Improving S/N

Yury Kuchura EU1KY

Current design suffers from the noise that comes from the 5V power line, which is awful if the device is powered from PC's USB port. This is because the schematic does not use symmetric output of the mixer. Modelling shows that a very simple change in connections between the mixer and filter can improve the power noise immunity by at least 12 dB.
See how the 2 kHz power line noise of just 2mV peak to peak interferes with the useful 10 kHz signal, and how it is decreased in the improved schematic. Existing schematic is at the top, improved is at the bottom. AC characteristics remain the same, only the gain of the filter is improved by 3 dB.


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