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LM258: Output voltage is abnormal

Part Number: LM258
Other Parts Discussed in Thread: TLV314

Hi team,

Below is LM258 SCH. We can see,  if it work,  LM258 U1 is output 0.5V. But actually it output 0.53V. When remove R492, it output=0.5V normally.

Keeping R392 in the board, I let U2 , U3 and U4 change the Gain resistance from 3KΩ / 15KΩ  to 10KΩ / 50KΩ. After that U1 output 0.5V normally. May I know why?


  • A voltage drop of 30 mV over R492 is to be expected when a current of 3 mA flows through it.

    If you want VCC0V5 to have the desired value, you must not take U1's feedback from a different node.

    What is the purpose of R492/C470? It might be possible to drop R492, move C470 in parallel with R494, and use a lower-noise opamp like the TLV314. Alternatively, replace R492 with a ferrite bead.

  • Hi Eggsy,

    what is connected to I1, I2 and I3?



  • Hi Eggsy,

    In addition to Clemens comments, please make sure that the output of the LM258 is not oscillating. A 10uF cap load can be difficult to drive for an op amp. I recommend connecting a scope probe directly to on the output pin of the amplifier to see if  it is oscillating.

    Lastly, I recommend looking at Figure 47 in the datasheet that shows the minimum output voltage swing vs output current. Notice that with only 20uA output current the minimum output voltage swing is slightly above 0.5V. If the op amp output current is greater than ~20uA the amplifier will not be able to swing lower than ~0.5V. 

    Thank you,

    Tim Claycomb