Tektronix TEKFAX ADA400A, Instruction Manual

Page: 26 / 80

Reference
14
ADA400A Differential Preamplifier
Single-ended measurements, however, often yield unsatisfactory information
because of interference resulting from ground-loop currents between the
oscilloscope and the device under test. In other cases, it is desirable to eliminate
a common-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 ADA400A 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
Amplifier
+
--
Figure 6: Differential Rejection of Common-Mode Signal
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 15 illustrates
this type of connection.