Stanford Research Systems RGA100, Operating Manual And Programming Reference

Page: 118 / 362

6-14 Programming the RGA Head
SRS Residual Gas Analyzer
Programming the Detector
Positive ions that successfully pass through the quadrupole filter are focused towards a
detector that measures the ion currents directly (Faraday Cup, FC) or, using an optional
electron multiplier (CDEM), measures an ion signal proportional to the ion current.
Use the Detection Control commands to choose the detector type (FC or CDEM), query
the CDEM option, recalibrate the electrometer’’s I-V response and set the electrometer’’s
averaging and bandwidth. Use the CA command to zero the ion detector at any time.
Important : Faraday Cup detection is the default setting when the RGA head is turned on.
Use the MO? command to determine whether an electron multiplier option (Option 01) is
available in the RGA head being programmed. The CDEM is available if the response is
1<LF><CR>.
The HV command activates/deactivates the CDEM detector. Its parameter value
represents the bias voltage applied across the multiplier, and determines the gain of the
amplifier. The null parameter, HV0, deactivates the CDEM and reconnects the FC
detector to the electrometer.
Note: The ElecMult LED reflects the status of the CDEM at all times.
A temperature compensated logarithmic picoammeter measures the ion currents collected
by the FC or CDEM. The microprocessor automatically configures the electrometer and
connects its input to the correct signal based on the type of detector being used. The sign
of the electron multiplier currents is reversed before the current value is sent out over
RS232 so that the computer does not need to do any sign flipping on the currents
received when the CDEM is activated. The accuracy of the ion current measurements is
insured by an internal calibration procedure that calibrates the output of the electrometer
against input current over its entire operating range. The I-V response can be recalibrated
by the user at any time with the CL command.
The bandwidth and detection limit of the electrometer are programmed with the NF
(Noise Floor) command. A decrease in the Noise-Floor setting results in longer
measurement times with cleaner baselines and lower detection limits. Histogram scans,
analog scans, single-mass measurements and total pressure measurements share the same
NF setting. The RGA automatically adjusts the scanning rate and averaging based on the
NF setting selected.
Important: The zero of the ion detector is automatically readjusted at the beginning of
each analog and histogram scan so that the baseline is always centered around zero. The
zero can also be readjusted at any time with the command CA. In both zeroing
procedures, the output of the electrometer is measured in the absence of input ion current
and stored, as a current value, in the RGA’’s memory. The current value (called offset
correction factor) is then automatically used by the firmware to offset-correct all ion
currents measured (i.e. including those from single mass and total pressure
measurements) under the same detector settings. The detector settings are the
electrometer’’s noise floor parameter value and the type of detector (i.e. FC or CDEM) in
use at the time the zeroing is performed. Offset correction factors for all the possible
combinations of detector settings can be generated and accumulated in the RGA’’s