# Low pass filter offset voltage question. LM358, INA129

I am designing a low-pass anti aliasing filter. I have set my cutoff frequency to be at 500Hz, and my sampling frequency at 1000Hz, but I'm not that far yet.

I designed a 4th order butterworth low-pass filter, but am having some issues at the output with some small offset.

Is this due to my op-amp's input offset voltage?

Here is my schematic.

I run a transient analysis and find there to be about 50mV of offset on my output.

Finally, here is a picture of the AC analysis frequency response. It matches my criteria.

My question really is why am I getting this offset in my final signal and how can I get rid of it? Do I need to compensate for component non-ideal values?

Thanks!

• LM358 uses bipolar transistors, that have some bias current....which must flow through those large-value resistors. Can you change the filter's RC to favour lower-value R's? – glen_geek Feb 25 '20 at 18:50
• @glen_geek I adjusted my values to be much lower (in the 1k-10k range) and it has dramatically improved. Thanks. :) – Punchki Feb 25 '20 at 19:15
• Multiply the input bias current in the datasheet by 1.75Megohms. Put the result in the question. – Brian Drummond Feb 25 '20 at 20:06

The LM358s have an offset of +/-3mV but also a bias current of as much as 35nA at room temperature, so you could get another 96mV from the unbalanced resistances. That's because there is a total of 2.56M$$\\Omega\$$ of resistance that adds voltage when multiplied by the input bias current.