Hi @timreyes, FYI –
I did another few experiments with a signal generator, with a ‘square wave’ output set to 2.5V peak-to-peak and 1.25V Offset and created an Excel chart with the combined CSV data exported (and zoomed in) from two separate captures:
Trial #1 (T01):
Saleae Logic Pro @50 MS/s analog
PicoScope @ ~3MS/s
Trial #2 (T02):
Saleae Logic Pro @3.125 MS/s analog
PicoScope @ ~3MS/s
Notice that the PicoScope analog data is apparently NOT as filtered even using a lower sample rate (i.e., ~3 MS/s vs. 3.125 MS/s).
I also noticed that the PicoScope 6 software has some sampling options, such as:
- Sin(x)/x Interpolation (I had this turned OFF in “Tools > Preference > Sampling”)
- Bandwidth Limit set to “None” (could have set to “20 MHz”) on the channel
I wasn’t sure if Saleae Logic2 software (or the FPGA firmware) is doing one (or both) of these settings (interpolation and/or bandwidth limiting) automatically, and without user-configurable method to turn the feature(s) OFF?
E.g., had the software/firmware done a simple sample decimation, then I expect the results would have been pretty similar to the PicoScope – the waveform would have been more ‘square’ than ‘sinusoidal’ and would NOT have been attenuated either.
I look forward to better understanding the analog signal chain design – as I use my Saleae Logic Pro more for its digital features, but it would be nice to more fully exploit the analog capabilities, too (without unnecessarily wasting memory @50 MS/s).
[Edit: update]
I did some additional analysis with the waveform generator and the Saleae Logic2 software ‘Measurement’ tool, and found some data to indicate that the decimation filter frequency response may be below 400 kHz for the 3.125 MS/s analog sample rate?
E.g., given waveforms w/ 2.5Vpp and +1.25V offset:
[Correction] 1.25V * SQRT(2)/2 + 1.25V = ~2.13V (~3 dB point)
[was: 2.5V * SQRT(2)/2 = ~1.77V – but 2.5V isn’t the right signal magnitude]
Set analog capture to 3.125 MS/s sample rate
400 kHz Square:
Vpp 1.5977706909179688 V
Vmin 0.45788028836250305 V
Vmax 2.0556509494781494 V (~0.64 gain)
Vavg 1.2542787790298462 V
Q5% 0.47309714555740356 V
Q95% 2.0404341220855713 V
490 kHz Square:
Vpp 1.0296744108200073 V
Vmin 0.7419284582138062 V
Vmax 1.7716028690338135 V (~0.42 gain)
Vavg 1.2574741840362549 V
Q5% 0.7520729899406433 V
Q95% 1.7614582777023315 V
490 kHz Sine:
Vpp 0.8064938187599182 V
Vmin 0.8535187840461731 V
Vmax 1.6600126028060913 V (~0.33 gain)
Vavg 1.2538232803344727 V
Q5% 0.8585910797119141 V
Q95% 1.6508824586868285 V
444 kHz Sine:
Vpp 1.0347466468811035 V
Vmin 0.7368561625480652 V
Vmax 1.7716028690338135 V (~0.42 gain)
Vavg 1.2547069787979126 V
Q5% 0.7470007538795471 V
Q95% 1.7614582777023315 V
Am I missing something, or analyzing this wrong … ?
[Edit: - added +1 waveform measurements, and updated gain calculations above]
297 kHz Sine:
Vpp 1.7499394416809082 V
Vmin 0.3817959427833557 V
Vmax 2.131735324859619 V (~0.71 gain)
Vavg 1.2577297687530518 V
Q5% 0.3970127999782562 V
Q95% 2.1215908527374268 V
Finally, an external reference for more about ‘decimation’ and ‘downsampling’ :