photodiode question

[R.Lewis]

Why is it common to use photodiodes in a circuit configuration that measures
the photocurrent at (near) zero Vf.
What advantage has this over a photovoltaic measurement arrangement.

 

[John Popelish]

"R.Lewis" wrote:

Why is it common to use photodiodes in a circuit configuration that measures
the photocurrent at (near) zero Vf.
What advantage has this over a photovoltaic measurement arrangement.

Improved linearity and speed. As the diode junction becomes forward
biased by the photo current (in photo voltaic mode) the output voltage
is proportional to the log of intensity. The forward biased junction
capacitance is also higher than the shorted junction capacitance, and
the capacitance doesn't change voltage much, so it has less effect on
the signal. If you reverse bias the diode the linearity and speed
improve even more, but the noise increases, also.

--
John Popelish

 

[Terry Given]

"John Popelish" <jpopelish@rica.net> wrote in message
news:40578A6E.81D8CA45@rica.net...

"R.Lewis" wrote:

Why is it common to use photodiodes in a circuit configuration that
measures
the photocurrent at (near) zero Vf.
What advantage has this over a photovoltaic measurement arrangement.

Improved linearity and speed. As the diode junction becomes forward
biased by the photo current (in photo voltaic mode) the output voltage
is proportional to the log of intensity. The forward biased junction
capacitance is also higher than the shorted junction capacitance, and
the capacitance doesn't change voltage much, so it has less effect on
the signal. If you reverse bias the diode the linearity and speed
improve even more, but the noise increases, also.

--
John Popelish

what he said.

Photodiode amplifiers, Jerold Graeme, McGraw-Hill, ISBN 0-07-024247-X

its a great reference, and contains more information than you can shake a
stick at.

 

[Rene Tschaggelar]

John Popelish wrote:

"R.Lewis" wrote:

Why is it common to use photodiodes in a circuit configuration that measures
the photocurrent at (near) zero Vf.
What advantage has this over a photovoltaic measurement arrangement.


Improved linearity and speed. As the diode junction becomes forward
biased by the photo current (in photo voltaic mode) the output voltage
is proportional to the log of intensity. The forward biased junction
capacitance is also higher than the shorted junction capacitance, and
the capacitance doesn't change voltage much, so it has less effect on
the signal. If you reverse bias the diode the linearity and speed
improve even more, but the noise increases, also.

Almost.
When the voltage over the diode is held constant at zero, the
capacitance doesn't matter. At least within the bandwidth of
the OpAmp. This is the slow setup.
The reverse bias is used for high speeds when the diode needs
an impedance as it may be connected through a 50 Ohm coaxial.
The junction capacitance becomes smaller with increasing voltage.

I couldn't put a boundary but guess it in the lower MHz or
even sub MHz.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

 

[Phil Hobbs]

John Popelish wrote:

"R.Lewis" wrote:

Why is it common to use photodiodes in a circuit configuration that measures
the photocurrent at (near) zero Vf.
What advantage has this over a photovoltaic measurement arrangement.



Improved linearity and speed. As the diode junction becomes forward
biased by the photo current (in photo voltaic mode) the output voltage
is proportional to the log of intensity. The forward biased junction
capacitance is also higher than the shorted junction capacitance, and
the capacitance doesn't change voltage much, so it has less effect on
the signal. If you reverse bias the diode the linearity and speed
improve even more, but the noise increases, also.

Low noise in zero bias mode is a myth.

When you apply a moderate reverse bias, the shot noise of the dark current
increases, but that's almost never the limiting noise source when you're
using ordinary photodiodes. Typical dark currents are in the tens to
hundreds of picoamps at room temperature, leading to dark current shot noise
densities of few femtoamps per root hertz. Your feedback resistor would have
to be > 5G ohms for this noise source to dominate.

A much more important noise source is the multiplied voltage noise of the op
amp, which generally dominates when f >>1/RfCd. Reducing the capacitance on
the summing junction (Cd+Cin) reduces this directly. Applying a reverse bias
can reduce this noise by up to 7x in voltage (17 dB), which is a huge
improvement.

Cheers,

Phil Hobbs

 

[Guy Macon]

Phil Hobbs <pcdhSpamMeSenseless@us.ibm.com> says...

Low noise in zero bias mode is a myth.

When you apply a moderate reverse bias, the shot noise of the dark current
increases, but that's almost never the limiting noise source when you're
using ordinary photodiodes. Typical dark currents are in the tens to
hundreds of picoamps at room temperature, leading to dark current shot noise
densities of few femtoamps per root hertz. Your feedback resistor would have
to be > 5G ohms for this noise source to dominate.

A much more important noise source is the multiplied voltage noise of the op
amp, which generally dominates when f >>1/RfCd. Reducing the capacitance on
the summing junction (Cd+Cin) reduces this directly. Applying a reverse bias
can reduce this noise by up to 7x in voltage (17 dB), which is a huge
improvement.

Not everybody is using "ordinary photodiodes." Some of us are using
large area photodiodes or even PSDs in low-speed applications.

From my Resume:

"The basic concept of NOx measurement is to mix the exhaust gas with
ozone. This causes a very faint glow as the NO reacts with the O3. You
then subject the gas to intense UV radiation, which converts any NO2
to NO, causing a faint but measurable glow. The glow is very dim
indeed - it takes days to get an image of it on film. I knew that our
only hope of measuring NOx within our cost constraints was to use a
photodiode instead of a photomultiplier tube. I calculated that a
photodiode with 250 square millimeters of light gathering surface
should be able to meet the specification. Normally, a photodiode can't
come close to the performance of a photomultiplier tube, but the glow
in question was largely outside of the spectral range of the
photomultiplier tube."

"I obtained some sample photodiodes, a good nanoammeter, and a supply
of gasses that had a mix of air and NOx at a level exactly ten times
higher than the legal limit after our dilution. I set up an experiment
with the reaction at the surface of the photodiode and measured
roughly one hundred picoamps with the gas turned on, and about ten
picoamps of noise with the gas off. Because I knew that the noise is
greatly influenced by temperature, I knew that we could meet the
specification by adding a thermionic cooler to reduce the noise. The
only question was whether I could design a system that met the
specification without the cooler."

"At this point, I decided to conduct a sanity check of my results using
alternative methods. I removed the photodiode from the chamber and set
it facing a seven-watt light bulb one meter away in a darkened room. I
then used a Variac to vary the brightness of the bulb. Everything
worked fine with a standard digital multimeter measuring the current
in the milliamp to microamp range, but the nanoammeter could not
measure currents greater than two nanoamps, and I couldn't get the
room dark enough to get the current that low. In order to get down to
the one hundred picoamp range, I had to cover the photodiode with two
layers of black electrical tape and move the bulb to three meters
away. This told me that my calculations were close and that I was
working with light levels that were roughly as low as expected."

"Now it was time to optimize the design. I used a coaxial feed tube to
reduce turbulence, designed a gold plated hemispherical reflector to
double the light level at the photodiode, found the lowest noise opamp
available, added a switched capacitor low pass bessel filter to reduce
the noise, and installed an optical filter to eliminate a small error
that I traced to an interaction between the ozone and carbon monoxide
in the exhaust stream. This gave me sufficient sensitivity to meet the
specification without a cooler, but the variation with temperature was
still too high, so I duplicated the entire photodiode and amplifier in
a dark chamber and fed the outputs of both chambers into a
differential amplifier. At this point, all of the NOx specifications
were met."



--
Guy Macon, Electronics Engineer & Project Manager for hire.
Remember Doc Brown from the _Back to the Future_ movies? Do you
have an "impossible" engineering project that only someone like
Doc Brown can solve? My resume is at http://www.guymacon.com/

 

[Al]

In article <-sCdnZhTarmW5sXdRVn-jg@speakeasy.net>,
Guy Macon <http://www.guymacon.com> wrote:

Phil Hobbs <pcdhSpamMeSenseless@us.ibm.com> says...

indeed - it takes days to get an image of it on film. I knew that our
only hope of measuring NOx within our cost constraints was to use a
photodiode instead of a photomultiplier tube.

Don't know if this will help, but a low cost photomultiplier tube is
available at www.surplushed.com. It's a 1P28. Cost is $10.00. Item 1435.
Shipping is $5.00.

Specs at: http://www.electricstuff.co.uk/1p28.html

Al

--
There's never enough time to do it right the first time.......

 

[Phil Hobbs]

Guy Macon wrote:

Phil Hobbs <pcdhSpamMeSenseless@us.ibm.com> says...


Low noise in zero bias mode is a myth.

When you apply a moderate reverse bias, the shot noise of the dark current
increases, but that's almost never the limiting noise source when you're
using ordinary photodiodes. Typical dark currents are in the tens to
hundreds of picoamps at room temperature, leading to dark current shot noise
densities of few femtoamps per root hertz. Your feedback resistor would have
to be > 5G ohms for this noise source to dominate.

A much more important noise source is the multiplied voltage noise of the op
amp, which generally dominates when f >>1/RfCd. Reducing the capacitance on
the summing junction (Cd+Cin) reduces this directly. Applying a reverse bias
can reduce this noise by up to 7x in voltage (17 dB), which is a huge
improvement.


Not everybody is using "ordinary photodiodes." Some of us are using
large area photodiodes or even PSDs in low-speed applications.

From my Resume:

"The basic concept of NOx measurement is to mix the exhaust gas with
ozone.
(snip)

Wow, that's one wordy resume.

Position sensing diodes are even noisier, assuming you mean lateral
effect devices and not split detectors--the resistance of the epi layer
appears in shunt with the summing junction, which means that it sources
its full Johnson noise into the summing junction. Ugly. We had a
discussion about this a couple of weeks ago.

The post I was replying to claimed that running the PD at zero bias
makes it quieter, which it doesn't. Zero bias *does* get you near-zero
dark current, which is what your nitrogen oxides measurement required,
but that's not the same thing at all. If the PD has horrible 1/f noise,
e.g. from poor passivation, zero bias can help that too. Otherwise it's
a snare and a delusion.

I agree that it was the right solution for your measurement, but you
aren't the OP, after all.

Cheers,

Phil Hobbs

 

[John Popelish]

Phil Hobbs wrote:

Low noise in zero bias mode is a myth.

The shot noise of the dark current increases, but that's almost never the
limiting noise source when you're using ordinary photodiodes.
(snip)

Thanks for the info. It would also be very helpful to know what is an
'ordinary' photo diode and what is not.

For instance, I have recently been working with InGaAs photo diodes,
that were much quieter than when operated with as little as a volt of
reverse bias.

--
John Popelish

 

[Winfield Hill]

John Popelish wrote...

Phil Hobbs wrote:

Low noise in zero bias mode is a myth.

The shot noise of the dark current increases, but that's almost
never the limiting noise source when you're using ordinary
photodiodes. (snip)

Thanks for the info. It would also be very helpful to know what
is an 'ordinary' photo diode and what is not.

For instance, I have recently been working with InGaAs photo diodes,
that were much quieter than when operated with as little as a volt
of reverse bias.

Oops, very VERY leaky devices, right? They need zero bias voltages.
WHOOP! WHOOP! TROLL ALERT. TROLL ALERT!! WHOOP! WHOOP!

Consider, a UDT InGaAs-1000 photodiode (0.8mm^2 area) has 2mA max of
leakage current at -2V, compared to a silicon PIN-040A with the same
area: 0.5nA max leakage at -10V. That's pretty clear! Come on now,
the InGaAs parts are near IR detectors (900nm to 1700nm), whereas the
Silicon detectors are used from 500nm to 1100nm, quite another scene.
OF COURSE the InGaAs photodiode with its 30M of shunt resistance needs
to be operated at 0V. We could expound on the issue of low-energy IR
photons, band gap voltages and kT, etc., but what's not helpful about
the basic info in the InGaAs photodiode's datasheet? Sorry, John,
but I think you knew all that, hence my TROLL ALERT alarm. :&gt

Thanks,
- Win

whill_at_picovolt-dot-com

 

[John Popelish]

Winfield Hill wrote:

John Popelish wrote...

Phil Hobbs wrote:

Low noise in zero bias mode is a myth.

The shot noise of the dark current increases, but that's almost
never the limiting noise source when you're using ordinary
photodiodes. (snip)

Thanks for the info. It would also be very helpful to know what
is an 'ordinary' photo diode and what is not.

For instance, I have recently been working with InGaAs photo diodes,
that were much quieter than when operated with as little as a volt
of reverse bias.

Oops, very VERY leaky devices, right? They need zero bias voltages.
WHOOP! WHOOP! TROLL ALERT. TROLL ALERT!! WHOOP! WHOOP!

Consider, a UDT InGaAs-1000 photodiode (0.8mm^2 area) has 2mA max of
leakage current at -2V, compared to a silicon PIN-040A with the same
area: 0.5nA max leakage at -10V. That's pretty clear! Come on now,
the InGaAs parts are near IR detectors (900nm to 1700nm), whereas the
Silicon detectors are used from 500nm to 1100nm, quite another scene.
OF COURSE the InGaAs photodiode with its 30M of shunt resistance needs
to be operated at 0V. We could expound on the issue of low-energy IR
photons, band gap voltages and kT, etc., but what's not helpful about
the basic info in the InGaAs photodiode's datasheet? Sorry, John,
but I think you knew all that, hence my TROLL ALERT alarm. :&gt

Sorry to get you so riled up. I was just trying to sort cases through
the generalizations. I have been working with some 2 mm and 3 mm
diameter InGaAs detectors that have leakage currents much less than a
microamp at 1 volt or two reverse bias. These are optimized for 1.7
um wavelength. But the 1.9 um units were way leakier. And the 2.2 um
units have so much leakage that they can hardly be called diodes,
unless they are cooled. So, cases vary. I just think that the
generalization that reverse bias lowers noise for photo diodes (in the
amplifier) circuit needs some limitations. Obviously, Phil has some
general sort of diodes in mind with his 'ordinary' descriptor, but
exactly what he has in mind may not be obvious to all readers. So I
invited him to expound a bit.

I would have sworn that some 100 mm^2 silicon detectors also ran quite
a bit quieter at zero volt bias than when reverse biased (and they got
really noisy as their breakdown voltage was approached. But their
bandwidth also increased quite a bit. But if I didn't need the
bandwidth, zero bias was quietest, overall (if I am not remembering
incorrectly).

--
John Popelish