Noise in CMOS image sensor

We reconstructed a couple of images from the sensor and we seem to see the effect of alternate brighter and dimmer pixels. what are the other noise sources ?

Hello TD,

It's been a long time that I designed a CCD camera but what you most likely see is the differences between adjacent readout columns. There will be "gain" differences as well as offsets. That can be corrected either analog or after the ADC. I did it analog but this was in the

Then there is the dark current which is temperature dependent. Also IR sensitivity which the mfgs tried their best to muffle.

Most important are squeaky clean row and column clocks with not a speck of jitter. This also means very quiet power rails. The real kicker was to know when to sample and then do a proper integrate&hold with tons of dynamic range. The only thing I found adequate for that task was a quad-diode switch driven via a nicely symmetrical toroid transformer. Which of course you couldn't buy back then. This got me about 15dB or so more dynamic range out of one of the "stone-age" sensors than the data sheet said.

Regards, Joerg

I did designs with a CMOS image sensor.

The pure 'pixel noise' can be calculated as the RMS value of a number of readings of the same pixel in a certain readout-mode, under certain lighting conditions.

Be careful to skip the first image(s) after powerup or initialization, use a repeatable control sequence, start in the (complete) dark, and get rid of 'dead' pixels before you do the calculation.

Noise is usually specified either as a number of electrons. Sometimes manufacturers only specify the SNR in dB.

I agree completely with Joerg about the cleanliness of the control signals. Also, optimal signal scaling (using a VGA, and automatically adapting the shutter speed) can be used to optimize the SNR.

So that is all about 'temporal noise'. Because of variations between pixels, there is also 'spatial' noise in the image. It is the same for each frame, so you may be able to correct for these effects by analog or digital (onchip/offchip) processing.

Spatial noise types:

- variation of the dark response from pixel to pixel (dark signal non-uniformity, DSNU)

- variation of the offset from pixel to pixel (fixed pattern noise, FPN)

- variation of the sensitivity from pixel to pixel (photo response non-uniformity, PRNU)

Does this help you?

Best regards,

Marco

thans for your replies. Its been really helpful

I did a calculation of the "pixel noise"/ temporal variation on a pixel and found it to be around 2-3 mV. We used a 12 bit A/D converter for digitizing, and so a 9 or 10 bit A/D seems right with these noise levels...Our voltage swing is 2V.

Also I estimated the DSNU. One thing I am not able to understand is when I take the data set on a completely dark condition. I see that every other pixel shows 0. I thought there will always be some noise in the sensor/ readout circuitry leading to a non zero value. And the alternate pixels are around 3mV.

And can you explain this, variation of the sensitivity from pixel to pixel/ offset from pixel to pixel. I see variations in the pixel values under uniform lighting conditions, making neighboring pixels brighter and dimmer.

thanks again, TD

KoKlust wrote:

Hello Joerg, Thanks again for your suggestions.

We are reading out 4 channels of data from the CMOS sensor corresponding to columns that are interdigitated. and we are reading out the data row wise. When I view the data, I see there are 0 digitized values on only one of the channels ( so it may have been some specified columns in the array) But we tried imaging a number of different objects and the reconstructed image looks very good except for FPN. So defintely the array doesnt seem to be damaged. aaha I think I may have found out why it happens.

The A/d converter digitizes differential input from -1 to 1V. and hence anything below -1v is reported as 0. I think what may have happened here is we have an offset control for the analog output swing, probably and hopefully its swinging from -1.2 to 0.8 or -1.1 to 0.9 instead of

-1 to 1V. and hence ADC reports these as 0s. (I also do not remember digitized value reaching 4096, always a little lower) But I have to see how I can control the offset and verify this....

But I see this pattern occuring even the pixels are not saturated. I see this effect at almost all light levels. I do not have the entire architecture and I am not allowed to post what I have due to proprietary reasons.

Regards, TD

Hello TD,

A 10 bit converter won't cut the mustard here. You need at least 12 bits. An honest and detailed data sheet will show you the effective number of bits (ENOB) versus frequency. Many converters drop almost two bits when being used at more than half the maximum clock frequency. So from a noise perspective that 10 bit converter might really behave like

Zero is not normal. There always is some noise and it should certinly no be this non-uniform. Could the array be damaged?

Offsets and variations are normal, see my previous post. If you observe neighboring pixels of an illuminated area increase in value even though they weren't illuminated that is usually caused by charge bleeding into their cells. Often this is called "blooming". Mostly that happens when you exceed the maximum charge in a pixel cell. I guess Shakespeare would have said "the bucket overfloweth".

It's hard to guess what really happens on your array because none of us knows how it's laid out. If you were at liberty to post the architecture that would help a lot.

Regards, Joerg

Hello TD,

That would explain it. Offset controls for CCD are a very touchy subject. They usually aren't meant to ease the life of the guy that has to design the interface. In my CCD days about 20 years ago I used a Philips NXA sensor. It was very particular about any of its DC levels.

Again, offset and transfer factor (often called gain) deviations are part of life with CCDs. Not just between columns but also rows if the readout occurs via more than one row at the bottom. In my case it was three rows. I don't remember how many columns I had. I think it was four like on your array.

Offsets can be measured. Usually (hopefully...) the manufacturer has provided a covered blank pixel for each column. Then you can detect and clamp on that value which will restore the DC bias for each columns.

Transfer function deviations are more difficult. Basically you'd have to calibrate for that, using the cal values as coefficients to scale the readout values.

That's understandable. But you as the circuit designer must have the entire architecture. I don't know how else you could achieve optimum performance.

Assuming you get raw and unsampled pixel output from the array: Do you have a proper integrate and hold circuit? That makes a huge difference in uniformity and also in dynamic range. Resist the temptation to just kind of add the row outputs together and send them through some RC. That's what the app notes for my array had originally suggested. The difference between their sample camera and the new one was stunning.

Regards, Joerg