oscilloscopeoversampling
A while back i was gathering some ECG data for a colleague at a reasonably high sample rate (around 20kHz – 100Hz is enough for ECG). She came back to me and asked me to regather the data at a lower sample rate – claiming that the high sample rate was introducing extra noise.
Is this plausible, realistic, or based in truth? I can't see how a higher sample rate would make a signal worse – especially considering the data was gathered on an expensive hi-spec oscilloscope
Ground loops are important and safety is always a major consideration BUT from a technical point of view, use of the ground AT the probe tip is essential in many circuits.
The oscilloscope "sees" a voltage between probe tip and some reference point.
If the reference point is the probe-ground the ground lead can be connected very close to the point under measurement at a point where "ground" is meaningful for the signal concerned.
If the circuit is operating at say 100 MHz and involves other than perfect infinite ground planes then local ground for a signal may be quite different in potential than the "ground" level on the oscilloscope itself. Failure to use probe ground will often produce a signal that would be ideal for use as a source of random numbers.
So, for technical reasons, in complex situations, use of a probe ground connected AT the point of relevance is utterly essential. Safety and ground loop and other considerations are not made any less important by this need, but must be worked out AFTER the technical ground point requirement is met.
Achieving anything worthwhile was never meant to be easy :-)
How real men (and women) do it - a picture from a Jim Williams application note:
More than adequate, as I think any scope will be for sampling 500 kHz. 500 kHz is a period of 2 µs, so at 250 Msps that gives you 500 samples per period, enough for measuring time with a 0.2 % accuracy.
Note that the scope's bandwidth is enough to capture up to the 50th harmonic of a 500 kHz signal. Even with a brick wall filter, cutting of all further harmonics (which doesn't happen), as a first approximation your square wave will still look like this:
So the edges of your signal won't suffer from the bandwidth limitation.
Best Answer
Your college does not seem to remember her statistics lessons. The additional higher frequency noise can be trivially filtered with a low pass filter, and the filtered signal might be better than one sampled at low frequency (oversampling).
Note that using a standard oscilloscope to record live ECG violates patient protection, as medical equipment requires better electrical isolation than those devices usually provide.