Acquiring Waveforms
3-6
TDS5000 Series User Manual
Coupling. All oscilloscopes and probes specify a maximum signal level. (Exact
limits for this instrument are provided in Appendix A, Specifications.) Do not
exceed the limit, even momentarily, as the input channel may be damaged. Use
external attenuators if necessary to prevent exceeding the limits.
Coupling determines whether an input signal is directly connected to the input
channel (DC coupling), connected through a DC blocking capacitor (AC
coupling), or not connected at all (GND coupling).
The input resistance of each input channel is 1 MΩ or 50 Ω. To properly
terminate signals when using coaxial cables, or to support active probes designed
fora50Ω environment, a 50 Ω termination resistance can be selected by
pushing the front-panel termination button for a particular channel.
All probes expect a specific coupling and input termination. Both coupling and
input termination resistance are displayed on the screen. If the oscilloscope
determines the coupling and termination required by the probe, either implicitly
because of the TEKPROBE interface or through performing a probe calibration,
the oscilloscope sets the required coupling and input termination.
Consider the following when you use 50 Ω termination with any channel:
H The oscilloscope does not accurately display frequencies under 200 kHz if
AC coupling is selected.
H The oscilloscope reduces the maximum volts per division setting for the
channel to 1 V from 10 V (10 V from 100 V with a 10X probe attached),
since input amplitudes appropriate for the higher settings would overload the
50 Ω input.
H The oscilloscope switches to 50 Ω termination and DC coupling if you
connect an active probe. Active probes also reduce the maximum volts per
division.
Scaling and Positioning. The scaling and positioning controls determine the
portion of the input signal received by the acquisition system.
H Set vertical scaling, positioning, and DC of fse ts to display the fea ture s of
intere st on your wavefor m and to avoid clipping. (Ref er to Note that
follows.) Vertical Acquisition Window Considerations on page 3--14
descr ibe s the vertic al acquisition window.
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