Lake Shore Model 625 Superconducting MPS User’s Manual
Service
7-15
7.13.3.3
Calibrate Output Voltage Reading Zero
This step assumes that the previous two steps are successful and the current through the shunt resistor is quite low
resulting in virtually no voltage across the Model 625 output terminals.
1. Send “CALZ 5, 0” to set the current reading offset constant to 0.
2. Get the Model 625 output voltage reading (by front panel or interface).
3. Calculate zero offset constant:
–
(output voltage reading).
4. Send “CALZ 5, zero offset constant”.
5. Verify the Model 625 output voltage reading to be 0 ±0.0001 V.
6. Send “CALSAVE” to write this calibration to non-volatile memory.
7.13.3.4
Calibrate Remote Voltage Sense Reading Zero
1. Short the remote voltage sense lines (pins 1 and 9 of the analog I/O connector).
2. Send “CALZ 6, 0”. To set the remote voltage reading offset constant to 0.
3. Get the Model 625 remote voltage sense reading (by front panel or interface).
4. Calculate zero offset constant:
–
(remote voltage sense reading).
5. Send “CALZ 6, zero offset constant”.
6. Verify the Model 625 remote voltage reading to be 0 ±0.0001 V.
7. Send “CALSAVE” to write this calibration to non-volatile memory.
7.13.3.5
Calibrate External Programming Voltage Reading Zero
1. Short the external current programming input lines (pins 3 and 11 of the Analog I/O connector).
2. Send “CALZ 7, 0”. To set the external programming voltage reading constant to 0.
3. Get the Model 625 external programming voltage reading.
NOTE:
To get this reading from the Model 625, press and hold the
Status
key on the front panel until the display
goes dark (
3 seconds). When the key is then released, a diagnostics display will be seen. The upper right
reading, “REMOTE I”, is the reading needed for this step.
4. Calculate zero offset constant: – (external programming voltage reading).
5. Send “CALZ 7, zero offset constant”.
6. Verify the Model 625 external programming voltage reading to be 0 ±0.0001 V.
7. Send “CALSAVE” to write this calibration to non-volatile memory.
7.13.3.6
Calibrate Output Current Gain (Span)
This calibration is the most difficult and most important of the procedure. It first requires measuring the span of the
trim
range
(which changes from unit to unit within 1
–
2%) to determine the trim adjustment range that corresponds to a 100%
calibration trim change. In other words, we adjust the calibration in % from –100% to +100% but the size of the
calibration trim range changes slightly for each unit. We first measure the span of the calibration trim range and then
measure the full span of the instrument with the calibration trim set to 0%. We have adopted the use of % as the unit
name for the trim calibration since it corresponds to only the percent of trim range and
not
the full span of the
instrument.
The 55 A set point for calibration is chosen because the maximum trim range at 60 A forces the current past the 61 A
internal analog limit and therefore creates significant error. Yet another source of lesser error is the 25
parallel
damping resistor that is part of the output circuitry of the Model 625. While at a 0 volt output, this resistance contributes
no error but when there is voltage created by the current, this error becomes significant. Even though small, wiring
resistance may allow the voltage across the output terminals of the Model 625 to reach 0.5 V or so at 55 A depending
upon length. This reduces the measured shunt current by 0.5 V/25
or 0.020 A and therefore, must be considered during
the calibration. The shunt resistor remains connected to the output terminals for this procedure.
Summary of Contents for 625
Page 2: ...Lake Shore Model 625 Superconducting MPS User s Manual A ...
Page 5: ...Lake Shore Model 625 Superconducting MPS User s Manual D ...
Page 6: ...Lake Shore Model 625 Superconducting MPS User s Manual C ...
Page 8: ...Lake Shore Model 625 Superconducting MPS User s Manual E ...
Page 100: ...Lake Shore Model 625 Superconducting MPS User s Manual 6 2 Options and Accessories ...
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