Has anyone used a photodiode with built in amplifier such as a TSL25X? If
necessary for resolution, scale the max 2v output up to Vdd?

StUart
----- Original Message -----
From: "Mike Singer" <m_singer@POLUOSTROV.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: 06 January 2004 00:00
Subject: Re: [EE]: How to measure light? Phototransistor?


John N. Power wrote:
> > By the way, "plus input" current is about 0.02 ma for LM2900. Ever
> > seen a phototransistor with the 0.02 ma output current?
>
>       This particular amplifier doesn't work that way. Each input
appears as
>       a diode to ground. A current into the positive input is
subtracted from
>       the current into the negative input by means of a current
mirror. The
>       current difference drives the output. Because of this current
sinking
>       property, this amplifier is good for sources which produce
current
>       directly, such as phototransistors...
>       The 3900 can sink current without inverting.
>
>       The maximum current into each input is 5mA max at 25C, and
>       reduces to 3.8 mA at 70C. Within this range, the inputs can
>       accomodate a perfectly reasonable amount of current.


Have a look at the LM3900 datasheet, please.
Mirror Gain is specified only for 0.02 ma and 0.2 ma input current. And
the difference of Mirror Gain for these two input currents could be as
big as 5%. For usual light intensity range 2^12=4096 one could suspect
the difference of Mirror Gain as big as 20 times. Pretty linear, yeah.

And you can't compress the signal, cause even National failed to
specify temperature drift of Mirror Gain.


> > Not to say, two transistors without negative feedback
(phototransistor
> > and "plus input" transistor) are real killers for the converter
> > linearity and even stability. Temperature signal shift will be
> > much more than working signal range.
>
>      Consider a current source like a phototransistor working into a
diode
>      load to ground. The current in the diode controls the output
voltage.
>      This  is quite linear.


Yeah, only 20 times change in amplification over the range.


>                                                             ... Try to
make a conventional
>       virtual ground input using a normal opamp and a single 5 volt
>       power supply. You might try to bias the positive input with a
divider,
>       but remember that this limits the voltage on the
phototransistor. Also,
>       the output is inverted which causes problems if the amp output
can't
>       go within a volt of ground.

I wouldn't. I'd rather simulate current mirror using xtal PIC's PWM,
precision resistor, capacitor.


> Bush is a Yale graduate, so I find that flattering.

Fattering to Bush? Does Bush deserve flattering? At his Yale
he even didn't learn how to comment code and use linker. :-)
PICList membership is better.


> Which "stile"[sic] is yours?

"Style", sorry for the typo, thanks for the hint.

Mike.

P.S. My current subscription options [OT], [EE]

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