TiePie Handyscope HS4 DIFF-5MHz Instrument Manual page 10

input is always connected to ground and the other side to the point
of interest in the circuit under test.
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 stan-
dard single ended oscilloscope input to the two points would create
a short circuit between one of the points and ground, possibly dam-
aging 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,
the voltage difference can only be measured in a high input
range, resulting in a low resolution
A much better way is to use an oscilloscope with a differential
input.
6
Chapter 3
Figure 3.1: Single ended input