Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Description of Schematics 10-13
SRS Residual Gas Analyzer
2.766VDC.) The emission current may be set to values as high as 50 mA during the de-
gas procedure.
The primary side current drive may be sensed in the 0.5
:
FET source resistor. The
voltage across this resistor is filtered and amplified by 6X, and may be measured by the
CPU. The duty cycle of the FET switches may also be measured by the CPU via the
voltage labeled "FIL_DUTY". A drop of about 40% in the primary side current for a
particular duty cycle value indicates that one of the two parallel filaments has burnt out.
A drop of 90% indicates that both filaments are missing.
If the source sense resistor voltage reaches 1V (when the primary side current reaches an
instantaneous value of 2A) the LT3525 will terminate the gate drive to FET for the
remainder of the cycle. This feature protects the circuit in the case of a short circuit or
other severe fault.
Bias Regulators
The regulated repeller and focus plate biases are controlled by the CPU. The set voltages,
RPL_SET and FOC_SET are multiplied by -20 and -30, respectively, by the bias
regulators.
If the control signal RPL_SET is increased, the non-inverting input (and so the output) of
the op-amp U502B will go up, increasing the current in the opto-isolator’’s LED,
increasing Q502’’s collector current, increasing the magnitude of the (negative) repeller
bias.
If the control signal FOC_SET is increased, current is injected via the 61.9k
:
resistors
R538 and R539 directly into the emitter of Q506 and away from the emitter of Q505
which are biased at ground. These currents are mirrored by Q503/504 and Q507/508 to
the output node, which has a 1M
:
resistance to ground via R531, providing an overall
gain of -30.
Imbalances in currents due to focus plate currents or current mirror mismatches are made
up by the op-amp U504A. If the output is smaller (less negative) than it should be, the
non-inverting input to U504A will be positive, causing its output to ramp up. This
increases the current into the emitter of Q506 and decreases the current into Q505. These
currents are mirrored to the output, increasing the current in Q508 and decreasing the
current in Q504, pulling the output more negative. The output of U504A will go up by
1V for each 1
P
A of ion current collected by the focus plate. The output may be digitized
by the CPU to monitor the ion current in order to watch for excessive operating pressure.
Since the focus plate may collect ions when set to any potential below the grid potential,
and since it may collect electrons when set to any potential above the repeller voltage, the
focus plate regulator is designed to sink or source current.
The grid voltage is supplied by the op-amp U502A. This op-amp is operated with a gain
of six to amplify the voltage selected by the analog switch U500. The op-amp is
protected from possible discharges, which would connect the grid to the repeller
potential, by R517 and D500.
For a high power de-gas, a DPDT relay is used to by-pass the bias regulators, connecting
the grid and repeller directly to the un-reg250Vdc and -150Vdc supplies. During