Measurement Computing DataShuttle - Manual (Page 98)

12-2 Applications Primer 11-12-01 DataShuttle and DynaRes
Sensor Connections
There are several general topics regarding sensor connections that must first be presented. These are single-
ended vs. differential connections (for analog input), auxiliary power output and voltage reference, auxiliary
components (sometimes needed to make certain sensors work), and digital line connections.
Single-Ended vs. Differential Connections
This concept refers to how analog input sensor wires connect to the terminal panel. A single-ended input
has only one wire from the sensor connected to each channel. The other wire from the sensor is connected
to a common return for all channels. In this case, the A/D converter measures the voltage difference
between the sensor wire and ground.
A differential connection means that there are two wires from your sensor that are connected to the terminal
panel. Here, the A/D converter measures the voltage difference between the two wires.
When using boards in differential mode, analog inputs connect to the + and – terminals on the terminal
panel. The reading is positive when the + terminal is positive with respect to the negative terminal. The
COM terminal is ground. Often, the COM terminal is connected to the – input with a jumper plate. When
the input voltage already has a ground reference, remove the connection to the COM terminal so that
current won’t flow between the two ground points, creating a voltage drop in the wire (a ground loop).
When using a data acquisition board in single-ended mode (if applicable), analog inputs 1 through 8
connect between the + terminal of channels 1 through 8, respectively, and GND on the terminal panel. The
reading is positive when the + terminal is positive with respect to GND. The reading is negative when the
positive terminal is negative with respect to GND. Channels 9 through 16 connect between the – terminal
of channels 1 through 8 respectively, on the terminal panel and GND.
The reading is positive when the – terminal is positive with respect to GND and the reading is negative
when the – terminal is negative with respect to GND.
Auxiliary Power Output & Voltage Reference
Many sensors, such as RTD’s, thermistors, strain gages, and pressure sensors, need a stable power source
because the output is proportional to the power applied. For this reason, a stable voltage reference is
available in all analog input and output boards (accessed via the terminal panel). Another advantage to the
voltage reference is that it can be used for easy recalibration of analog inputs (see your hardware manual).
All other analog input boards have a 6.6-6.9 Volt reference with a low impedance that can supply 20mA of
current (refer to the hardware manual for specifications). The ACAO-12 (analog output) has a 10 Volt
reference, also with a low impedance that can supply 20mA of current.
Alternatively, you can use the +5 Volt or ±12 Volt lines to provide power for your applications. These
sources, however, are typically noisy,
and may vary, so they do not make good reference voltages. Yet, they
are an excellent source of relatively large amounts of power for sensor electronics.
Auxiliary Components
For some sensors, it is necessary to install auxiliary components. These components provide signal
conditioning and can easily be installed in the terminal panel for convenience.
Installing auxiliary components requires soldering and some familiarity with electronics. Most common
sensors can be connected without the use of auxiliary components.
Refer to the your Terminal Panel manual for more information on installing these components.
Digital Connections
Wires for digital signals connect to either the IN (+) or OUT (–) terminal and GND. Most software
automatically changes each line to be either an input or an output when you use the line.