Electronics and Electrical Engineering Design Forum



Getting Noise on a mV range Square Wave Generator

I’m in the process of designing a Square Wave Generator that is capable of operating at millivolt levels.

The circuit uses a MIC1557 timer/oscillator to generate a 5V square wave.

Three op-amps are being used to divide the voltage down by increments of 10. A fourth op-amp is attached to a 10-turn pot to enable adjustment of the amplitude of the wave from 0 to 50mV.

I’ve attached a photo of the oscilloscope showing the Square Wave Generator set to a 1mV output.

The waveform looks ok overall but is relatively noisy and that is where I am having problems. I’m not used to working at these voltage levels, so any advice or help would be most welcome.

Attached Files:
Asked By:
William Croft
3 years ago
Score: 0

Oscilloscope probes often have poor common-mode rejection and any noise potential between the probe ground and the oscilloscope AC ground will show up as a signal.

To measure this attach the probe ground lead to your circuit and attach the probe tip to the same spot. This is probably what is shown on you oscilloscope pic.

One way around this is to float the circuit under test or to float the oscilloscope by powering either one from a battery. Isolate one or the other (or both) from any ground other than the connection between the circuit and the oscilloscope.

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