Noise in the amplifier

Thread Starter

pooya_b

Joined Feb 25, 2021
50
Hi
I designed a photodiode amplifier, which I put below the schematic and photo board.
Capture.JPG
IMG_2804.JPG
I used IC LT1931A to make its negative power supply. I put this part in the datasheet attachment.
My problem is with the noise that is seen on the output. This noise starts from the base of the photodiode, which you can see this noise below.
1.jpg
After amplifying this noise, it amplifies and becomes the following shape.
2.jpg
How can I remove this noise?

pooya
 

Attachments

Ian0

Joined Aug 7, 2020
2,192
Why so little gain at each stage? Op amps are generally more stable at higher gains. What value are your capacitors really? You have them measured in Coulombs!
With three stages, you will have enough phase shift for any coupling between output and input to make an oscillator.
 

andrewmm

Joined Feb 25, 2011
1,466
What noise are you getting from the LT1931A ?
Thats not the quietest DCDC on the block ,

Can you share the picture of the dcdc switcher ?
 

ericgibbs

Joined Jan 29, 2010
12,902
hi pooya,
How are you light stimulating the photodiode.??

I would say your 1st stage OPA has a problem, it is causing some inversion of the negative swing of the signal, ref this image.
Image shows the two waveforms super imposed for comparison.

On the PCB, the -3.3v supplies the power to the OPA's and the PD negative bias.!!
The long cross board wire link is the -3.3V source, it is a very crude way to bias the PD, it will introduce noise into the amplifiers.

E
ESP_ 33a1.png
 

Attachments

ericgibbs

Joined Jan 29, 2010
12,902
hi andrew,
Is that query for me or the TS.??:)

E

Aside:
I have observed that by not using a simple addressee header, we can get confused about whom is being asked the question.E
 

Thread Starter

pooya_b

Joined Feb 25, 2021
50
Why so little gain at each stage? Op amps are generally more stable at higher gains. What value are your capacitors really? You have them measured in Coulombs!
With three stages, you will have enough phase shift for any coupling between output and input to make an oscillator.
I did not understand what you meant. Can you explain more?

I designed this circuit to be able to receive very low light intensities, but my photodiode does not work exactly according to the data sheet and passes more current, so I had to reduce the gain of each step.
What do you think I should do better?
 

Thread Starter

pooya_b

Joined Feb 25, 2021
50
hi pooya,
How are you light stimulating the photodiode.??

I would say your 1st stage OPA has a problem, it is causing some inversion of the negative swing of the signal, ref this image.
Image shows the two waveforms super imposed for comparison.

On the PCB, the -3.3v supplies the power to the OPA's and the PD negative bias.!!
The long cross board wire link is the -3.3V source, it is a very crude way to bias the PD, it will introduce noise into the amplifiers.

E
View attachment 239366
Hi ericgibbs
I made this circuit to receive pulses of about 100 nanoseconds. And with the TORLABS photodiode I compared, it reports about 20 nanoseconds more bandwidth.
I have already asked for the first part of OPA inside another site and it says that this will create less noise.
I was able to receive the wave correctly by inverting it 3 times.

What do you think about preventing noise from being transmitted from the source to the photodiode?
 

DickCappels

Joined Aug 21, 2008
7,426
There are some application in which it is just too much trouble to use a switching power supply.

I would suggest at least temporarily moving the LT1931A to a separate assembly separated by about 30 cm of wire, then experiment with various types of filtering to see what it takes to remove the noise from the output. Once that is accomplished, try holding the DC to DC supply close to the preamp and see whether you are going to need shielding.

Given that your input stage is a transimpedance amplifier it will be particularly sensitive to capacitively coupled noise.

Edit: How much bandwidth do you need with that amplifier? Would filtering the 2.2 MHz solve the problem?
 
Last edited:

ericgibbs

Joined Jan 29, 2010
12,902
hi pooya,
I will bring your attention again to the layout and routing of the -3.3V supply line.

The -3.3v PD bias line is being used to bias the PD and power the following OPA's.

When the -3.3v current demand for the OPA's change, due to the changing OPA signal levels.
The long wire from the -3.3v voltage regulator to the OPA minus supply causes the bias to the PD changes, which injects a signal into the PD circuit.

If you look closely at the superimposed waveforms I posted, it clearly shows this effect on the negative swing of the signal noise.

Use a separate -3.3v supply for the PD, also rework the layout of the -3.3v line.

I am not saying that the 3.3v generator is not causing the switching 'sine wave' noise, but the layout and by using the -3.3v for PD and OPA's is making the problems worse.

E
ESP_ 330.png
 

DickCappels

Joined Aug 21, 2008
7,426
Most trandconductance amplifiers I have encountered were shielded from stray electric fields. I am not familliar with LT1931 but I would bet that one of those capacitors in the circuit is being switched between being in parallel with the 12 volt input and being in parallel with the output capacitor. With that capacitor hopping up and down 2.2 million times per second it would radiate a very significant electric field.
 

Thread Starter

pooya_b

Joined Feb 25, 2021
50
There are some application in which it is just too much trouble to use a switching power supply.

I would suggest at least temporarily moving the LT1931A to a separate assembly separated by about 30 cm of wire, then experiment with various types of filtering to see what it takes to remove the noise from the output. Once that is accomplished, try holding the DC to DC supply close to the preamp and see whether you are going to need shielding.

Given that your input stage is a transimpedance amplifier it will be particularly sensitive to capacitively coupled noise.

Edit: How much bandwidth do you need with that amplifier? Would filtering the 2.2 MHz solve the problem?
Hi DickCappels
Thanks for your complete tips.

I need frequencies less than 2.2 MHz.
What filter do you suggest?
 
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