Keithley SourceMeter 2425, Руководство по обслуживанию

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Model 2425 Service Manual Troubleshooting 4-11
A/D converter
The SourceMeter unit uses a multi-slope charge balance A/D converter with a single-slope
run-down. The converter is controlled by gate array U610. Commands are issued by the
MPU on the digital board through communications opto-isolators to U610, and U610
sends A/D reading data back through opto-isolators to the digital board for calibration and
processing.
Active guard
The Model 2425 has an active guard or “six-wire ohms” circuit used to measure complex
devices. This circuitry provides a low-current (50mA) equivalent of the voltage on output
HI. If the unit is in the SV mode, the low-current equivalent of the source voltage will
appear on the guard terminal. If the unit is in the SI mode, the voltage on output HI is
equal to the source current multiplied by the external resistance value. An equivalent
voltage will be generated by the guard circuit, and a guard sense terminal is provided to
sense around the voltage drop in the guard leads since significant current can flow
(50mA).
Digital circuitry
Refer to Figure 4-5 for the following discussion on digital circuitry.
The core digital circuitry uses a Motorola 68332 microcontroller running at 16.78MHz.
The memory configuration includes two 256K
×
8-bit flash EEPROMs and two 128K
×
8-
bit RAMs used in parallel to utilize the 16-bit data bus of the MPU. The RAM is battery
backed-up, providing continued storage of data buffer information during power-down
cycles, and flash ROM support allows internal firmware upgrades using either the serial or
GPIB port for downloading new firmware. All calibration constants and the save 0 setup
are stored in a separate serial EEPROM. Setups 1 through 4 are stored in battery backed-
up RAM.
External communication is provided via GPIB and serial interfaces. A 9914 GPIB IEEE-
488 standard interface IC is used for the GPIB, and a 68332 Queued Serial Module (QSM)
provides the serial UART. For internal communications, the Time Processing Unit (TPU)
is used for serial communications with the front panel display module, and both the TPU
and QSM handle digital-to-analog interfacing.