From: Bob Newman on 23 Jul 2008 13:29
On 23 Jul, 16:05, ejmartin <ejm_60...(a)yahoo.com> wrote:
> BTW, something puzzles me -- if the cutoff is imposed by 1/f type
> noise sources, won't those be decreased by reading out the pixel
> faster? Which confuses me, I've always understood that read noise is
> reduced by slower readout (or was that a limitation of ADC's?), and is
> counter to the efforts to reduce read noise by using column parallel
> readouts that increase read times.
I think that there are two effects working against each other. High
speed will limit 1/f noise, but capture chains tend to perform worse
at high speed. The problem tends to be 'settling time'. Every stage
has a limited output current and some capacitance. If you want to
achieve a good sample, you need to sample it when that capacitance is
fully charged. Push too much against the speed limits, or cut the
drive current too much, and you tend to sample while it's still
charging, and then you get jitter noise, due to irregularities in the
sampling period. This is why, I suspect, the D3 A to D chain is so
massively overspecified (one of those six AFE chips provides
sufficient bandwidth according to the specs). The best answer would be
to use a very high specced, high drive sample and hold (which actually
determines the read time), which can capture the sample in a very
small window, and thus minimise 1/f noise, followed by a nice
leisurely ADC. I suspect that is what Sony is doing, and possibly
Canon too. The two papers I've seen (on the D10 sensor and the 52MPix
showcase) have on sensor sample and hold. I think the real advantage
of the Sony column ADC architecture is not mainly noise, but cost. The
very low spec ADC's can be integrated quite easily onto the sensor
chip, and it saves a costly AFE off chip. Higher spec ADC's would
require a costly BiCMOS process, which might not be available on CIS
From: Bob Newman on 23 Jul 2008 13:42
On 23 Jul, 15:30, ejmartin <ejm_60...(a)yahoo.com> wrote:
> It does however seem to be the case that the best Canon can achieve
> with their current approach (and what they're willing to spend on fab
> costs per chip) is ~4 e- for read noise, a figure that has been rather
> constant since the 20D over a range of pixel sizes from 8.2µ (1D2) to
> 5.7µ (40D) but we'll see what transpires with the impending small-
> pixel, small-sensor offerings that are supposedly imminent.
Something that strikes me about this is that Canon's fab line is now
quite old. Presumably, the new one is not simply a duplicate of the
old one, and can work at finer geometries. There's a wealth of
interesting stuff in Eric's workshop papers, and one of the things
that also seems to be a myth is that CMOS sensors are not very
demanding process-wise. That might be true if they don't want to
explore the limits of pixel size/noise, but if they do, they need fine
drawing. Maybe that's an advantage the newer MOS lines (Panasonic,
Sony, IBM/Kodak, Toshiba) have. The 52MPix sensor seems to have some
feature that look like saving space (like the 4T 4S architecture -
maybe Canon has to because of the limitations of its current fab.
From: ejmartin on 23 Jul 2008 14:14
On Jul 23, 12:42 pm, Bob Newman <bob.csx...(a)gmail.com> wrote:
> There's a wealth of
> interesting stuff in Eric's workshop papers, ...
Indeed. Here's one that proposes some theory for 1/f noise, extending
Look at section 4.
From: John Sheehy on 23 Jul 2008 16:01
rfischer(a)sonic.net (Ray Fischer) wrote in news:4886dd50$0$17165
> Don't try to feed me bullshit in order to justify your weak argument.
I haven't even told you what my weak argument is yet.
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John P Sheehy <JPS(a)no.komm>
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From: Bob Newman on 23 Jul 2008 16:09
On 23 Jul, 19:14, ejmartin <ejm_60...(a)yahoo.com> wrote:
> On Jul 23, 12:42 pm, Bob Newman <bob.csx...(a)gmail.com> wrote:
> > There's a wealth of
> > interesting stuff in Eric's workshop papers, ...
> Indeed. Here's one that proposes some theory for 1/f noise, extending
> your model:
> Look at section 4.
Yup, and look at figure 5. There is predictable behaviour there that
could be used to identify the flicker noise and process it out - real
One of the other papers is about 'backside illuminated sensors' -
presumably they need to be made of Gallium Arsenide :D