Lake Shore Model 218 Temperature Monitor User’s Manual
7-10
Service
7.13 CALIBRATION PROCEDURE
Both groups of sensor inputs require calibration. Sensor Input groups consist of 4 separate current
sources which can supply 10 µA or 1 mA of current. They are calibrated by adjusting pots on the
Model 218 main board. The Sensor Input groups consist of 4 inputs each with multiple gain
configurations to accommodate the various sensors the Model 218 supports. The input circuitry is not
adjusted during calibration. Instead, precision voltages and resistors are attached to each input and
mathematical calibration constants are calculated and programmed into the Model 218 to use to
compensate for input offset and gain errors. Refer to Paragraph 7.13.15 for details on calibration
specific interface commands.
7.13.1 Required Equipment List
1. PC with software loaded which provides serial command line communication.
2. DE-9 to DE-9 cable. Pin to pin connections on all 9 pins. Female connectors on both ends.
There is a 3 foot minimum.
3. DE-9 null modem adapter.
4. DVM with minimum 5 digits resolution.
5. Precision reference pro2.5 ±0.00001 V and -2.5 ±0.00001 V.
6. Four 200 k
Ω
resistor calibrated to ±2.0
Ω
.
7. Four
250
Ω
resistors calibrated to ±0.001
Ω
.
8. Four
5
k
Ω
resistors calibrated to ±0.025
Ω
.
9. Eight 100 k
Ω
resistor calibrated to ±0.5
Ω
.
7.13.2 SENSOR INPUT CALIBRATION SETUP
Allow the Model 218 to warm up for at least one hour with 100 k
Ω
resistors attached to all eight
inputs. Configure both input groups to the 2.5 V Diode range. Connect the Model 218 to the PC via
the serial port. Verify operational serial communication by sending the
Q
IDN? command and
receiving the proper response from the Model 218. During the calibration process leave four
100 k
Ω
resistors attached to the input group not currently being calibrated. Calibrate input group A
first then repeat the process for input group B.
7.13.3 Clear Calibration
Send the CALCLEAR command to return all calibration constants to their default value. Once cleared,
send the CALSAVE command to save the constants in the E
2
Prom. CAUTION: Once this step is
complete, the Model 218 sensor inputs must be completely calibrated for proper operation.
7.13.4 A/D Linearity Calibration
PURPOSE: To provide ground, positive, and negative full scale voltages to the input of the A/D to
allow it to self-calibrate linearity.
CONFIG:
Attach the pre2.5 V to the 1
st
inputs voltage terminals, the positive side
attaches to the positive terminal. Attach the precision -2.5 V to the 2
nd
inputs voltage
terminals, the negative side attaches to the positive terminal. Connect the ground of the
voltage reference to the negative input terminals of both inputs. Short the positive
current source terminal to the negative current source terminal on the 1
st
and 2
nd
inputs. On the 3
rd
and 4
th
inputs short all 4 terminals together. Do not tie the 4 terminals
to ground. Input group configured for 2.5 V input, all inputs of the group are enabled.
PROCESS: Via the interface send the ADCAL command specifying the input group to be
calibrated. The CALSAVE command must then be issued to save the A/D calibration in
the E
2
prom. Finally, the
Q
RST command is issued to reload both A/Ds with the
calibration data stored in the E
2
prom to both A/Ds.
