Model SR715 Model SR720 LCR Meters

CALIBRATIONINTRODUCTIONCalibration of the SR715/720 is composed of several parts: open and short circuit calibration, standard resistorcalibration, and amplitude calibration. The open and short circuit calibrations are offset corrections to correctfor any stray impedances of the test fixture. These calibrations may be performed at any time and should bedone whenever the fixture is changed. The standard resistor calibration sets the accuracy of the SR715/720since it allows the LCR meter to determine the values of its internal standard resistors. Standard resistorcalibration need only be performed periodically to account for component aging and drift. Amplitude calibrationsets the amplitude of the ac test signal, and only needs to be done periodically. The amplitude calibration doesnot affect the SR715/720's accuracy.Calibration EnableCalbytesThe SR715/720 is shipped with standard resistor and amplitude calibrationdisabled. When calibration is disabled only the open and short circuit calibrationsare allowed. The internal calibration enable jumper must be set to enableamplitude and resistor calibration. To set the jumper, remove the SR715/720'stop cover by removing its four retaining screws (this will break the calibrationseal). In the rear center of the circuit board is a three pin jumper labelled JP1001.Use the jumper to connect the center pin and the "cal" pin to enable calibration.Connecting the center pin and the "normal" pin will disable calibration.The SR715/720's calibration is controlled by calibration constants ("calbytes") thatthe firmware uses to adjust the impedance calculations. These calbytes arestored in the SR715/720's RAM. Recalibration of the SR715/720 involvesdetermining the values of the calbytes and storing the new values in RAM. Thecalbyte values determined at the time of the SR715/720's manufacture are alsostored in ROM and may be recalled at any time. The standard resistor calbytesare automatically determined by the standard resistor calibration subroutines. Theamplitude calbytes must be manually determined.A list of the SR715/720's calbytes is shown on the following page. The open andshort circuit, and resistor calibration calbyte values are automatically determinedby the SR715/720's calibration subroutines. These calbytes cannot be directlychanged from the front panel (they may be changed via the computer interfaces).The amplitude calbytes are directly adjustable from the front panel.For the SR715 only: The values for the 100 kHz calbytes are set to zero and donot need to be adjusted.5-10