Lake Shore Model 370 AC Resistance Bridge User’s Manual
8.12 MODEL
370
CALIBRATION
The following paragraphs provide calibration information for the Model 370 AC Resistance Bridge. Calibration
information for the Model 3716 Scanner follows in Paragraph 8.13. There are three features of the Model 370 that can be
calibrated, the resistance measurement, analog voltage outputs and heater output. All of these calibrations are done
through computer interface and the calibration constants are stored in non-volatile memory. There are no “trim-pots”
inside the Model 370 and the cover does not have to be removed to calibrate the instrument. In contrast, the Model 3716
scanner is calibrated with a single trim-pot that is accessible from the scanner front panel.
8.12.1 Calibration
Interface
Computer interface commands are included in the Model 370 specifically for calibration. These commands work with
either the IEEE-488 or RS-232C interface. A complete description of each command is included in Paragraph 8.12.5.
Communication must be established between the instrument and a computer before calibration can be attempted. Some
time should be spent becoming familiar with the following commands before beginning a calibration. The RDGR and
RDGRNG commands are also important to calibration.
CALZ/CALZ?
Send or query the zero offset calibration coefficient for a measured input or calibrated output. (The
measurement current source does not require zero offset coefficients.) The zero offset is in the same
units as the input or output being calibrated and the default value is 0.
CALG/CALG?
Send or query the gain coefficient for a measured input or calibrated output. The gain coefficient is a
ratio of actual and ideal input or output and the default value is 1.
CALVIN?
Query the voltage measured at the instrument input for calibration of voltage measurement ranges.
Note: This value is not filtered.
CALSAVE
Issue the CALSAVE command to save any new calibration coefficients in non-volatile memory.
It is always recommended to read out old calibration coefficients using the CALZ? and CALG? interface queries before
attempting to calibrate. This will give the operator experience with the interface command, data formatting and typical
values. If the old values are saved, they can be reloaded in the case of accidental loss of data during calibration. New
calculated calibration coefficients should be very similar to the old values. Discrepancy between the old and new values
of more than 0.1% of gain calibration coefficients or 0.1% of range for zero coefficients could indicate an error in the
calibration procedure or a hardware failure. Do not attempt to recalibrate a damaged instrument.
The instrument will use the new calibration coefficients as soon as they are sent with the either the CALZ or CALG
interface command but they are not saved permanently until the CALSAVE command is issued. If a mistake is made in
the calibration process, turn the instrument power off and on again before CALSAVE is issued to restore the old
calibration constants. Once CALSAVE is issued, old values cannot be retrieved from the instrument.
If calibration coefficients are left at default or are outside of the normal calibration range, the following error message
will appear in the instrument display when the instrument is turned on: “Instrument Uncalibrated.” This error message
must be bypassed to allow calibration of the instrument. Press both Enter and Escape keys simultaneously to bypass the
error message. Operation in this state is possible but at least one calibration is known to be out of proper range and
measurement is likely to be erroneous.
8.12.2
Resistance Measurement Calibration
A complete measurement calibration of the Model 370 is very complex and time consuming. Make sure calibration is
necessary before returning the instrument for calibration or attempting to calibrate it. Verify the instrument accuracy by
comparing measurements of resistive standards to the specifications in Chapter 1 of this manual. Calibrating only the
current settings and voltage ranges that are actually being used can save significant time and effort. Lake Shore
recommends returning the instrument to the factory whenever an instrument is damaged or a complete calibration is
necessary.
Calibration coefficients for the measurement are used by the instrument firmware to compute an accurate resistance
reading. The sourced current and voltage at the instrument A/D converter do not change as a result of calibration. The
equation for calculating resistance is as follows:
Calibrated Resistance (RDGR?) = [(A/D Voltage/Ideal Gain) – CALZ(V)) × CALG(V)]/(Ideal Current/CALG(I))
8-16
Service
