I'm trying to sample a(low frequency) signal I'm producing using Microchip's PIC16F1783 (which has an internal DAC). I connect its output and ground my PC's audio jack (GND+Right Channel). I then watch the waveform using an application which is basically a scope for audio signals. The strange thing is that the waveform doesn't change whether I plug the GND or not! Does it mean that the I can't really trust the results I see? Is there another (cheap) way for me to sample the output of the PIC?
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3\$\begingroup\$ You might be establishing a degree of common grounding through the power supply or programmer cable to your circuit. If you can battery power your circuit with no other connection, try that, and you will probably see your intentional signal swamped by noise picked up from your house/office wiring. \$\endgroup\$– Chris StrattonCommented Dec 31, 2012 at 16:30
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Most likely the ground is already connected to the PC by other means. This could be a power supply with a ground lug that connects ground to it, via USB, RS-232, thru a scope, etc. It's pretty easy to end up with ground not floating unless you are careful to ensure it is floating.
However, relying on some other ground connection path is not a good idea for a signal. If you listened to the audio, you will probably hear some background noise when the ground is not connected thru the audio cable. There will probably be even less noise if you eliminate the other ground path so that the PIC circuit is only grounded to the PC thru the audio cable. With both ground connections, you have a "ground loop", which has a good chance of adding noise to your signal as seen at the PC input.
answered Dec 31, 2012 at 17:37
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\$\begingroup\$ your'e right... The ground is connected via PICkit. Thanks \$\endgroup\$– DanielCommented Jan 2, 2013 at 8:04