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Reducing noise from power supply

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watertreader

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Hi,

I am currently using an amplification board based on some op-amps. Have tried to do amplification with the signal but however, the board seem to amplify signal from the power supply too.. Signal from 50Hz is shown when i am reading off the spectrum. How do i reduce this noise?

Btw, i am sharing power supply with another control board with the amplifiication board

Thanks for your help!
 

you can connect your amplifier's ground with the control board's ground at a single

point, and use decouple capacitor to eliminate much of power supply noise on

your amplifier's VDD.

best regards





watertreader said:
Hi,

I am currently using an amplification board based on some op-amps. Have tried to do amplification with the signal but however, the board seem to amplify signal from the power supply too.. Signal from 50Hz is shown when i am reading off the spectrum. How do i reduce this noise?

Btw, i am sharing power supply with another control board with the amplifiication board

Thanks for your help!
Click to expand...
 

The ground of the amplifier board and the control board are grounded at the same point. Also I have tried to use a capacitor to connect the positive supply to the ground.

Btw, by using the decouple capacitor on a dual supply op-amp.. should we decouple for both the positive and negative supply?

thanks
 

ya we are supposed to use decoupling capacitors for both the supplies........
 

Hi,

Use this circuits.
 

You can check opamp's power and measure 50 Hz on it (though opamp PSRR is high at 50 Hz ).
But it could also be due to amplifier impedance is too high and some input circuitry exposed to surrounding electromagnetic field (e.g. ECG).

You can do shield or filtering or change PCB layout or change to differential mode connection or ...
But you have to post your device internals and describe its use cases more precisely to help others to help you .
 

The schematic of the circuit is as follows
 

- proposed bandwidth of amplifier ?
- output impedance of input source (IN1-IN2)
- input cable length
- Is the gain of 1000 000 really needed for this circuit

Related to schematics :
- input offset voltage temperature coefficient is 18 uV/C , for gain 1000000 it means 18 V per degree - so this opamp wont work stable for high gains
- it is better to adjust offset by offset adjustment terminals of opamp N1 and N2 , and connect noninverting opamps inputs directly to ground
- the gain adjustment is done poorly and is not immune to external electromagnetic field if pcb was not properly routed and shielded
- try to use differential mode to connect to signal source - for long cables this will reduce 50 Hz noise .
- C1 to C4 need to be increased , the order of increase must be decided by max gain and measurement . I can try to estimate but leave it to you .
- circuit bandwidth must be reduced as much as possible .
- if that above wont help 50 Hz notch filter must be used after first amplifier.
- low noise instrumental opamps is better choise , Check out also opamps with chopper stabilization for noise and bandwidth related requirements .
- search internet for ECG circuits - those are designed for high gain and exposed to outside electromagnetic field. Look also to Analog's app notes and seminars - it is good source to find answers for your problem
 
watertreader said:
Hi,

I am currently using an amplification board based on some op-amps. Have tried to do amplification with the signal but however, the board seem to amplify signal from the power supply too.. Signal from 50Hz is shown when i am reading off the spectrum. How do i reduce this noise?

Btw, i am sharing power supply with another control board with the amplifiication board

Thanks for your help!
Click to expand...

If you use IC's, you should decouple each IC with 2 ceramic caps value 0.01uF is sufficient. If your board has + and - voltages you should decouple each to ground.
Those caps are cheap and it doesn't harm to use overkill in cases where there is noise...
 

artem said:
- proposed bandwidth of amplifier ?
- output impedance of input source (IN1-IN2)
- input cable length
- Is the gain of 1000 000 really needed for this circuit

Related to schematics :
- input offset voltage temperature coefficient is 18 uV/C , for gain 1000000 it means 18 V per degree - so this opamp wont work stable for high gains
- it is better to adjust offset by offset adjustment terminals of opamp N1 and N2 , and connect noninverting opamps inputs directly to ground
- the gain adjustment is done poorly and is not immune to external electromagnetic field if pcb was not properly routed and shielded
- try to use differential mode to connect to signal source - for long cables this will reduce 50 Hz noise .
- C1 to C4 need to be increased , the order of increase must be decided by max gain and measurement . I can try to estimate but leave it to you .
- circuit bandwidth must be reduced as much as possible .
- if that above wont help 50 Hz notch filter must be used after first amplifier.
- low noise instrumental opamps is better choise , Check out also opamps with chopper stabilization for noise and bandwidth related requirements .
- search internet for ECG circuits - those are designed for high gain and exposed to outside electromagnetic field. Look also to Analog's app notes and seminars - it is good source to find answers for your problem
Click to expand...

Hi, actually i am using a premade board(used previously for photodetection with transimpedance amplifier settings and built in voltage amplifer) at this moment. So there is very little much I could really alter.

I am currently only using one of the amplifier(configured as a voltage amplifier) instead of 2 and I am only setting for a gain of around 50. The unit gain BW is around 3MhZ. So I guess there should be no problem since I am only required to detect signal up to 15Khz. Have adjusted the offset and differential mode is used

Probably would consider building another circuit myself since there is very little way i could alter this board. Just have a few question from the tips you have given

1) output impedance of input source -> Does this mean the input impendance to the board and the impedance of the signal source i am detecting?
2) If we used decouple capacitor for the power supply, what determines the farad value we should set?

Thanks a lot!
 

For 50 gain and 0 - 15 KHz input bandwidth there should not be problem. Can you set the gain to such value, short circuit amplifier input and measure 50 Hz noise ?

For input source impedance i meant - the signal source impedance connected to amplifier input . If it is high and cables are long you can have radiated 50 Hz noise on connection cable.
The way to solve that is to use differential input amplifier and input cable shielding .
 

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