Tektronix ADA400A Instruction Manual page 22

Reference
acommon-mode DC voltage without using a DC blocking capacitor or the AC
coupling capacitor of the preamplifier. These capacitors limit low-frequency
response. Connecting the signals differentially virtually eliminates these effects.
Differential. Figure 6 shows two signals connected to the ADA40O0A Differential
Preamplifier. One signal is connected to the + input and the other is connected to
the ~ input. Because the preamplifier inverts the signal at the — input, the
preamplifier amplifies only differential voltages between the inputs and rejects
(cancels) voltages that are common to both inputs. The oscilloscope displays the
waveform of the differential signal only.
NOTE. To avoid creating a fictitious differential offset, set both AC GND DC
coupling switches to the same AC or DC switch position.
+
Differential
Figure 6: Differential Rejection of Common-Mode Signal
12
To avoid ground loops in a differential setup it is important to leave the grounds
of the shielded coax disconnected and to route the chassis ground to the
oscilloscope ground using a separate ground lead. Figure 7 on page 13 illustrates
this type of connection.
ADA4OO0A Instruction Manual