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Schematic

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)

low0 low1

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

  1. R2 = 40k, gain = 5

high0 high1

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.

circuitBefore I changed it to a unity gain buffer.

Before adding the decoupling capacitor to the negative terminal, Pulse

the same phenomenon was shown.

circuitAfter As you see my pen pointing, adding 100nF cap, pulse after 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?

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  • \$\begingroup\$ How is this physically constructed (breadboard? 2-layer PCB? or what)? How long are the wires from the power supply to the op-amp board? Is there any load on the output other than the scope probe? And what are your scope's probe's capacitance? \$\endgroup\$ – The Photon Feb 3 at 3:49
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    \$\begingroup\$ FWIW, that looks to me more like an unstable output than noise. \$\endgroup\$ – The Photon Feb 3 at 3:50
  • \$\begingroup\$ You are getting noise from a high performance op-amp. The reasons would be local power decoupling at the op-amp Vcc and Vee pins and general board layout. You cannot use a breadboard or sloppy layout with a high performance op-amp. It will tend to ring and oscillate, or distort rising and falling edges. Just a thought... \$\endgroup\$ – Sparky256 Feb 3 at 4:14
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    \$\begingroup\$ Please either take a picture of your breadboard and post it, or add 100nF decoupling caps from each power rail to ground and report back. Keep your wires short -- putting big loops of wire right next to each other on a circuit with a high-gain part in it is like getting on your knees and begging the Oscillation Gods to give you trouble. \$\endgroup\$ – TimWescott Feb 3 at 4:48
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    \$\begingroup\$ Hi, all. I used a breadboard with adaptor boards to mount SOIC chips. I have to admit that I have used long jumpers (50~70mm long) and I didn't add the decoupling caps from each power rail to ground. I will try it again! Thanks! \$\endgroup\$ – David Lee Feb 4 at 1:01
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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.

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  • \$\begingroup\$ There is also (on a breadboard in particular) a sneaky pole at the inverting input due to parasitics. A small cap across R2 might also help. \$\endgroup\$ – Peter Smith Feb 3 at 14:30
  • \$\begingroup\$ A few puff 🧢 can make a difference on 20MHz ringing near null phase margin. This can be verified with a light touch across an R or on inputs or outputs to see the effects while touching gnd. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 3 at 14:34

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