SRS Labs QMS 100 Series, Руководство пользователя

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B-14 Description of Schematics
SRS QMS Gas Analyzer
DE_GAS bit) to the voltage seen on the 100
Ω
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
Ω
, and Vref (+5V)/30k
Ω
. All of these components sum. With a repeller voltage
of -60V, the two non-emission sources sum to 0.60+0.166=0.766mA. So, to set an emission current of
2mA, EMIT_SET, which provides 1mA/V, is set to 2.766VDC.) The emission current may be set to
values as high as 50mA during the de-gas procedure.
The primary side current drive may be sensed in the 0.5
Ω
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
Ω
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
Ω
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
µ
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.