Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Electrometer 4-9
SRS Residual Gas Analyzer
setting available in the Scan Parameter Setups of the Scan menu is used to set the NF
parameter value in the RGA Head according to the equation: NF = ScanSpeed - 1.
The following table summarizes the performance of the RGA electrometer during mass
measurements as a function of the Scan Speed and NF settings. Please refer to this table
to estimate minimum detectable partial pressures and scan rates for different scanning
conditions.
Scan Speed
parameter
NF
parameter
Scan rate
(msec/amu)
Single mass meas.
time (msec)
Baseline noise
(a)
(std.dev.)(Amps)
1 0 2000 2200 7e-15
2 1 1000 1100 1e-14
3 2 400 440 1.5e-14
4 3 200 220 2e-14
5 4 126 139 4e-14
6 5 45 50 1.2e-13
7 6 30 33 2.5e-13
8 7 15 16.5 5e-13
(a) The values in this column are the result of averaging a large number of data points
from several different units. Use this values
for reference only . Variations are to be
expected from unit-to-unit, as a function of operating conditions and even as a function of
time.
Adjusting the Zero of the Ion Detector
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 memory. However, all offset correction factors
are cleared after a recalibration (CL) of the electrometer is performed, and when the unit
is turned off.
Please consult the RGA Command Set section for details on the CL and CA commands.