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5725A
Instruction Manual
4-30
to approximately 33Ω/°C by R615. Op amp U603A provides a reference voltage of
about -0.77V. This reference voltage is applied to the circuit containing summing
amplifier U603B, R611, and the linearized thermistor. The output of U603B is signal HV
TEMP M, equal to 10 mV/°C.
A voltage proportional to the temperature rise from the heat sink to the junction of the
transistor is added to HV TEMP M through R610. This is done via +400V IM from the
Power Supply assembly. Resistor R610 is selected to properly scale this contribution to
the 10 mV/°C output of U603B. Capacitor C610 provides averaging for the ripple on the
current-sensing resistor, and CR604 prevents C610, an electrolytic capacitor from being
reverse charged.
4-54. High Voltage Sense Assembly (A6)
In addition to ac voltage sensing for which it is named, the High Voltage Sense assembly
provides four major functions for the 5725A. These additional functions are the
following:
• Analog monitoring of the 5725A status
• Interface with the 5700A controller
• Switching input signals from the 5700A
• AC Line Power switching
Because it performs all the above functions, the High Voltage Sense assembly plays a
primary role in controlling the instrument, attaining ac voltage specifications,
performing 5725A calibration, and maintaining instrument safety.
The High Voltage Sense assembly has the following six sections: sense amplifier, analog
monitor, serial interface/guard crossing, analog input switching, control, and mains
switching. Each section is first briefly described, then described in detail. For reference,
see Figure 4-5, the block diagram for the High Voltage Sense assembly.
• Sense Amplifier Section
The sense amplifier section consists of a precision ac amplifier configured as an
active attenuator, a buffer amplifier that isolates the 5725A high quality ground from
the 5700A ground reference, and a precision true rms converter circuit for 5725A
calibration. Together, these circuits accurately attenuate the output of the 5725A in
ac voltage function to drive the 5700A Oscillator control circuitry. The sense
amplifier section is essentially a gain-determining feedback element of a loop whose
forward path consists of the 5700A Oscillator and the High Voltage Amplifier
assembly (A3).
• Analog Monitor Section
The analog monitor section is comprised of a bipolar output dac, a comparator, and a
16-channel multiplexer. The inputs to the multiplexer are scaled voltages
representing critical parameters in the other 5725A assemblies, particularly those
involving high voltage and/or current. Each input can be compared to programmed
limits set by the dac under microprocessor control. The comparator gives simple
go/no go decisions, which are then acted on in a more sophisticated way by the
software. An additional function of the analog monitor section is to provide access to
the B-RCL line used by the 5700A during 5725A calibration.