amplifying a photodiode signal

[Jamie Morken]

Hi,

I have a photodiode with the anode connected to a 100K pullup to 12V,
cathode to ground, and an RC highpass filter (47pF, 2.2Mohm) driven from
the photodiodes anode, what is the best way to amplify the signal to 1V
peak to peak from the highpass filter assuming a frequency of up to
500MHz at 0.1mV amplitude? I am thinking of an MMIC RF amplifier, or a
high speed comparator but I am not sure what would be best. I would
like to feed this signal into a high speed serial to parallel converter
to interface it to 5Volt logic.

cheers,
Jamie

 

[John Popelish]

Hi,

I have a photodiode with the anode connected to a 100K pullup to 12V,
cathode to ground, and an RC highpass filter (47pF, 2.2Mohm) driven from
the photodiodes anode, what is the best way to amplify the signal to 1V
peak to peak from the highpass filter assuming a frequency of up to
500MHz at 0.1mV amplitude? I am thinking of an MMIC RF amplifier, or a
high speed comparator but I am not sure what would be best. I would
like to feed this signal into a high speed serial to parallel converter
to interface it to 5Volt logic.

cheers,
Jamie

How much capacitance does the photo diode have?
What is the impedance of that capacitance at 500 MHz?

I doubt you have any chance of getting a usable signal out of a 100k
pull up resistor at 500 MHz. You need a much lower load impedance
across the photo diode. I think you should look into using a
transconductance amplifier or common base amplifier at the front end
and incorporate the high pass filter into that.

 

[Jamie Morken]

Hi,

How much capacitance does the photo diode have?
What is the impedance of that capacitance at 500 MHz?

I am not sure about the capacitance, it isn't in the 2page datasheet I have.

It is used in the Ronja optical datalink project:
http://ronja.twibright.com/main.php


400MHz 820nm photodiode
digikey part#: 425-1029-5-ND manufacturer part#: PD101SC0SS

cheers,
Jamie

 

[John Woodgate]

I read in sci.electronics.design that Jamie Morken <[email protected]>

Hi,

I have a photodiode with the anode connected to a 100K pullup to 12V,
cathode to ground, and an RC highpass filter (47pF, 2.2Mohm) driven from
the photodiodes anode, what is the best way to amplify the signal to 1V
peak to peak from the highpass filter assuming a frequency of up to
500MHz at 0.1mV amplitude? I am thinking of an MMIC RF amplifier, or a
high speed comparator but I am not sure what would be best. I would
like to feed this signal into a high speed serial to parallel converter
to interface it to 5Volt logic.

500MHz and 2.2 Mohm are incompatible. You will not get the filtering you
expect. The stray capacitance across your 100 kohm also attenuates such
fast signals severely.

 

[John Popelish]

Hi,



I am not sure about the capacitance, it isn't in the 2page datasheet I have.

It is used in the Ronja optical datalink project:
http://ronja.twibright.com/main.php

400MHz 820nm photodiode
digikey part#: 425-1029-5-ND manufacturer part#: PD101SC0SS

The data sheet (from Digikey):
http://rocky.digikey.com/WebLib/Sharp/Web Data/PD101SC0SS.pdf

Says that with a 5 volt reverse bias, the capacitance is 3.5 pf.

At 500 MHz that has an impedance of about 90 ohms. That is in
parallel with your 100k pull up resistor.

The RC 3db down corner for 100k and 3.5 pf is 450 kHz. Then you have
to parallel all the other stray capacitance of what else is connected
to that node. Probably a few more pf, at least.

 

[CBarn24050]

Hi, you won't get anything like 500MHz from such a simple circuit. There is
good site somewhere that tells you how to do it, I can't remember where but it
didn't take me long to find using the search engines. Your other option is to
read the diodes application notes, usually the best place to start anyway.

 

[Rene Tschaggelar]

Jamie,
you need to make the diode fast by a moderate reverse voltage
of say 12V or so. What photodiode do you have and what is its
rise time ? Then how much light do you have ? And how
much jitter can you tolerate ?

Rene

 

[Rob Gaddi]

Go get yourself a copy of Jerald Graeme's book "Photodiode Amplifiers:
Op Amp Solutions" It's got lots of pretty equations that you can use
for the first stage transimpedance amp. All of the posts regarding the
fact that you won't be able to get that kind of signal swing from a
passive I/V converter are dead on the money.

That said, how much current are you expecting to be able to get from
your photodiode? Your specs as provided indicate that you're expecting
about 1 nA signal amplitude. In my experience, if you're trying to gain
up that small of a photodiode signal across that wide of a bandwidth
you're going to be lucky to get 3 bits of data above the noise floor.

 

[Jamie Morken]

Hi,

I found a good site that goes through this topic very well:
http://www.elecdesign.com/Articles/Index.cfm?ArticleID=4346&pg=1

cheers,
Jamie

 

[Jamie Morken]

Hi,

Jamie,
you need to make the diode fast by a moderate reverse voltage
of say 12V or so. What photodiode do you have and what is its
rise time ? Then how much light do you have ? And how
much jitter can you tolerate ?

I am thinking of using a circuit like this:
http://www.elecdesign.com/Files/29/4346/Figure_03.gif

I am looking for a photodiode with a high A/watt light sensitivity
rating and also a low capacitance - and a max price of $10!
This is one possible photodiode:
digikey part#: 425-1029-5-ND manufacturer part#: PD101SC0SS

This is the current to voltage circuit I am looking at:
http://www.elecdesign.com/Files/29/4346/Figure_03.gif

I don't fully think this is the best circuit to use as I would like to
get rid of any DC current coming out of the photodiode from ambient
light. I would like a circuit that can detect a very small, high
frequency, lightsource from the photodiode. Would it be possible to AC
couple the photodiode right to the input of an RF low noise amplifier
and get something useful out of it?

cheers,
Jamie

 

[Rene Tschaggelar]

Hi,




I am thinking of using a circuit like this:
http://www.elecdesign.com/Files/29/4346/Figure_03.gif

I am looking for a photodiode with a high A/watt light sensitivity
rating and also a low capacitance - and a max price of $10!
This is one possible photodiode:
digikey part#: 425-1029-5-ND manufacturer part#: PD101SC0SS

I tend to doubt you're going to reach 500MHz with a 10$ diode.
After having had a look at the datasheet, it is specified for 400MHz.

This is the current to voltage circuit I am looking at:
http://www.elecdesign.com/Files/29/4346/Figure_03.gif

I don't fully think this is the best circuit to use as I would like to
get rid of any DC current coming out of the photodiode from ambient
light. I would like a circuit that can detect a very small, high
frequency, lightsource from the photodiode. Would it be possible to AC
couple the photodiode right to the input of an RF low noise amplifier
and get something useful out of it?

Sort of, provided you have sufficient light. The source
impedance for an RF amp is not to be much more than say 100 Ohms.

Rene

 

[[email protected]]

Yes, you can AC couple the photodiode and use an RF amplifier.
Typically, you'd use a 50Ohm load. However, photodiode will have to be
quite small to have sufficiently small capacitance for 500MHz
operation, AND, you'll have to begin by understanding that the CATHODE
is positive for reverse bias!

Paul Mathews