Keithley 2750, Service Manual

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4-12 Troubleshooting Model 2750 Multimeter/Switch System Service Manual
Multiplexer and A/D converter
All input signals, except FREQ, are routed to the A/D MUX & Gain circuit. The multi-
plexer (U163) switches the various signals for measurement. In addition to the input sig-
nal, the multiplexer also switches among reference and zero signals at various phases of
the measurement cycle.
When the input signal is selected by the MUX, it is amplified by U132 and U166. Gain is
controlled by switches in U129 and associated resistors.
The multiplexed signals of the measurement cycle are routed to the A/D Converter (U165)
where it converts the analog signals to digital form. The digital signals are then routed
through an opto-isolator to the MPU to calculate a reading.
The multiplexer amplifier has an overload protection circuit. The circuit reduces the mul-
tiplexer gain to unity during overload conditions. This circuit is made up of primarily
CR101, CR113, Q103, Q107, Q117, U128, U140, U143, VR110, and VR111.
Scanner card signals
Scanner card input signals are connected directly to installed scanner cards. Scanner card
output signals are routed internally to the INPUTS switch, which selects between the front
panel terminals and the scanner card outputs.
The Model 7700, 7706, and 7708 modules contain CJC (voltage) temperature sensors.
These sensors are multiplexed into U163 at various phases of the measurement cycle.
Dry circuit ohms
Figure 4-4 shows a simplified schematic of the dry circuit ohms circuitry. Dry circuit
ohms measurements are made similar to those for conventional ohms in that a 10

A to
10mA current source (depending on ohms range) forces a current through the DUT resis-
tance, and the voltage across the device is measured. In the case of dry circuit ohms, how-
ever, there are two clamps that act to limit the voltage across the DUT to 20mV. Normally
the voltage is sensed across the SENSE HI and LO terminals, and the main clamp restricts
the DUT voltage to 20mV. A secondary clamp monitors the voltage across INPUT HI and
LO and acts to restrict the voltage to 50mV if the SENSE terminals are not connected to
the DUT. When either clamp acts to limit the voltage across the DUT, the current source
goes into compliance, effectively turning the current source into a voltage source thus lim-
iting the DUT voltage to 20mV or 50mV as the case may be. Note that a 0.8

F capacitor
and 100k¾ resistor are connected across the DUT during dry circuit ohms measurements
for stabilization.