DayTronic IL78A Manual - Page 10

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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

(1)

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)

(2)

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),

(3)

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 diagram section of this

manual in the area of CAL SENSE.

AC Strain Gage Conditioner Module

IL78A

Calibration IL78A.3