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2.5 Probe Compensation
Before taking measurements with the USBscope50, the attached x10
scope probe must be compensated to ensure that it is matched to the
scope input stage
8
.
This ensures that the probe+scope act together to keep the input
frequency response as flat as possible. Using a scope probe that has
not been adjusted to match the USBscope50 can lead to inaccurate
readings for higher frequency signals.
The USBscope50 includes a probe compensation output
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.
This must be turned on using the USBscope50 software. Once
enabled, the compensation output is a 0 to 3V (approx) square wave
with 50% duty cycle and 1KHz frequency. Note that the
compensation output will switch off automatically when you close
the channel’s tab
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.
The USBscope50 can work with scope probes that list an adjustment
range that includes 16pF. It is best to use probes that have a range
that puts the 16pF point near the middle of the range, for example
the HP10071A type probe lists an adjustment range of 9-17pF and
our tests show that whilst it is easy to compensate this probe against
the USBscope50, the trimmer in the probe body is certainly quite
close to the top end of its range. There are many probe choices on
the market, and lots of them list ranges that span 16pF.
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To compensate your probe follow these steps:
1. Connect your x10 probe to the USBscope50 software and run
the USBscope50 software
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This does not apply to x1 scope probes
9
Later models have a dual pad compensation point that includes a GND pad
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This is to avoid the fast edges of the square wave possibly coupling into your measurement
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Some very high frequency low capacitance probes may not be suitable for use with the USBscope50
Elan Digital Systems Ltd. 15 USBscope50 USER’S GUIDE Iss5