# LTSpice, add real noise to opamp

If I will build this circuit in real life, I will probably see a mV noise with the scope due to high cascaded gain and probably some oscillations.

I use LPC358 which is specified to (Input-referred voltage noise) 178nV/SQRT(HZ). I am trying to figure out how small can be the input signal (now is GND at C3) to be masked by noise generated by circuit itself.

My simulation is clearly wrong, showing a clean output with some nV flicker.

How can I add some noise in LTSpice, based on the fact that any opamp has specified noise, that will emulate real worlds noisy output so I can select from several opamps, gain, etc?

• Have you tried a LTspice noise analysis? Even with this tool, many opamp models are crude. You get output noise optimistically low. – glen_geek Jul 9 '17 at 15:45
• Yes, I tried, but this shows nothing to me compared to real life that I can measure. I try to estimate for a real prototype. – user1797147 Jul 9 '17 at 16:00
• The specifics of your physical measurement can fool you; make sure you are probing immediately next to the ground reference. Ground loops in measurements are surprisingly easy to create. – Peter Smith Jul 9 '17 at 16:15
• Does LTSPICE provide a transient-random-noise mode? – analogsystemsrf Jul 10 '17 at 4:24
• @analogsystemsrf If you search in the LTspice Yahoo Group for tranoise, you'll find one Robert Macy who made a calibrated transient+noise analysis. You'll also find steps to make it. – a concerned citizen Jul 10 '17 at 5:58