ACR Electronics OWL series Reference Manual page 36

OWL Data Loggers
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Maximum Impedance
An important consideration when choosing transducers is the requirement for input or loop
impedance. This simply refers to how much resistance your instrumentation (for example, the
OWL logger) can have before it will begin to cause problems. For voltage inputs, a very high
impedance is usually required (greater than 10k ohms). This prevents the unnecessary
draining of current from the transducer circuit. The OWL DC Voltage logger's input is high
impedance (greater than 100k ohm). Check the specifications for the transducer you intend to
use to ensure the OWL's input impedance is acceptable.
Three-Wire Transducers
Three-wire transducers have three connections: a supply (excitation) voltage input (V+IN), an
output voltage (SIG+) and a common (COM-). A three wire transducer can be identified by
reviewing the wiring diagram and looking for a single common connection. These types of
transducers are the most popular and are recommended for use with the OWL - see Figure
4-2.
Figure 4-2: Three Wire Transducer Connection
Four-Wire Transducers
Four-wire transducers fall into two categories: line-type and Wheatstone bridge types. The
isolated supply appears as a line operated or isolated common that has no electrical
connection to the output - see Figure 4-3. The bridge transducer provides a differential output
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