# Photodiode High Pass Filter

#### DaveS180

Joined May 1, 2020
13
Hi
Im relatively new to electronics however I designed the circuit in the picture I was wondering if the schematic configuration is correct for a photodiode circuit with a high pass filter .
If so does the C1 & R1 have any effect on Cf & Rf in terms of cut off frequency
(C1 is connected to VCC)
Thanks

#### Attachments

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#### Audioguru again

Joined Oct 21, 2019
2,059
I think C1 and R1 do nothing since the inverting input of the opamp is a virtual ground and the input photodiode is a current source, not a voltage source,
C2 and R2 form a lowpass filter with a -3dB cutoff at 330.6kHz

#### DaveS180

Joined May 1, 2020
13
I think C1 and R1 do nothing since the inverting input of the opamp is a virtual ground and the input photodiode is a current source, not a voltage source,
C2 and R2 form a lowpass filter with a -3dB cutoff at 330.6kHz
Thanks , I thought Cf and Rf in are put in place to compensate phase shift and help prevent oscillation , I didn't know it works at lowpass filter too . I'm trying to supress noise interferences from external light sources ; sun etc . I've seen few designs online that place a high pass on the output of pin 6 would this be worth doing ?

#### Audioguru again

Joined Oct 21, 2019
2,059
C1, R1 would be a highpass filter if a signal was feeding into C1.
Cf has a lower reactance at high frequencies that reduces the opamp gain at high frequencies.
Old fashioned incandescent light bulbs produced 100Hz or 120Hz (half the electrical mains frequency) but modern compact fluorescent light bulbs have an oscillator that blinks them at around 38kHz.
A capacitor to ground at the output of an opamp would cause it to oscillate. Many opamps oscillate if they drive the tens of pF of an output cable, without using a 100 ohm resistor to isolate the output of the opamp and its negative feedback from the capacitance of the cable.

#### DaveS180

Joined May 1, 2020
13
C1, R1 would be a highpass filter if a signal was feeding into C1.
Cf has a lower reactance at high frequencies that reduces the opamp gain at high frequencies.
Old fashioned incandescent light bulbs produced 100Hz or 120Hz (half the electrical mains frequency) but modern compact fluorescent light bulbs have an oscillator that blinks them at around 38kHz.
A capacitor to ground at the output of an opamp would cause it to oscillate. Many opamps oscillate if they drive the tens of pF of an output cable, without using a 100 ohm resistor to isolate the output of the opamp and its negative feedback from the capacitance of the cable.
I've seen been playing around with circuit and from your reply I think the changes I made link with what you have said?. However I put this into practice in real-life and the photodiode is not picking up anything now .

Joined Mar 10, 2018
4,057
The MCP6021 has good phase margin with a 60 pF load. AC specs all characterized at 60 pF.

Here is a sim as seen by the current source representing the photo diode.

Regards, Dana.

#### DaveS180

Joined May 1, 2020
13
The MCP6021 has good phase margin with a 60 pF load. AC specs all characterized at 60 pF.

Here is a sim as seen by the current source representing the photo diode.

View attachment 205999

Regards, Dana.
Is that suppressing anything 120K below or have I completely misinterpreted that graph?

#### Audioguru again

Joined Oct 21, 2019
2,059
On your huge new schematic the photodiode and negative feedback are not connected to the opamp inverting input pin 2 with a dot.
Also, the opamp is not DC biased properly, its + input pin 3 must be biased at half the supply voltage.

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#### DaveS180

Joined May 1, 2020
13
On your huge new schematic the photodiode and negative feedback are not connected to the opamp inverting input pin 2 with a dot.
Apologize on the size it uploaded like it , it didn't show on the picture but there is a junction present there .

Joined Mar 10, 2018
4,057
Is that suppressing anything 120K below or have I completely misinterpreted that graph?
Basically yes, although G flat in that region at 60 db down.

Regards, Dana.

#### MrAl

Joined Jun 17, 2014
7,849
Hi
Im relatively new to electronics however I designed the circuit in the picture I was wondering if the schematic configuration is correct for a photodiode circuit with a high pass filter .
If so does the C1 & R1 have any effect on Cf & Rf in terms of cut off frequency
(C1 is connected to VCC)
Thanks
An easy way to accomplish that is to make a multi stage passive RC high pass filter then amplify the output. You could get good results with four or more stages. The ambient light can be relatively high amplitude so you should really have multiple stages.

#### sparky 1

Joined Nov 3, 2018
383
help mr. wizard
I use an online filter program called Filter Wizard: https://tools.analog.com/en/filterwizard/
looks like you want artifacts of an led that are above 50 kHz.

I searched" looking for noise " got this

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#### DaveS180

Joined May 1, 2020
13
An easy way to accomplish that is to make a multi stage passive RC high pass filter then amplify the output. You could get good results with four or more stages. The ambient light can be relatively high amplitude so you should really have multiple stages.
Thanks for the help , @Sparky1 provided a link to a filter wizard which included the multi-stage filtering you spoke about. Unfortunately I currently only have 1 additional MCP6021 at the moment, so I altered the frequencies slightly to involve one additional stage which I then incorporated into the new the schematic

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#### DaveS180

Joined May 1, 2020
13
help mr. wizard
I use an online filter program called Filter Wizard: https://tools.analog.com/en/filterwizard/
looks like you want artifacts of an led that are above 50 kHz.

I searched" looking for noise " got this
Thanks for the filter wizard link , it also lead to me a photodiode wizard which look rather useful. Yeah I'm using an old DMX lighting fixture to see what sort of distance I can get the photodiode to receiver outdoors.

Ill keeping searching for noise to a minimum then haha !

#### sparky 1

Joined Nov 3, 2018
383
The wizard has tabs, select components next select I want to choose my own components. Continue find the tab has the schematic that was completely designed. Use the first stage if you want. You can mouse around on the chart. Download the finished circuit. I believe wizard connects you to a system cloud and that the at home software for this type design mode has become time consuming when you don't do this all the time.

#### DaveS180

Joined May 1, 2020
13
The wizard has tabs, select components next select I want to choose my own components. Continue find the tab has the schematic that was completely designed. Use the first stage if you want. You can mouse around on the chart. Download the finished circuit. I believe wizard connects you to a system cloud and that the at home software for this type design mode has become time consuming when you don't do this all the time.
Yeah thanks again for that! The download PDF that comes with it is very handy. I implemented the design earlier with the additional stage included , and I saw a huge reduction in detection by the photodiode from the LED im using. I thought I would have been more likely to have seen in increase in gain from the photodiode ? Any reasons why

#### sparky 1

Joined Nov 3, 2018
383
Let me look at "that old lime light DMX light fixture " it likes to converse with other light fixtures with 0's and 1's

looking further at the elaborate controllers and I see that I am not well enough informed
however there probably is a manual somewhere.

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#### MrAl

Joined Jun 17, 2014
7,849
Thanks for the help , @Sparky1 provided a link to a filter wizard which included the multi-stage filtering you spoke about. Unfortunately I currently only have 1 additional MCP6021 at the moment, so I altered the frequencies slightly to involve one additional stage which I then incorporated into the new the schematic
Well actually what i was talking about really only requires ONE op amp.
The rest of the circuit is just a string of RC high pass filters connected together. There are no op amps between them, just one op amp at the end of the string.

Code:
o---C---+---C----+---C---+---OPAMP---o
|        |       |
R        R       R
|        |       |
o-------+--------+-------+-----------o