Circuit Discussion; Microcontroller Interface; The Amplifier - Stanford Research Systems SIM983 Operation And Service Manual

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5.1 Circuit Discussion

5.1.1 Microcontroller interface

5.1.1.1 Digital control and clock stopping
5.1.1.2 Power and grounds
5.1.2 The amplifier
5.1.2.1 Input voltage buffer
The following sections correspond to schematic pages at the end of
the manual.
The SIM983 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 U105. This pin is driven by synchronizing flip-flop U102B to
ensure that no "runt" clock pulses are produced that would violate
the minimum clock period of U107. Four separate clock starting
signals are combined by U103 and U104, as discussed in Section 2.4.
The fast start time of the RC oscillator ensures that incoming se-
rial data will be correctly decoded by the microcontroller's UART,
even when the clock is started by the serial start bit of the incoming
data. When the microcontroller has completed all pending activity,
it drives the STOP signal HIGH (Pin 71 of U107), e ectively halting
its own processor clock. In this way, the SIM983 guarantees that no
digital clock artifacts can be generated during quiescent operation.
A separate clean 5 V source is provided by voltage regulator U109
to power the analog circuitry of the amplifier. Each point in the
circuit that connects to Ground 2 (Section 2.1.2) is separately routed
to Pin 8 of interface connector J101, forming a star ground on Layer 3
of the circuit board.
The signal path in the SIM983 Scaling Amplifier consists of five
stages: the high-impedance input voltage bu er, the summing am-
plifier, the voltage inverter, the programmable gain stage, and the
passive LRC filter. Other parts of the amplifier circuit are the preci-
sion voltage reference, the o set voltage generator, and the output
microvoltmeter, used for autocalibration.
The input bu er is a high-impedance (40 pA max bias current), high-
slew-rate (1200 V s typ.), high-speed (105 MHz typ. small-signal
Circuit Description
SIM983 Scaling Amplifier