Bogaerts he told me that the camera is using a mechanical shutter (that is no secret I guess), but one should realize that in that case a CCD is neither showing any smear issues.

Hirofumi Totsuka of Canon presented a 250 Mpixel APS-H size imager : 1.5 um pixel pitch (4 sharing) made in 0.13 um technology node.   The device is consuming 1.97 W at full resolution 5fps.  An interesting build-in feature of this sensor is the following :  ALL pixel signals are converted by column SS-ADCs with a single ramp, but in front of the ADC, each column has its own PGA that can be switched to 4x or 1x gain, depending on the signal level.  So when the pixels are sampled, a first check is done to look whether the signal is above or below a particular reference level, and then the right gain of the PGA is set to 1x or 4x.  Simple method, but I think that the issues pop up in the reconstruction of the signal at the cross-over point between the two settings of the PGA.

 

Kei Shiraishi of Toshiba presented a stacked sensor with 1.2 e of noise with a comparator-based multiple sampling PGA.  The most important characteristic is the multiple sampling in the analog domain.  This goes much faster than the multiple sampling in the digital domain.  After 32 samples of each signal, a noise level of 1.2 e could be reached for 1 M pixels at 20 fps.  The device is realized in 65 nm, both for the sensor as well as for the circuit on the second silicon level.  It was mentioned in the paper, but I guess that the noise floor without the multiple sampling should be around 5 e at 30 fps, going down to the 1.2 e reported at 20 fps.

Charles Liu of TSMC showed the results obtained by a 33 Mpixel, stacked device with a negative substrate bias.  The idea is actually pretty simple, maybe the implementation is more complicated. “Simply” bias the substrate of the sensor to -1.3 V and you can lower all other supply voltages by 1.3 V.  So instead of having a power supply of 3.3 V, the device has now a supply of 2.0 V.  But the large pixel swing is maintained by means of the negative substrate bias.  The sensor is fabricated in 65 nm 1P5M technology.

Albert, 05-02-2016.

ISSCC 2016 (1)

February 4th, 2016

Already quite a bit of words are spent on the organic conductive sensors presented in the Panasonic papers.  Nevertheless, here is some more info.

Kazuko Nishimura presented the paper on the large HDR sensor with a low noise level.  A few remarks about this sensor :

–          HDR is obtained by two light sensitive areas within one pixel : one with low and one with high sensitivity.  This is a very similar method as proposed long time ago by Fuji in their SuperCCD,

–          The pixels do suffer from kTC noise, but by means of a cleaver circuit/feedback, they are able to reduce the remaining kTC noise to 1.2 e reset noise and to 5.4 e overall.  In combination with a full well of 600 ke, it creates a gorgeous dynamic range,

–          The process used to fabricate the sensor is 65 nm CMOS, 1P3Cu1Al,

–          The results mentioned are overall not bad, but there was no information provided about dark current, about quantum efficiency, about uniformity and about reliability of the material.  So this suggests (to me) that there are still some issues to solve.

Sanshiro Shishido presented the paper on the global shutter version of the organic photoconductor sensor.  The topplate of the photoconductor is made out of ITO and needs to be biased to larger voltages.  But the overall light sensitivity of the organic photoconductor depends strongly on the exact voltage on the ITO gate.  A lower voltage on the ITO gate lowers the light sensitivity and actually 0 V on the gate makes the sensor even blind.  In this way one can create a global shutter functionality to the sensor.  Moreover, one has the possibility to modulate the sensitivity during the exposure time, for instance, the exposure time can be split in parts in which the sensor will be sensitive and in parts in which the sensor will be insensitive.  Even one has the option to modulate the sensitivity during the periods the sensor is sensitive by means of adapting the high voltage set to the ITO gate.  Overall a nice technology !

The third paper of Panasonic, presented by Yoshihisa Kato had nothing to do with the organic conductive layer, but the sensor presented was provided with an EM-function.  The latter is built in the vertical direction of the silicon.  This is new and is never shown before in an imager (to my knowledge).  The EM-functionality can be switched on and off by means of the voltage biasing the substrate (around 23 V).  Amazing images were shown (shot at extremely low light levels).  From the data shown, it looks like the EM is very strongly depending on the exact voltage on the substrate.  In comparison to the well-known EM-CCD and EM-CMOS devices (which are on the market), the EM-multiplication in the latter is done with very small gain steps, but by doing multiple EM-steps finally a large gain can be reached.  In the case of the Panasonic paper, the EM is done only once, so all the gain needs to be created in one step.  Has this way of working advantages or disadvantages compared to EM-CCD and EM-CMOS ?

Albert, 04-02-2016.