Stanford Research Systems RGA100, Operating Manual And Programming Reference

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10-12 Description of Schematics
SRS Residual Gas Analyzer
DC Potentials
In addition to the RF, DC potentials of about
r
1/12th the RF peak-to-peak value is
required for the two rod pairs. The op-amp U403A is the error amplifier which maintains
the negative potential equal to a fraction of the set RF level. When DC_SET goes up, the
output of the op-amp goes up, increasing the current in Q406, increasing the current in
Q407, bringing down the collector of Q407, which is the output of the negative potential
voltage regulator.
The op-amp U403B is the error amplifier for the positive potential. The positive potential
is compared to the negative potential by R431 and R432. When the negative potential
increases, the non-inverting input to U403B will go down, lowering the op-amp output,
increasing the current in Q405, increasing the current in Q404, thereby increasing the
positive potential until it matches the magnitude of the negative potential.
Schematic name: QMSE_B2.
Filament Heater Supply
The sensitivity of the quadrupole depends on the filament emission current, which
depends on the filament temperature and bias with respect to the grid. As the grid to
filament bias is fixed to optimize the electron impact ionization cross section, the
filament emission current is controlled by controlling the filament temperature via the
filament heater current’’s duty cycle.
A switching power supply, which is synchronized to a sub-multiple of the master clock at
172.8 kHz, is used to provide heater power to the filament. (Note that two clocks are
required for each cycle, so the primary drive current is at 86.4 kHz.) The switcher’’s
transformer has a 6:1 turns ratio, reducing the +24V input to a 4V output, which leaves
about 3.5V after rectification in the dual Schottky diode, D501.
The primary of the transformer is operated from the +24VDC supply, and is driven
differentially by a pair of IRF530 MOSFET’’s. An LT3525 switching controller, U501, is
used to drive the FET gates. The duty cycle of the gate drive depends on the voltage at
the COMP pin (pin 9). A voltage of about 0.9V or below will set the duty cycle to zero.
The duty cycle increases to a maximum near 50% (for each gate) as the voltage on this
input is increased to about 3.3V.
The analog switch, U500, controls the source of the signal to the COMP pin. If
EMIT_CTL is low (as it is after a reset) then the DAC signal EMIT_SET is connected
directly to the COMP pin allowing for direct control of the filament heater’’s duty cycle
by the CPU. This mode is used when the filament is first turned ““on”” to verify that the
pressure is low enough to allow the filament to remain ““on””.
When EMIT_CTL is set high, the COMP pin is connected to the compensation capacitor
C500, which will be charged and discharged by the transconductance error amplifier in
U501. The error amplifier will compare the EMIT_SET voltage (or the attenuated
EMIT_SET voltage, depending on the state of the DE_GAS bit) to the voltage seen on
the 100
:
emission current shunt resistor, R522. (The current sensed by this resistor is
actually composed of three components: the filament emission current, the repeller
voltage/100k
:
, and Vref (+5V)/30k
:
. All of these components sum. With a repeller
voltage of -60V, the two non-emission sources sum to 0.60 + 0.166 = 0.766 mA. So, to
set an emission current of 2 mA, EMIT_SET, which provides 1 mA/V, is set to