Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Description of Schematics 10-15
SRS Residual Gas Analyzer
the quantization noise of the A/D is smaller than the shot noise of the I/V converter's bias
source, and to provide sufficient resolution.)
At the core of the detection system is the log I/V converter. A standard arrangement is
used, where the current is applied to the inverting input of a very low bias current op-amp
(U700) and a diode is used between the output and the inverting input of the op-amp.
Small signal JFETs transistors (Q700/701) are used as diodes, as they have well
characterized (and very low) reverse gate leakage currents.
A programmable current source is also applied to the inverting input of the op-amp. This
source serves two purposes: it is used to calibrate the gain of the I/V converter (which
changes with temperature), and it provides a bias current to the diodes during data
collection. As a calibrator, the current source can output currents from 0 to 5 nA. As a
bias source, it is programmed to provide 500 fA to 64 pA depending on the speed and
sensitivity required for a scan. (High speed scans require high bias currents which reduce
the time-constant though the diode, and high sensitivity scans require low bias currents to
reduce the shot noise of the bias current source.)
At 500 fA bias current, the Johnson noise current of the 1G
:
current source resistor
(about 4 fA/
—
Hz) establishes the noise floor for ion current measurements. At higher bias
currents, the shot noise of the bias current establishes the noise floor. For example, shot
noise associated with a 500 pA bias current is about 12.7 fA/
—
Hz. In addition, the
bandwidth of the detector increases with bias current, increasing the noise.
Back-to-back diodes are used to allow either ion currents (Faraday cup case) or electron
currents (electron multiplier case) to be measured. During calibration procedures, the ion
current is shunted to ground by a latching relay, U701. A latching relay is used so that the
relay coil will not be active during either calibrations or measurements.
The input offset voltage of the I/V converter's op-amp is nulled prior to calibrating the
converter's gain. This is done by strobing the MISC port bit (LINEAR) high, which
activates the relay U702, configuring the op-amp to a linear amplifier with a gain of 1001
for it's own input offset voltage. The CPU then aOFFSET to null the output of the
op-amp, nulling it's input offset voltage. The I/V converter is returned to the log mode by
strobing the LOG bit high.
A second and identical I/V converter resides next to the I/V converter described above.
The second I/V converter is operated and calibrated in the exact same manner, however,
collected ion currents are not applied to its input. The primary purpose of the second
channel is to correct the output of the first I/V converter against temperature effects.
Schematic name: QMSE_V2
Electron Multiplier High Voltage Power Supply
The electron multiplier option extends the operation of the RGA to much lower pressures.
By multiplying the ion current before detection in the I/V converter, the signal to noise
ratio is not affected by the bias current noise of the I/V converter. Since the gain of the
electron multiplier varies rapidly with applied bias, well regulated negative high voltage
supply is required.
A switching power supply, synchronized to a sub-multiple of the master clock at 43.2
kHz, followed by a voltage multiplier, is used to generate a negative high voltage. The