LM358: Generating Noise on O/P Side

Hi Nilesh, 

Mayank Mirajkar said:

but it generates noise on output side and isolation issue when neutral is connected with main supply. 

In 3-phase system, the neutral terminal should have voltage near zero, if the AC winding is balanced. The noises likely come from the 3-phase; you may minimize the noises by coupling a capacitor between Neutral terminal to Earth GND. I am not sure that LM358 is able to withstand the transient voltage at the Neutral terminal for the application. For high voltage transient events at neutral terminal, LM358 op amp may not be able to survive the voltage variation at the node (over temperature and time). What is the application?

What does it mean when Neutral terminal is connected to main supply. Please clarify. 

Mayank Mirajkar said:

We have developed a metering application which use dc offset voltage to measure the ac voltage and current. 

What is the total system power in the application? Typically, AC voltage is measured via voltage divider using differential circuit and AC current is measured via current sensing transformer, shunt resistor or Hall effect sensor. Is this a motor application?

Best.

Raymond

Hello,

FYI, this circuit is designed for metering application for measuring three phase voltage and current.

We took reference of the meters which used the same topology, and it is working as expected. However, it is generating noise in our design.

The main functionality of an op-amp in design is to generate offset voltage and sum that voltage to ac voltage divider circuit.

I am assuming that this issue has happing because of the two different grounding of SMPS circuit. GND and GND_ISO. Does it could generate the noise?

Hi Nilesh,

Mayank Mirajkar said:

I am assuming that this issue has happing because of the two different grounding of SMPS circuit. GND and GND_ISO. Does it could generate the noise?

You did not mention if the 3-phase is motor or transformer. The neutral connection can be any potential it wants to be. If all the phase and load are balanced, the neutral terminal should be close to GND. For transformer, the neutral terminal can not be DC biased, since it may saturate the magnetic core of the transformer. 

Did you tie a capacitor to Earth_GND? The common-mode noise at neutral terminal provides a discharge path to ground via the capacitor, which acts as a low impedance path and shunt noise to Earth_GND (high frequency noise filter). What types of noise are you able to observe in your design?

Please let me know if you have questions. 

Best,

Raymond

Hi Nilesh,

I am going to close this query, since I have not heard from you. 

If you have other questions, please let us know. 

Best,

Raymond

HVA Application Team

Nothing is connected with 3phase like motor or transformer. It is 3phase voltage measurement circuit. 

I am going to attach the output of op-amp which is measured at R_Phase side which is connected with microcontroller.

Kindly find it and let me share the resolution.METER_PROBLEM_DISCUSSION.pdf

Hi Nilesh, 

In 3-phase system, the Neutral phase should have 0 AC voltage and current if the 3-phase is in a balance system. 

3-phase AC 03122025.TSC

In an unbalanced 3-phase system, the magnitudes of the phase voltages are not equal. As a result, the vector sum of the phase voltages will not be zero, which can lead to a non-zero neutral voltage and current. This is what you observed from the scope shot. 

Here is another likely possibility as well. What type of scope probe do you use? You need to get a probe without the long flexible GND return leads. Probe the signal next to a GND signal in your measurement. And these spikes will be likely gone. 

The voltage spikes near the peaks and valleys are resulted from Ldi/dt events in your unbalanced 3-phase system. It is not from the op amps. 

If you have other questions, please let me know. 

Best,

Raymond

Thanks for response.

I would like to inform you that we have the same meter which is the reference meter which is working as expected without any noise with the same supply. Why are these noises not observed with it?

I want to know that how much voltage should apply on VCC pin of Op-amp if i want to get 3V on Output side?

I have provided 3.3V on VCC; However, 5V is provided on VCC in reference meter.

Does it may create an issue?

Moreover, Where I can measure the 3-phase impedances?   Can you share the procedures?

Hi Nilesh, 

Mayank Mirajkar said:

I want to know that how much voltage should apply on VCC pin of Op-amp if i want to get 3V on Output side?

Mayank Mirajkar said:

I have provided 3.3V on VCC; However, 5V is provided on VCC in reference meter.

Yes, this could be part of your problem, see the Vcm requirements and op amp's output droop in this part. 

For LM358, if you want to get out of 3Vdc at the output, the op amp's supply voltage has to above 7-8Vdc with some design margin (assume single supply rail configuration). The input Vcm has some limit and output voltage has some droop, depending on the output current. Prefer to use 10 or 12V LDO with low ripple voltage.  

Mayank Mirajkar said:

Moreover, Where I can measure the 3-phase impedances?   Can you share the procedures?

If you have LCR meter, you need to configure and match the frequency range or close to it and compare the inductance figures. 

Can you send me a picture of your 3-phase setup? I am not able to picture what you have in your configuration. 

Best,

Raymond

Hello,

I have provided 5V supply to op-amp's VCC. But the problem remained same.

Can you review our layout?

It would be very helpful for resolving the issue. 

Hello, 

Could you send me the schematic? So far, we have exchange multiple replies, but I have no ideas how you setup the op amp. 

Best,

Raymond

Hello,

I have already sent you the schematic. 

For more analyses, kindly review the Gerber files and let me know your comments.

Seansing Gbr_v06.zip

Hello,

I made the circuit on the breadboard which is providing the output as expected.

However, In designed PCB, I am getting the noise.

Please review the attached Gerber and let me know if any modifications is needed.