Scion Instruments 436-GC Service Manual page 88

436-GC/456-GC
The electrometer amplifier (AR1) is configured as an integrator by feedback capacitor C16, so any
imbalance between the input currents from the ECD cell and R27 at pin 2 causes the output voltage at pin
1 to slew up or down in response. The idealized output voltage should therefore be a linear ramp
downward due to the constant current from the DAC, with sudden rises in voltage when the pulses occur.
Because there is a large capacitive feedthrough of the pulse through the cell, which is much faster than the
amplifier can follow, there is a large glitch in the amplifier output at each pulse. C15 helps to absorb these
rapid changes in current, but the output waveform of AR1 is dominated by the glitches, which obscure the
idealized waveform at frequencies above a few kilohertz. R35 and C17 smooth the output waveform
further.
Exponential gain compensation.
When the instrument is operating at baseline conditions, a relatively large charge is delivered with each
pulse, resulting in a pulse frequency of about 1.6kHz. As the delivered charge per pulse decreases, the
frequency may rise as high as several hundred kilohertz to keep the average current constant. The gain of
the feedback path thus varies in inverse proportion to the pulse frequency, since the charge per pulse must
be inversely proportional to the pulse rate in order to maintain constant average current.
In order to keep the feedback loop gain constant as the pulse frequency changes, an element must be
introduced into the circuit whose output varies exponentially with its input.
The collector current of a bipolar transistor varies exactly this way with respect to its base-emitter voltage.
The filtered output of the electrometer is buffered by AR 1 (pins 5-7) to provide a low-impedance drive for
the emitter of Q4. Since the collector current of Q4 drives the current-to-frequency converter, and both the
transconductance of Q4 and the pulse frequency are proportional to the collector current, the desired
compensation is achieved.
CR7 and R29 limit the maximum frequency which can be set for the current-to-frequency converter.
Frequency settings which approach or exceed the reciprocal of the pulse width (1.6MHz) would cause
irregular pulse intervals, which must be avoided.
Diagnostic feedback circuit.
Because the electrometer is configured as an integrator, its output will inevitably swing to one of the power
supply rails if a feedback current (proportional to pulse frequency) is not applied to its input. Such a
feedback source is provided on the board, so the board may be tested without having an external
connection to an ECD cell. To enable this diagnostic feature, analog switch U3 (pins 14 and 15) is closed
and switch section U3 (pins 2 and 3) is opened. The output pulses are "stretched" by CR4 and C38, and
then summed with the Cell Current DAC output through R49 and R22. Since the average voltage at the
bottom end of R27 must remain at zero volts for the integrating electrometer to function, the circuit will
adjust the pulse frequency to make the average voltage at the anode of CR4 equal to the opposite of the
voltage at AR5-7. Linearity of the diagnostic circuit is improved by C39, which absorbs the pulses without
letting the voltage rise significantly. R23 holds the voltage at U3 (pins 2 and 14) below -15V even at the
highest frequencies, but has no effect on the operating point of the loop, since it has no DC voltage across
it. Stretching the pulses allows operation at lower frequencies at the desired voltage levels from the Cell
Current DAC.
Output amplifier.
The information from the electron capture detector is contained in the pulse frequency, so this frequency is
converted to a voltage for system use. The output pulse stream is averaged by R21 and C21. Since the DC
offset applied to the pulser output carries no signal information, its effect must be removed from this
averaged signal. The offset voltage, which appears at AR6-3, is inverted by AR6 (pins 5-7), R52, and R36.
Because R41 is equal to R21, the inverted voltage from AR6-7 cancels the pulser DC offset voltage, which
appears at J2-1 between pulses. AR4 amplifies the averaged pulse signal, with C20 providing further
smoothing. Resistor R40 can be paralleled with feedback resistor R37 through U3 (pins 6 and 7) to select
one of two gains for the output amplifier.
R42 adds an offset to the signal, so the normal resting frequency of 1.6kHz produces zero output voltage at
AR4-6. R39 and C28 prevent glitches from the system ADC input multiplexer from disturbing AR4.
SCION Instruments
436-GC/456-GC Service Manual Revision B February 2019
610 Hardware description
Page: 87