ICS Advent ADIO1600 Series, Руководство по эксплуатации продукта, Страница: 46 / 72

Chapter 6: A/D Converter Applications
Manual Number: 00650-004-5 Page 6-1
Chapter 6: A/D Converter Applications
Connecting Analog Inputs
The ADIO1600 offers optional switch-selected eight-channel differential or 16-channel single-ended
input configurations. Single-ended configuration means that you have only one input relative to
ground. A differential input uses two inputs and the signal corresponds to the voltage difference
between these two inputs. Although the single-ended mode provides ability to accept 16 inputs
rather than eight, this configuration is suitable only for "floating" sources; i.e., a signal source that
does not have any connection to ground at the source.
Thus, if the local ground for the signal source is isolated from the system ground, the eight channel
differential configuration should be used. A differential input responds only to the difference in
signals between the high and low inputs. In practice, the signal source ground will not be at exactly
the same voltage as the computer ground where the ADIO1600 is because the two grounds are
connected through ground returns of the equipment and the building wiring. The difference be-
tween the ground voltages forms a common mode voltage (i.e., a voltage common to both inputs)
that a differential input rejects up to a certain limit. In the case of the ADIO1600, the common mode
limit is ±10V.
If you have a combination of floating and ground-referred signal sources, use the differential con-
figuration. For the floating signals, connect a jumper between the low input and the low level ground
at pins 28 or 29. The jumper connected between the low input and the low level ground effectively
turns that differential input into a single-ended input.
It’s important to understand the difference between input types, how to use them effectively, and
how to avoid ground loops. Misuse of inputs is the most common difficulty that users experience in
applying and obtaining the best performance from data acquisition systems.
Comments on Noise Interference
Noise is generally introduced into analog measurements from two sources: (a) ground loops and (b)
external noise. In both cases, use of good wiring practice will reduce and sometimes eliminate the
noise. A key point with regard to ground or return wiring is that in an analog/digital "system", digital
circuits should have a separate ground system from analog circuits with only a single common point.
The reason for separate ground busses is that digital circuits, by their very nature, generate consid-
erable high frequency noise as they rapidly change state.
Ground Loops
AC noise and DC offset can be added in series with a grounded signal source if the source ground
is at a different potential than the A/D’s analog ground. If there is an ohmic resistance between the
source ground and the A/D’s ground, the resultant current flow causes a voltage to be developed
and a "ground loop" exists. If the signal is measured in a single-ended mode, that voltage is added to
the source signal thereby creating an error. The best way to avoid ground loop errors is to use good
wiring practice as described above. If this is not possible, use of a differential measurement mode
will minimize errors.