3 GENERAL DESCRIPTION AND CALIBRATION
The Model 5M78 is useful in applications involving tranformer coupling to the strain gage
bridge (for example, rotary transformer torque sensors) and in certain appications that require high
sensitivity with optimum signal-to-noise characteristics. Carrier amplifiers offer higher sensitivity
than dc amplifiers and, since they respond only to the modulated carrier frequency, they reject certain
extraneous voltages that can cause error in dc systems. These error sources include thermocouple
or galvanic voltages in the cable-connector system, homopolar voltages from rotating machinery,
low-frequency pickup, and 1/f noises of various origin.
Calibration of the 5M78
is accomplished by the conventional shunt technique, using an internally
(or externally) installed calibration resistor, or via the "dead-weight" method.
Calibration Resistor.
If a fixed resistor is shunted across one arm of a strain
gage bridge, it produces an unbalance equivalent to that of a particular value of
mechanical input. If this Equivalent Input value is accurately known, it can be used
as a reference point for shunt calibration of the system. Upon completion of
installation of the transducer and its associated cabling, the user can:
Perform an overall dead
weight calibration using a precisely known
value of mechanical input. The calibration can then be transferred to the
installed calibration resistor for convenience in subsequent checking.
Replace the installed calibration resistor with one (or an equivalent
resistance value) supplied by the transducer manufacturer to achieve a
precisely known Equivalent Input allowing the instrument sensitivity to
be adjusted correctly.
Determine the Equivalent Input value for the installed calibration resis-
tor, knowing the transducer sensitivity, and adjust the instrument sen-
sitivity accordingly.
A one percent 59-kilohm calibration resistor is installed in the 5M78 at the factory.
The installed resistor can usually be used even though the transducer calibration
data mentions some other resistance value. In Section 4 of this manual, the tech-
niques described above are demonstrated. If, however, the installed value of the
calibration resistor is not appropriate for the transducer and measurement range to
be used, the 59-kilohm resistor should be replaced at this time. The calibration
resistor is mounted on terminals located internally to the 5M78 conditioner's
printed-circuit board. It can be accessed by removing the instrument case (Fig. 5).
Note:
A variety of Rotary Transformer Torque Transducers are supplied
with the appropriate calibration resistor integral to the transducer. When this type
of transducer is used it will be necessary to place a short across the 59-kilohm
resistor internal to the instrument. The transducer calibration resistor can be appropri-
ately connected to the 5M78 calibration circuit via the transducer cabling. Refer to
cabling diragam section of this manual in the area of CAL SENSE.
7
(1)
(2)
(3)
Calibration 5M78.3
