Data Translation DT300 Series, User Manual

Page: 151 / 164

Using Your Own Screw Terminal Panel
151
Analog Inputs
DT300 Series boards have three different types of analog input configurations that you can
use:
• Single-ended
• Pseudo-differential
• Differential
Single-Ended Inputs
With single-ended inputs, you have the maximum number of inputs but have the worst-case
noise immunity without external signal conditioning.
The major problem with this configuration it that you need a common ground between the
external inputs and the data acquisition board. Even with conditioning, consideration must be
given to the cable length and how the cable is routed. If the cable is over 3 feet, you must
consider the ringing and cross-talk in the cable. A typical cable has 30 pF per foot of
capacitance. If the source impedance is 1,000
Ω
and the cable is 3 feet, then the cross talk based
on the source impedance is 1,000
Ω
x (30 pF x 3 ft) = 90 ns.
This seems negligible, but when you consider that it requires nine time constants to settle
within 0.01%, the cross-talk becomes almost 10% of the time required to settle when switching
channels at 100 kHz.
Coupling must also be considered when adjacent channels have high-speed signals, especially
if these signals are TTL-type with high-speed edges.
Pseudo-Differential Inputs
Pseudo-differential inputs allow one common-mode voltage for all single-ended inputs. With
this type of connection, the low side of the instrumentation amplifier is used to sense an
external common-mode voltage. For example, if you have a signal-conditioning rack, the
AMP LOW signal connects to the analog common of the external rack.
The pseudo-differential configuration allows you to use the maximum number of input
channels, while placing an impedance between the external ground and the data acquisition
ground or analog common. Even if it is 100
Ω,
this impedance provides the bias return
currents for the inputs and causes only 10 mA of current to flow with a ground potential
difference of 1 V. (The input bias current is typically in milliamperes.) This is usually
manageable by the common-mode range of the instrumentation amplifier and analog ground
system. Consider the problems with 1
Ω
of impedance between 1 V of potential difference.
The resulting 1 A of current causes many problems in the analog signal integrity.
If it is provided and not used, ensure that you connect AMP LOW to the analog common of
the data acquisition board or to ground when running in single-ended mode.