Re: Photodiode TIA

On 23 Nov 2006 07:02:11 -0800, wellies@xxxxxxxxx wrote:



On Nov 16, 4:43 pm, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On 16 Nov 2006 01:50:42 -0800, well...@xxxxxxxxx wrote:







John Larkin wrote:

On 15 Nov 2006 12:49:20 -0800, bill.slo...@xxxxxxxx wrote:

well...@xxxxxxxxx wrote:
John Larkin wrote:

On 15 Nov 2006 08:57:13 -0800, well...@xxxxxxxxx wrote:

I realise there is quite a bit of advice in this group on TIA's but i
thought i'd save myself some time and get an off the shelf TIA,
designed to work with PIN photodiodes in high speed applications.
However i'm now confused. I'm only working with 160Mbps, but everything
either stops at 155 or goes up to 622 due to various fibre optic
standards. I'm looking at using the max3665 which runs up to 622Mbps,
reading the data***, the device has a bandwidth of 470MHz. Reading on
it then states:

"In general, the bandwidth of a fiber optic preamplifier
should be 0.6 to 1 times the data rate. Therefore, in a
622Mbps system, the bandwidth should be between
375MHz and 622MHz. Lower bandwidth causes pattern-
dependent jitter and a lower signal-to-noise ratio,
while higher bandwidth increases thermal noise."

Does this mean that the thermal noise would be large in my system if i
used it for 160Mbps? Or is this not really an issue? I was going to use
the Optek OPV480 and reverse bias to around 20V and feed it straight
into this chip, or it is easier just to design my own transimpedance
amp around a single opamp and feedback resistor and capacitor?

Thanks

Andrew

A lot depends on how much light you have. If it's microwatts, the
noise budget will be tight. If's it's in the milliwatt range, you'll
probably overload the Maxim and might be better off making your own
TIA. I've had good results using a current-mode opamp as a TIA when
there's lots of light.

What's the capacitance of the OPV480? That's a serious issue at high
speeds.

John

Bill - thanks for the info, i'll check out that data***.

John - the VCSEL i'm using is
http://www.ulm-photonics.de/docs/products_new/datasheets/VCSEL-ULM850...

I've not used it before or anything but the spec looks good. The
optical output is 1mW @ 5mA, but then the photodiode will be 1cm away.

Its actually a OPF380 Optek part (sorry, typo i should have spotted)
and the capacitance is max 2pF at 5Vr so i was hoping it would drop
right off when reverse biasing at 20 odd volts?

You don't usually seem to get much reduction in capacitance in going
from 5V to 20V of reverse bias. Since you are starting off with only
2pF, the capacitance you need to plug into your equations is likely to
be dominated by stray capacitances anyway - the OPA656 has about 2.8pF
of common mode input capacitance.

The capacitance will drop somewhat from 5 to 20 volts (check the
data***, probably less than 2:1) but in fact higher voltages make
the critters faster independent of the capacitance dropoff... carriers
are swept out of the I region faster, I guess. 50 ohms and 2 pF is
only a 100 ps tau, so capacitance isn't the speed limit here.

You could do a quick DC experiment to see how much photodiode current
you're going to get, and then plan the amp. The Maxim TIA will
probably work OK. I'm getting about 200 MHz bandwidth here,

http://www.highlandtechnology.com/DSS/J720-730DS.html

using an Optek silicon PIN diode driving an AD8014 as a tia, with only
6 volts on the photodiode. I guess I should have built a little boost
converter to get some more bias voltage. Oh well, next time.

John

Sorry for sounding ignorant but what quick DC experiment would i do? Is
this just turning the VCSEL on at the required working distance and
seeing how much current i get? Can i just add a load resistor and check
the voltage across it?Yup. Run the nominal DC current into the laser, set up the mechanics
like the real thing, bias the PIN with a 9-volt battery and measure
the pin current with a microammeter, or measure the voltage drop
across a 10K or some such series resistor. Expect maybe a milliwatt
from a cheap vcsel laser, and roughly 0.5 amps/watt (0.5 ma/mw) from
the photodiode.

Now the issue becomes, how much of the laser's light is landing on the
active area of the photodiode? If it were fiber-coupled, it would be
close to 100%. In free air with no optics, it might be 1% or it might
be 0.01%.

John- Hide quoted text -- Show quoted text -

OK so i'm back for more!

Experimentation shows i'm getting about 7uA at my 1cm free air with the
stated 1mW output of the VCSEL. I ended up getting in the AD8011 as it
was easy to get hold of and i'm trying to plan my TIA around it.

From a quick calc if i want a reasonable signal out (read at least 4v)
i'm going to need an Rf of standard resistance 620K. Now reading the
suggested reading material on the OPA656 data*** it goes into a
calculation for Cf, but this requries the GBP which i can't decipher
from the AD8011 data***. It suggests that for a gain of 10 the
bandwidth is 57Mhz - does this mean the GBP is 570Mhz? Or am i missing
something, i mainly ask because the GBP at G=+1 and G=+2 don't tally up
with this at all.

Thanks

Andrew

Hi, Andrew,

To work at 160 MHz, with the 7 uA pd current, you're going to need
multiple gain stages. The stray capacitance across a 600K resistor
would squash the gain, and the opamp doesn't have enough gbw anyhow.

An official tia, like one of the Maxims, would be a good first stage.
They can achieve effective transresistance in the kilohms and still
keep the bendwidth up. Or a current-mode opamp like the AD8011 with a
feedback resistor in the < 1K range, with maybe a pF of shunt
capacitance if you need it for stability. That will give you under 7
mV of signal, so you'd need later gain stages to get up to the volts
level. This much gain-bandwidth desperately wants to be an oscillator,
so the input node needs to be well shielded from the output.

Improving the optics, with a lens somewhere, could increase your pd
signal by a huge factor and simplify the situation. 7 uA ain't a lot
of signal at these speeds.

John


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