LM324: Long output of operational amplifier leads to self-excited oscillation

liujun

Part Number: LM324
Other Parts Discussed in Thread: TMS320F28035

The operational amplifier LM324 collects three-phase electrical inputs, and the output of the operational amplifier is sent to the AD port of the chip TMS320F28035. When there is no three-phase electrical input, the operational amplifier forms oscillations, and the output line of the operational amplifier is too long. What measures can be taken to eliminate the self-excited oscillation of the operational amplifier? Explanation: What measures should be taken when the layout of the circuit board has been determined, that is, the distance from the output of the operational amplifier to the AD sampling port remains unchanged?

Thank you!

9 months ago

0 Jerry Madalvanos 9 months ago

TI__Mastermind 26471 points

Hello Liujun,

Could you please share the schematic? Please make sure to include the supply voltage and the load R/C on the output.

Best,
Jerry

0 liujun 9 months ago in reply to Jerry Madalvanos

Prodigy 220 points

The schematic diagram is shown in Figure 1, and the PCB layout is shown in Figure 2. The same circuit diagram can be used in some projects, while in some projects, the AD port has a sine wave when there is no input

Figure 1

Figure 2

+1 Jerry Madalvanos 8 months ago in reply to liujun

TI__Mastermind 26471 points

Hello Liujun,

You have multiple components adding excessive phase lag to your circuit. I do not know what the values of any of the components are, but unless they are less than 10pF, they are almost certainly causing your instability.

I have circled in red or black all the components that can add phase lag to your loop. I would consider reducing these values or eliminating them completely.

All the capacitors circled in black should be separated from the output of the amplifier with an isolation resistor. An example of how this would look is included below. Depending on the value of the capacitor for each loop (c1 in the below picture) you may need to adjust R1 for appropriate rise time or stability. 100 ohms and 1uF does not work for every op amp. Information on how to test loop stability in simulation can be found in the below video series.