Figure 5: Selecting The Input Impedance - Tektronix ADA400A Instruction Manual

Input Configurations
ADA400A Differential Preamplifier
H Ensure that the source has some impedance to ground. The internal input
amplifier requires some input bias current to operate. Typically this amount
of input bias current is less than 25 pA.
H Select the DC coupling position when operating the preamplifier. The AC
coupling position blocks the bias current that the preamplifier input requires
to operate.
H Take steps to reduce interference from other sources (see page 19). Because
the input impedance is essentially infinite you may notice an increased
susceptibility to noise and DC drift.
Selecting the Infinite Ω 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 1 MΩ to the
infinite Ω position as Figure 5 illustrates.

Figure 5: Selecting the Input Impedance

There are two basic types of input 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.
Reference
- - +
Input Input
Infinite
Ω
1
MΩ
13