SRS Labs QMS 100 Series, Руководство пользователя

Страница: 255 / 280

B-10 Description of Schematics
SRS QMS Gas Analyzer
-IRQ, the CPU sets CS_VETO high (which sets -CS_ADC16 high) and sets R/-C high to allow the
data to be read. The -BUSY output will remain low for up to 20
µ
s during a conversion. When -BUSY
goes high, the CPU returns CS_VETO low, which again asserts the -CS_ADC16 (this time with R/-C
high), and reads the data from the ADC via the SPI. Since the ADC shifts its data on the rising edge of
the data clock (i.e. data should be read on the falling edge of the data clock) the inverted SPI clock is
used as the data clock to the ADC. This will also permit simultaneous transmission of data to the 18-bit
RF amplitude control D/A. The CPU sets R/-C low after the data is read to prepare the A/D for the
next conversion cycle.
Power-up Conditioning
The A/D reference output (+2.50VDC) is amplified, buffered, and inverted by U302A&B to provide
system references of +/-5.00VDC. These references are set to zero volts when the system is reset so
that other analog signals (such as the HV and filament control signals) will stay at zero until set by the
microcontroller.
The triple 2:1 analog switch U301, a 74HC4053, is used to make certain that various circuits remain off
when the system is reset, until the CPU can establish the system environment. Upon reset the dual flip-
flop U300 is reset. The output of the second flip-flop, which controls all three channels of the analog
switch, will return high with the second port strobe from the CPU to the MISC bits port. Immediately
following a reset, EMIT_CTL will be low (to multiplex EMIT_SET to control the duty cycle of the
filament heater circuit directly), the
±
5.00REFs will be set to zero (so that all the analog control signals
will be zero), and the output current from the 18-bit DAC which controls the RF amplitude will be
shunted to ground.
Schematic name: QMSE_T3
DC Control Voltages
U307, a MAX528 octal 8-bit DAC, provides DC voltages to control various parameters in the system.
Each output has a range from 0 to 4.98V with a step size of about 19.5mV. The eight outputs and their
functions are listed here:
0 RES_CTL This output (-2.5V and times 1) increases the rods' DC.
1 EMIT_SET This output (times 1 or 10mA/V) sets the emission current.
4 RPL_SET This output (times -20) sets the repeller and filament bias.
3 FOC_SET This output (times -30) sets the focus plate bias.
7 HV_SET This output (times -500) sets the electron multiplier bias.
5 +OFFSET This output (-2.5 & times .001) fixes the detector offset.
5 -OFFSET This output (-2.5 & times .001) fixes the reference offset.
6 CAL This output (-2.5V then times -2) is the calibration source.