Stanford Research Systems RGA100, Руководство по эксплуатации и программированию

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10-14 Description of Schematics
SRS Residual Gas Analyzer
de-gas, the filament emission current is set to 20 mA, which will provide about 8W of
power to heat the grid, in addition to 15W of filament heater power.
Schematic name: QMSE_B3
Power Supplies
The unit is operated from +24VDC, and requires up to 2.0A. (The current draw will be
the highest when the unit is scanning at the high end of the mass range, or during de-gas.)
The unit is protected against power supply reversal by D610. A 5A circuit board mounted
fuse limits damage in the case of a severe circuit failure. The DC input is filtered by
C615, L601 and C616, which also reduce conducted emissions from the unit.
A DC-DC inverter is operated from the +24V supply to provide -140V for the repeller,
r
250V for the mass filter bias sources, and -20V for the negative linear voltage
regulators. The primaries to the inverter are driven by FET’’s. A switching power supply
regulator, U601, provides 12V square wave drive voltages to the FET’’s which are sync’’d
to a sub-multiple of the master clock. Overcurrent protection is provided by U601 on a
cycle by cycle basis. U601 does not regulate the outputs of the inverter, rather, its COMP
pin is tied high to provide the maximum duty cycle.
The first of three secondaries is full wave rectified by D611-D614. This output is used
only to bias the repeller, which allows the filament emission current to be monitored and
controlled with high precision. The next secondary has a pair of half-wave doublers
(C635, C636, and D615-D618) to provide
r
250V. The third secondary has a full wave
rectifier (D619-D622) to provide -20VDC, which is filtered by C622,C623 and L603.
All three secondaries are balanced so that no DC current is drawn in order to avoid
saturation in the un-gapped transformer core of T601.
The positive power supplies (+15 for the op-amps, +12 for the RS232 and MOSFET
drivers, +5_CLEAN for the signal multiplexers and A/D converter, and +5 for the logic
circuits) are derived via linear three terminal regulators (U602-605) from the +24V
supply.
The negative supplies (-15 for the op-amps, -12 for the RS232, and -5 for the A/D and
analog multiplexers) are provided by the linear regulators U606-608 from the -20V
generated by the main inverter.
Schematic name: QMSE_V1
Signal Conditioning
The output from the mass filter is a very small ion current which is proportional to the
pressure of the selected mass. The lowest pressure which may be measured by the
instrument is proportional to the lowest current which may be detected. Extreme care is
required to achieve low drift and low noise current measurements at these very low
current levels. In addition, the instrument needs to measure a wide range of pressures,
which requires current measurements over a wide range.
The sensitivity of the ionizer and mass filter is about 100
P
A/Torr, and is nearly constant
from 0 to 5x10
-5
Torr, so we expect ion currents from 0 to 5 nA. To accommodate this
large dynamic range, a log I/V converter is used, whose output is amplified, filtered, and
digitized by a 16-bit A/D converter. (The high resolution A/D is required to assure that