TiePie Handyscope TP450, Руководство пользователя

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3.1 Differential input
Most oscilloscopes are equipped with standard, single ended inputs,
which are referenced to ground. This means that one side of the
input is always connected to ground and the other side to the point
of interest in the circuit under test.
Figure 3.1: Single ended input
Therefore the voltage that is measured with an oscilloscope with
standard, single ended inputs is always measured between that
specific point and ground.
When the voltage is not referenced to ground, connecting a
standard single ended oscilloscope input to the two points would
create a short circuit between one of the points and ground, possi-
bly damaging the circuit and the oscilloscope.
A safe way would be to measure the voltage at one of the two
points, in reference to ground and at the other point, in reference to
ground and then calculate the voltage difference between the two
points. On most oscilloscopes this can be done by connecting one
of the channels to one point and another channel to the other point
and then use the math function CH1 - CH2 in the oscilloscope to
display the actual voltage difference.
There are some disadvantages to this method:
•
a short circuit to ground can be created when an input is
wrongly connected
• to measure one signal, two channels are occupied
•
by using two channels, the measurement error is increased,
the errors made on each channel will be combined, resulting
in a larger total measurement error
•
The Common Mode Rejection Ratio (CMRR) of this method
is relatively low. If both points have a relative high voltage,
but the voltage difference between the two points is small,
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Chapter 3