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Taskuraha
Joined: 04 Jan 2003
Posts: 22
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 04 Jul 2008 13:46 Dividing junction capacitance of photodiode to smaller piece |
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I am trying to compare is it possible to get better results ( noise, BW...) dividing large junction capacitance of photodiode to smaller sections.
I attached an example picture, which shows the idea.
As you can see ( picture ) there has used separate TIAs for all of the smaller photodiodes. I think to reduce power dissipation there could be better ways of doing that " current summation " ( for example using basic common gate transistors for separate diodes and then one TIA...)
Does anybody have any other options/ideas how to make that photocurrent summation from invidual photodiodes?
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Old Nick
Joined: 14 Sep 2007
Posts: 360
Helped: 39
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| 04 Jul 2008 14:32 Re: Dividing junction capacitance of photodiode to smaller p |
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I can't really see what you're trying to do.
I don't think you'll get better noise performance by adding more amplifiers.
Also, is there a reset switch going from Vdd to the photodiode tho charge it up? Or do you have it in a logarithmic configuration?
A log pixel will give you a higher bandwidth! (you just have to live with the log response.)
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pixel
Joined: 16 Sep 2004
Posts: 556
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| 04 Jul 2008 14:40 Re: Dividing junction capacitance of photodiode to smaller p |
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I am not sure that this is an good idea. Of course depends on system characteristics.
Seperated capacitance is not equal to C/4, because of edge capacitance.
Note that if noise at the amplifier output is decreased by factor 4 you had nothing improoved, because signal is also decreased by factor 4.
If you decrease Cf, you could have problem with stability, but that depends on your amplifier speed. You will probably need Cf in parallel with Rf.
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FvM
Joined: 22 Jan 2008
Posts: 2219
Helped: 370
Location: Bochum, Germany
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| 04 Jul 2008 18:21 Re: Dividing junction capacitance of photodiode to smaller p |
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| It's a matter of noise matching to my opinion. If you are required to use an existing TIA, that has optimal noise performance with smaller photodiodes, paralleling amplifiers can be expected to give an inprovement. If the TIA noise parameters are known, the noise behaviour can be estimated rather precisely.
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leo_o2
Joined: 03 Sep 2004
Posts: 307
Helped: 16
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| 05 Jul 2008 14:49 Dividing junction capacitance of photodiode to smaller piece |
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| In my opinion, separated TIAs will lead to worse noise characteristic.
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Taskuraha
Joined: 04 Jan 2003
Posts: 22
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| 07 Jul 2008 11:08 Re: Dividing junction capacitance of photodiode to smaller p |
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I am trying to find some other way to join photocurrent from separated diodes ( something else than using parallel TIA:s ) Maybe like in picture which i attached or some kind of other current sum-structure ( ideas? )
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