Stanford Research Systems RGA100, Operating Manual And Programming Reference

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4-8 Electrometer
SRS Residual Gas Analyzer
Electrometer
Detection limit vs. scan rate
A unique, temperature-compensated, logarithmic picoammeter built into the ECU box
measures the ion currents collected by the Faraday cup (FC), or electron multiplier
(CDEM). The output voltage of the electrometer is equal to the logarithm of the ion
current so that several decades of signal can be read on the meter without any gain
switching being necessary. The microprocessor automatically configures the electrometer
and connects its input to the correct signal based on the type of detector being used.
The electrometer is completely autoranging and measures both positive and negative
currents with the same accuracy and resolution. Its operating range covers current
magnitudes between 1.32
u
10
-7
and 10
-15
A.
The accuracy of the measurements is insured by an internal calibration procedure that
calibrates the output of the electrometer against input current over its entire operating
range, and stores a calibration table in the non-volatile memory of the RGA Head.
Instead of simply relying on the intrinsic logarithmic I-V behavior of the electrometer, a
digital logarithmic interpolation algorithm calculates the currents from the calibration
curve.
The I-V response of the electrometer can be recalibrated at any time through a single
command (CL) supported by the RGA Windows software (Auto Calibrate option in the
Head menu), making it possible to generate accurate readings under different operating
conditions.
Excellent resolution is achieved using a 16 bit A/D converter to digitize the output of the
electrometer. The bandwidth and detection limit of the logarithmic electrometer are fully
programmable. This programmability is achieved biasing the electrometer with extra
current from a microprocessor-controlled current source. The biasing current is added to
the ion current and sets the gain and bandwidth of the electrometer during measurements.
An increase in the biasing current results in lower I-V gain and larger bandwidth, but also
means increased baseline noise due to the additional shot noise introduced by the extra
current. During regular mass scans the RGA automatically adjusts the scanning rate and
averaging to match the bandwidth of the detector. The microprocessor calculates the total
current flowing into the meter using the interpolation algorithm described above, and
subtracts the biasing current from the total to provide the actual ion current values used
for averaging. Obviously, the amount of biasing current must be chosen keeping in mind
the strong interplay between bandwidth (i.e. acquisition rates) and baseline noise (i.e.
minimum detectable partial pressures).
The single command NF (Noise Floor), supported by the RGA Windows program, is
used to program the electrometer’’s response. Histogram scans, analog scans, single-mass
measurements and total pressure measurements all share the same NF setting during
measurements. A decrease in the Noise-Floor setting reduces the biasing current resulting
in longer measurement times with cleaner baselines and lower detection limits. As
mentioned above, the NF parameter value must be chosen keeping in mind the
compromise between detection limit and acquisition speed.
When using the RGA Windows program to operate the RGA, the Scan Speed parameter