Not so much how you feed it but something is wrong with your sound card.
The sound card is working correctly in general, and the soundcard waveform problem was fixed by further increasing the ratio of the voltage divider (R1=88k, R2=2.2k) per Terry Bites' suggestion.
extreme overload of PC audio input.
Indeed, further reducing the voltage being fed to the soundcard fixed the problem.
Maybe we need to go back to the beginning. You say you have power fluctuations, how do you know? where do you see this? what happens?
Ahh, yeah, good point; I had not mentioned what _kind_ of fluctuations I had been having and wanting to measure.
The electric power in my neighborhood is fairly unreliable. This usually manifests itself in frequent prolonged total outages, or in brief (3-5 second) total outages during which a temporary fault presumably trips a protective relay which is then closed after a few seconds by an automatic recloser.
In the past two days, however, I have been seeing a brand-new-to-me type of power problem. The new problem has
not occurred again (yet) since setting up the soundcard based monitor, so all I can go by to describe what's happening are my own observations.
What happens is that for a period of somewhere around 4 seconds, the lights in my house flicker deeply and rapidly, at what appears to be a steady pace (maybe somewhere around 8-10 Hz, but again, this is estimated from my visual observation, not measured). At the end of the event, power returns to normal. Further observations:
* This affects lights on multiple branch circuits, on both legs of the 120/240 V, 60 Hz split-phase service
* When I say the lights "flicker deeply", I mean they (visually) appear to turn all or almost-all of the way off during the flicker cycle. This gives a strobe light-like appearance, quite unlike the momentary brightness fluctuation you might see if you turned on a large motor or other non-resistive load in the house.
* When this happens, it causes both of my double-conversion UPSes to transfer their loads to battery and record "ConditionInputFrequencyDeviation" and "Input Power Supply Fail" messages to their logs. This seems notable because these UPSes are quite tolerant of variations in input voltage and frequency. Their spec sheets indicate that they can accept input voltages between 70-140 V and input frequencies between 40-70 Hz without having to transfer the load to battery.
I have never seen a power problem like this before, and I have no idea what conditions on the distribution system might cause something like this. This is my reason for wanting to measure the utility voltage waveform. If this particular problem happens again, I would like to be able to characterize what is happening in more detail than my "the lights flickered deeply and rapidly" observation above.
At present, I am using Audacity to record the waveform on an ongoing basis after you all helped me correct my soundcard input circuit yesterday. But if I could get the Digilent Waveform software to do the needed type of capture, I could almost certainly get a better analysis out of the captured waveform if the problem happens again in that software than I can in Audacity.
Thanks again for everyone's input and help; I truly appreciate it.