Using Your Own Screw Terminal Panel
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Analog Inputs
The DT9820 Series function modules provide differential analog
output channels only.
Differential inputs offer the maximum noise rejection. For the best
results, shielded twisted pairs are a must. The shield must connect at
one end so that ground currents do not travel over the shield. In
low-level voltage applications, differential inputs reduce problems
not only due to electrostatic and magnetic noise, but due to cross-talk
and thermal errors.
One problem to consider with differential inputs is the bias current
error. The differential impedance is usually hundreds of megaohms.
With a very small bias current multiplied by this high input
impedance, the voltage produced is out of the common-mode input
range of the instrumentation amplifier.
An external resistor must be provided to return this bias current to
the analog common of the data acquisition board. This resistor is
typically in the order of 1 k
Ω
to 100 k
Ω
from the input low side to
analog common. Alternatively, the external common can be returned
through a 10
Ω
to 100 k
Ω
resistor to analog common (it cannot be 0
Ω
due to ground currents).
