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custom probes and input voltage ranges used?

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custom probes and input voltage ranges used?

Postby hillp » Wed Aug 13, 2008 11:27 am

We have a 2203 scope and have set up a custom probe for a pressure transducer. It puts out 0.5 VDC at 0 psi and 4.5 VDC at 150 psi. These were entered as 2 points in a lookup table. The automatic ranges subsequently generated by PS6 for this are 0 - 20 psi, 0 - 60 psi, and 0 - 200 psi. The "custom range" (also seemingly called "scaled range") 0 - 20 psi reportedly uses 13% of the "input range", while the 0 - 60 and 0 - 200 psi ranges use 16% and 40%, respectively, of the "input ranges".

If I convert psi to volts from my lookup table, I get 0.533 volts being the width of the "scaled range" for 0 - 20 psi, 1.6 volts for 0 - 60 psi, and 5.333 volts for 0 - 200 psi. Then applying the 13%, 16%, and 40% factors reported above, I get the scope voltage input ranges as about 4 volts, 10 volts, and 13.3 volts, respectively. Presumably PS6 generates the custom/ scaled ranges so it can pick the best hardware input voltage range to use (by actuating the correct relays?) for each scaled range. 4 volts (0 - 20 psi) must correspond to the +/- 2 volts hardware input range (of which I'm using the subrange 0.5 - 1.033 volts), 10 volts (0 - 60 psi) must be +/- 5 volts, but what about 13.3 volts? This doesn't seem to fit any of the voltage range specs for the 2203. The next one bigger than +/- 5 volts is +/- 10 volts.

And a clarification on terminology - I'm able to choose 0 - 20, 0 - 60, and 0 - 200 psi from the dropdown menu for each channel, but these drop down boxes are notated as "input range" when one holds the mouse over them. These are really scaled ranges, not hardware input ranges, correct? That is, the scope has only the voltage ranges listed in the manual to work with, breaking each hardware input voltage range into 256 (2 to the 8th power) discrete values (the 2203 is an 8-bit scope).

I'm asking because I believe this has implications for triggering with a given choice of transducer. I couldn't get a given pressure spike with the pressure transducer described above to trigger on the 0 - 60 psi range, but it triggered reliably on the 0 - 20 psi range. I need to understand the range options before investigating further. I'll take into account the triggering hysteresis info provided in a recent thread in this forum. I may need to get different pressure transducers. Thanks.
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Postby ziko » Thu Aug 14, 2008 10:44 am

Hi hillp,

The voltage ranges are as follows:

for 0-20 psi it is +/-1V
for 0-60 psi it is +/-2V
for 0-200 psi it is +/-5V

I understand the confusion, you see your maximum psi in your scaling file would be 150psi which equates to 4.5V, this is below +/-5V. If you changed your scaling file to show 200psi then it would actually show a different voltage range. (+/-10V).

You can actually go to the advanced settings and choose your own ranges.

Yes you are correct in that those ranges are not hardware input ranges and are just scaling. The voltage ranges available can be found when no scaling is chosen. Yes the device is 8 bits and as such you have 0 - 255 vertical points in each of the voltage ranges available.

I hope this helps.

Kind regards
Ziko

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Postby hillp » Thu Aug 14, 2008 1:24 pm

Thanks for the advice; you've helped me with my confusion about the 0-200 psi range. But it seems that the statement

for 0-20 psi it is +/-1V
for 0-60 psi it is +/-2V

is incorrect with the "% of input range used" as reported by the software for the automatically generated ranges. For instance 0 to 60 psi would be 38% of the +/-2V range, not 16% as reported (see attachment).

Your info, though, has enabled me to set those ranges manually as specified just above; this will help a good bit. In fact I now see that the ranges picked by the software are shown, but grayed out when auto range selection is enabled.

Are there any downsides to exceeding the hardware input range with a signal? The scope itself is protected. Seems it would just peg the signal at the top of the display. Will it crash the software?

Thanks.
Attachments
custom probe wizard screenshot 0-60.jpg
custom probe wizard screenshot 0-60.jpg (25.23 KiB) Viewed 1485 times
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Postby ziko » Thu Aug 14, 2008 3:52 pm

Hi hillp,

Yes I apologise you are correct about the voltage ranges I should have said

+/-2V and +/-5V for the 0-20psi and 0-60psi respectively.

If you exceed one of the voltage ranges then it would clip the readings and show an exclamation mark. For example using your scaling file on 0-200psi range if you put a voltage above 5V it would clip at approx 170psi.

The software should not crash. Please note not to go above the max input voltage range.

Kind regards
Ziko

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