3 GENERAL DESCRIPTION AND CALIBRATION
The Model IL78A 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 IL78A
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.
Install an external
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.
Move jumper
from INT to the ETX terminals, install resistor between Cal Sense and + or - Signal.
Determine the
Equivalent Input
value for the installed calibration resistor
(59K)
,
knowing the transducer sensitivity, and adjust the instrument sensitivity accordingly.
Jumper to be in the INT location.
A one percent 59-kilohm calibration resistor is installed in the IL78A 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 techniques 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
disabled by placing the Shunt jumper in the "EXT" position and an external shunt resistor
should be used
.
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
move the Shunt jumper to the EXT position and connect the CAL Sense wire
to the shunt activation pin of the transducer
. Refer to
Fig 1
cabling di
a
g
r
am section of this
manual in the area of CAL SENSE.
7
(1)
(2)
(3)
Calibration IL78A.3
AC Strain Gage Conditioner
Module
IL78A
