Tektronix ADA400A Instruction Manual page 21

Reference
ADA4OOA Instruction Manual
m= Select the DC coupling position when operating the preamplifier, The AC
coupling position blocks the bias current that the preamplifier input requires
to operate.
mTM Take steps to reduce interference from other sources (see page 17). Because
the input impedance is essentially infinite you may notice an increased
susceptibility to noise and DC drift.
Selecting the Infinite Q Position. To select the infinite impedance, remove the
knobs and top housing of the preamplifier. Do not remove the inner shield. With
a pair of needle-nose pliers or tweezers, move the jumpers from the | MQ to the
infinite Q position as Figure 5 illustrates.
g o o / o o
Infinite Q
1MQ
Figure 5: Selecting the Input Impedance
There are two basic types of put configurations on the ADA400A Differential
Preamplifier: single-ended or differential. Whether one or the other of these
configurations is appropriate depends on the type of signal and the electrical
environment of the test setup.
Single-Ended. To monitor a ground-referenced signal you only need to make a
single input connection. All the controls on the preamplifier still operate. The
only difference is that the preamplifier cannot cancel out common mode noise
without using the other input connection. When making single-ended measure-
ments, set the coupling for the unused input to GND.
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
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