Actually the speed limit of normal application is more on
the trans-impedance amplifier and Photo-detector. We never
need to worry about the LED speed (pulsing at 4us or less,
even less than 1us at high current >100mA and low duty ratio).
Photo-transistor is pretty slow but photo-diode is much
faster (typical rise and fall time 20ns). It will be
quite hard to develop a low cost >1MHz **TRANSIMPEDANCE=20
BANDWIDTH** trans-impedance amplifier though. Just take=20
note that TRANSIMPEDANCE BANDWIDTH << GAIN BANDWIDTH.

Regards,
Xiaofan

----------------------------------------------
Xiaofan Chen
R&D Engineer, Photoelectric Sensor Development
Pepperl+Fuchs Singapore
http://www.pepperl-fuchs.com
Signals for the world of automation
--------------------------------------------

>From TI/Burr-Brown OPA380 datasheet (expensive part!):

"FEATURES
 1MHz TRANSIMPEDANCE BANDWIDTH
 EXCELLENT LONG-TERM VOS STABILITY
 BIAS CURRENT: 50pA (max)
 OFFSET VOLTAGE: 25=B5V (max)
 INPUT CURRENT RANGE: 10nA to 1mA
 DRIFT: 0.1=B5V/=B0C (max)
 GAIN BANDWIDTH: 90MHz
 QUIESCENT CURRENT: 6.5mA
 SUPPLY RANGE: 2.7V to 5.5V
 SINGLE AND DUAL VERSIONS
 MicroSize PACKAGE: MSOP-8

The OPA380 family of transimpedance amplifiers provides
high-speed (90MHz Gain Bandwidth [GBW]) operation, with
extremely high precision, excellent long-term stability, and
very low 1/f noise. It is ideally suited for high-speed
photodiode applications.

The signal bandwidth of a transimpedance amplifier depends
largely on the GBW of the amplifier and the parasitic
capacitance of the photodiode, as well as the feedback
resistor. The 90MHz GBW of the OPA380 enables a transimpedance
bandwidth of > 1MHz in most configurations. The
OPA380 is ideally suited for fast control loops for power level
on an optical fiber.

As a result of the high precision and low-noise characteristics
of the OPA380, a dynamic range of 5 decades can be
achieved. This capability allows the measurement of signal
currents in the order of 10nA, and up to 1mA in a single I/V
conversion stage. In contrast to logarithmic amplifiers, the
OPA380 provides very wide bandwidth throughout the full
dynamic range. By using an external pulldown resistor to
-5V, the output voltage range can be extended to include 0V.
The OPA380 (single) is available in MSOP-8 and SO-8
packages. The OPA2380 (dual) is available in the
miniature MSOP-8 package. They are specified from
-40=B0C to +125=B0C."

-----Original Message-----
From: Russell McMahon
Sent: Friday, September 16, 2005 1:57 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] LED Speed Test


>I did a quick round of tests to see how fast LEDs are.
...
> http://bobblick.com/led_speed_test/

Very interesting.
What could be useful if you are doing any more of these if to
normalise the maximum output amplitude and denormalise the timescale.
eg set change in amplitude to between 3 and 4 vertical units and then
alter the timebase so the output decay more or less fills the screen.
Triggering off the falling edge of the input signal and placing the
trigger point at the left of the screen would give an excellent idea
of performance.

At present the red LED looks like it may have a fall time of around 20
ns! Expanding the scale would be most informative. (Of course, I could
always do it myself :-).)

So, red LED high speed coms it is then :-).
An el cheapo LASER diode response would be interesting.

Red LED rise time is substantially slower than fall time (as are
others). I wonder how much the detector response affects the lower
measurement limits. It doesn't seem to do too badly for fall time at
least.

        Russell McMahon

--=20
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