Op Amp output's noise changes when the gain changes

Question

I use Arduino DUE to generate a 100 Hz pulse.

I wanted to remove the DC value so I added a high pass filter and this is connected to the Op Amp(ADA4000-1)'s input. A RIGOL's DP832 power supply is giving +/- 9V to the Op Amp.

These are connected to the breadboard and I noticed when I change R2's value.

The following screenshots are the Op Amp's output.

1. R2 = 0 (Unity gain)

Some noise is discovered from the signal's positive peak.

2. R2 = 40k, gain = 5

When the gain becomes bigger, reversely, the signal's negative peak shows noise and the flat line after the positive peak seems noisy.

• What seems to be the reason for the noise when the gain changes?
• Also, what could be the solution to remove these noises?

EDITED: You guys are right; the output was unstable due to the missing decoupling caps.

I changed it to a unity gain buffer.

Before adding the decoupling capacitor to the negative terminal,

the same phenomenon was shown.

As you see my pen pointing, adding 100nF cap, now it is giving a stable output! Thanks!

Two more things;

• When I touch the Op Amp's output with my finger, I discovered that the output becomes stable (by looking at the oscilloscope) without the decoupling capacitor. Why did this happen?

• This happened due to the unpopulated decoupling capacitor. Is there a name/technical jargon of this phenomenon?

Answer 1

The oscillation is a result of instability in the layout, perhaps inductive power or ground leads without decoupling 🧢’s on power to gnd .

Unity gain gives the widest BW (from fixed GBW) but also the lowest phase margin which degrades with any capacitive load, unless compensated with reduced BW.

There is also a slope on the pulses, which suggests another reactive effect in your layout not shown on your schematic but could be uncalibrated 10:1 probe error.

So correct that with the probe trimmer and scope test pulse , add ceramic caps. to both supply rails and use a very short probe ground lead to get textbook waveforms from DAC then OpAmp.

Even Alkaline batteries make random noise which you can hear with 1.5V on a speaker, spurious effects depend on the phase margin which can improve with gain or in your case be asymmetric while improving.

Define your capacitive load! Is there a long cable output and measure your 9V supplies with a divider.