With no information about your circuit, it's necessary to guess.

Several things can be at work here. (1) is your oscilloscope probe properly grounded to the ground of your circuit?

(2) Have you got lots (one per power and one per GND of each IC and located within 1 mm of each connection) of small (10nF) bypass capacitors from power to gnd and one larger one ~4.7 uF tanatalum at the power entry to your board?

(3) Has your board got a good, low impedance ground plane throughout?

(4) Has your power supply got the capacity to provide at least 10x the maximal current your circuit demands?

(5) Have you tried putting a small (10 ohms) resistor in series with the power supply, before the entry capacitor?

If your power supply has more than 5 mV of random noise or of ripple with no load, it's too noisy!

If your power supply has more than 25 mV of random noise or or ripple with 10x full load, it's too noisy!

Keep the ground strap on the oscilloscope probe short (10 CM or less). Use the shortest probe (e.g. 1.5 meter) you can practically use, and shield it from external EMI. I often use aluminum foil, grounded to earth to help with this.

Be sure your power supply wires are large, not as thick as a pencil, but thicker than a pencil lead, (1.5 mm).

Use a carefully regulated, well-filtered linear supply. Switchers seldom have low enough ripple (<5mV at no load, <25 mV at full rated load).

Once you figure out what the source of the offending noise is, you can set about to remedy it. Remember to keep the bypass capacitors close to the IC power connections. If power is routed through individual wires, be sure the power and GND wires are at least 1/2 mm in diameter.

Good luck and happy hunting!

RE