Scion Instruments 436-GC Service Manual page 97

436-GC/456-GC
Gated Integrator
The output of the electrometer is fed to the integrator, AR2 (pins 5-7), through a set of resistors and analog
switches. Between pulses, section 4 of U4 (pins 14-15) is open, while section 1 of U4 (pins 2-3) is closed.
This keeps C22 discharged and holds the output of AR2 (pin 7) near ground. When the desired integration
time begins, the shunt switch across C22 is opened, and section 4 of U4 is closed.
A current proportional to the output voltage of the electrometer then flows into C22 through the resistors
connected between AR2-1 and U4-14. The integrator output ramps in a negative direction until U4 (pins
14-15) is opened again. The current then stops flowing, and the integrator output remains constant.
Optimum dynamic range exists when a voltage of +10V at the electrometer output results in a peak
integrator output voltage of -10V at the end of the integration time.
Since the integration time varies with the mode selection, the resistance between the electrometer output
and the integrator input must vary in direct proportion to the integration time, in order to keep the peak
integrator output voltage constant. As the microcontroller adjusts the integration time, it also turns on the
appropriate analog switches, shunting some combination of resistors R22-R25, R31, and R32, to leave a
net resistance which is proportional to the integration time. Since the integration time is much shorter in
"SWEEP" mode than in normal operation, a dedicated resistor (R30) is switched in for this mode, with all
other integrator gain-setting analog switches open.
Sampler and Filter
At the end of the integration period, switch U5 (pins 14-15) is turned on for 500µs, charging C24 to the
output voltage of the integrator. The integrator can then be reset without affecting the detector output. The
high input impedance of AR3 (a unity-gain buffer) at pin 3 isolates C24 from the input of the following filter,
but leakage currents of a few picoamps may still exist. The saw tooth waveform produced by the leakage is
not visible at the detector output at normal pulse rates. If no pulses are detected for one second, the
microcontroller initiates a new cycle of integration and sampling anyway, preventing the sampler from
slowly drifting off into saturation.
A filter consisting of AR3 (pins 5-7) and its associated resistors and capacitors smooth's the steps coming
from the sampler. It is a two-pole filter, having repeated real poles, with a bandwidth of 0.9Hz. The DC gain
of the filter is -1, to compensate for the inversion which takes place in the integrator. R26 and C25 prevent
glitches due to switching of the system ADC multiplexer from disturbing AR3.
Trigger Comparator
The ouput of the electrometer (AR2-1) connects to the inverting input of the trigger comparator, U6.
Between pulses, the electrometer output voltage is lower than the threshold voltage at the non-inverting
input, and the open-collector output of U6 is pulled high by R43. When the electrometer voltage rises
above the threshold voltage, the comparator output goes low, generating an interrupt to the microcontroller
and starting the timing period. R42 provides about 15mV of hysteresis to prevent oscillations during the
relatively slow variations of the electrometer output voltage.
The desired trigger voltage at the non-inverting input of U6 is set by section A of dual DAC U13. AR4 (pins
1-3), together with R40 and R41, inverts the +10V reference voltage, applying -10V to the reference input
of U13. The DAC output (at AR4-7) is then 10 * N / 256, where the DAC setting N varies between 0 and
255. R35 and R36 divide the DAC output by about 4, giving a trigger voltage range at U6-2 of 0 to 2.55V.
Since the comparator is fast enough to respond to narrow noise glitches, C30 is placed inside the
electrometer shield to remove digital noise. R34 prevents C30 from slowing down the hysteresis feedback
excessively.
Triggering always occurs at 10nA input current in the normal operating modes, since the electrometer is
always set to the most sensitive range while waiting for a trigger pulse. If
a lower DC level is applied to the electrometer input, the comparator output will remain high, and the
microcontroller will initiate integration cycles once per second. At higher DC input levels, the comparator is
constantly in the triggered state. The microcontroller then starts a new cycle as soon as the 70ms holdoff
period from the previous cycle expires, producing 14 cycles per second. This faster rate makes viewing
waveforms on an oscilloscope much easier and speeds the reaction of the output voltage to operator
actions.
SCION Instruments
436-GC/456-GC Service Manual Revision B February 2019
610 Hardware description
Page: 96