Scion Instruments 436-GC Service Manual page 84

436-GC/456-GC
Amplifier AR1 holds the collector of Q1 (pin 1) at ground potential and forces the collector current to flow
through R3. The final output voltage at P1-1 is then
where v is the sum of the input offset voltage of AR1 and all of the current-related voltage errors. Putting
os
in the nominal value of 25 microamps for I
This formula will not be accurate if the temperatures of all of the transistors are not equal. The two halves
of each dual transistor are always at the same temperature, since they are fabricated on a single silicon
die. Q1 and Q2 track each other in temperature only by being in close proximity in an isolated environment,
however, and any thermal disturbance will cause the output of the electrometer to drift. Simply touching
one of the transistors can cause a minute or two of drift, and the output may not stabilize for 5 or 10
minutes after soldering in the vicinity of the transistors.
The time constant of the amplifier is set to 45ms by Cl, and C2 eliminates high-frequency noise from digital
sources. R8 and C10 prevent glitches from the ADC multiplexer (on the main system board), which reads
the output voltage of the electrometer, from disturbing the amplifier output. Such glitches can couple
through C1 and the capacitance between the collectors of the two halves of Q1 to disturb the sensitive
input node, resulting in a much longer settling time than would be expected.
Electrometer diagnostic features.
In order to determine the input current from the output voltage, the value of vos in the equation in the
preceding paragraph must be determined. This is done during operation by closing the switch section of U1
(pins 1-3) which is connected to the output of the reference log amplifier. Current through the anti-log
transistors is thus cut off, leaving the output voltage at P1-1 equal to v
Though it is not immediately obvious from the equations presented here, closing the switch section of U1
(pins 14-16) which is connected to the output of the input log amplifier results in an output voltage at P1-1
which corresponds to an input current equal to the saturation current of Q1. This measurement is used for
diagnostic purposes only. Both of the diagnostic switch sections of U1 are left open in normal operation.
Because the input stage of the electrometer has a logarithmic response, its behavior becomes
unpredictable as the input current approaches zero. All of the voltages applied to the circuit are above
ground potential, so leakage currents should never cause the input to reverse in polarity and saturate the
output positively. However, the response time of the circuit is inversely proportional to the current level,
becoming very slow at currents below a few hundred femtoamps. For this reason, it is impractical to test
the noise and drift of the electrometer by "capping off' the input with no input current.
To test the electrometer when no external picoamp source is available, an input current source in the
picoampere range has been provided. This source relies upon the fact that the current through a capacitor
is equal to the product of the capacitance and the rate of change of the voltage across the capacitor. C4
(10pF) is connected to the electrometer input, which remains near ground potential. Applying a positive
sawtooth waveform to the other end of C4 should thus produce a steady input current, with negative spikes
on the negative transitions of the sawtooth voltage. This waveform (modified somewhat to optimize the
recovery of the electrometer from the negative spikes) is generated by an external DAC and applied to C4
through a filter consisting of R10 and C9. The filter removes noise from the external input, while slowing the
transitions somewhat. In normal operation, transistor Q3 is turned on by writing a logic 1 to U6-12, shorting
C4 to ground. Voltage variations at P1-10 will then not create an undesired current at the electrometer
input.
High voltage power supply.
The high voltage power supply provides a stable source of 190VDC at low current. It consists of a linear
regulator operating from the +24V supply, an unregulated flyback converter, and a zener diode regulator.
SCION Instruments
6
v = 2 .05 x 10 x sqrt ((I
out
4
v = 10 x sqrt(I + I
out in
436-GC/456-GC Service Manual Revision B February 2019
+I )(I )) +v ,
in sat ref os
gives the following approximate formula for the output voltage:
ref
) v
sat os.
610 Hardware description
.
os
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