Stanford Research Systems SIM984, Руководство по эксплуатации и обслуживанию

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3 – 2 Circuitry
3.1 Circuit Descriptions
3.1.1 Isolated power
Page 3 of the schematics shows the isolated supply that powers the
input stage. A spread-spectrum oscillator (U302) provides the AC
drive signal for power amplifier U301 to drive the isolating trans-
former T301. The large capacitor C305 prevents any runaway DC
current from saturating the transformer, which could be generated
by o
ff
set voltages at U301.
3.1.2 Input amplifier
The upper portion of Page 2 shows the floating input amplifier. Gain
is controlled through latching relays U214 and U215, the control coils
of which are earth-referenced.
The (amplified) signal is optically coupled through U205, U206, U207,
and U208.
3.1.3 Output circuitry
The (earth-referenced) output circuitry includes the output band-
width control. This circuitry is on the lower portion of Page 2 of
the schematics. The overall AC gain is trimmed using VR202 at the
factory, and should not require user adjustment.
DC o
ff
set can be adjusted with VR203, accessed from the front panel
of the instrument. Note that the o
ff
set trim is referenced to the output,
and comes after the gain is applied.
3.1.4 Digital control
The SIM984 is controlled by microcontroller U107.
A critical aspect of the design is the clock-stop circuitry implemented
by U102 and U105. A simple RC-oscillator is enabled or disabled at
pin 1 of U102, which is driven by synchronizing flip-flop U105B to
ensure that no “runt” clock pulses are produced that would violate
U107’s minimum clock periods. Four separate clock-starting signals
are combined by U106:
•
Power-on reset
•
Amplifier overload
•
Incoming serial data
•
Front-panel button press
SIM984 Isolation Amplifier