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In the attached circuit, with input of 30mV at 40kHz, the reference DC voltage at pin 2 of the comparator exhibits noise at the same frequency as the input.

At low input amplitude (4mV p-p) the noise does not appear.

A scope shot of the reference signal is shown below.

What might be causing this noise?

enter image description here

enter image description here

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  • \$\begingroup\$ Try giving a better picture of the schematic. \$\endgroup\$ – Andy aka Nov 3 '15 at 8:52
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If the circuit is exactly as shown you may have signal noise on the supply line that is getting into the reference. Add bypass filter capacitors from each supply line pin to each ground pin, (as close as possible to to the physical pins). Also place a capacitor from the comparator pin 2 to ground.

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  • \$\begingroup\$ The supply is from a 50Khz switching regulator and it has a bypass capacitors on its output. but the reference noise frequency is same as the input frequency, I check that with different input frequencies. \$\endgroup\$ – HochKonik Nov 3 '15 at 7:46
  • \$\begingroup\$ Also when I entered the comparator input from the function generator, that noise on the reference is disappeared!! \$\endgroup\$ – HochKonik Nov 3 '15 at 7:52
  • \$\begingroup\$ The high gain from the first amp will likely put noise on the supply lines and cause noise else where. So the filter caps need to be right at the component pins to filter out the frequency being amplified. You also seem to be over driving the first amp, (.030v x 200 = 6v), is that needed? Your supply seems to be only 5v, (hard to see values in your graphic). \$\endgroup\$ – Nedd Nov 3 '15 at 7:57
  • \$\begingroup\$ You could also place a series resistor of 1k from the amp output to the comparator input, this will equalize the input currents and minimize offset voltages that could also transfer some noise. \$\endgroup\$ – Nedd Nov 3 '15 at 8:07

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