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on the CURRENT HI terminal. The output of this comparator is used to turn off the output
stage of the current source when the selected open-circuit voltage limit is exceeded.
Analog-to-Digital Converter
The A-to-D converter is basically a high-impedance voltmeter that is used to measure the voltage drop
across the unknown load. This voltage is converted to digital form to represent the resistance value of
the load. The A-to-D converter circuitry is shown in the drawing number 4100-074 sheet 2.
A-to-D Converter
The A-to-D conversion is performed by the IC pair IC103-IC104 and their associated components.
IC103 is the digital portion of the converter and IC104 is the analog portion. The required reference
voltage is generated by IC106. IC102 is used to demultiplex the digital outputs of the converter and to
condition it for use by the display LEDs DS1-DS5.
Preamplifier
Ohmmeter Models 4150ATC, 4165, and 4165-1344 use a preamplifier stage consisting of IC101 and its
associated components to boost the voltage input to the A-to-D converter up to the required levels. The
preamplifier is a chopper stabilized x10 amplifier. This stage raises the 20mV full-scale test voltage of
these instruments up to the 200mV full-scale voltage required by the A-to-D converter.
Optional Interfaces
The BCD interface takes multiplexed digital information from the voltmeter and converts it to a parallel
BCD 8-4-2-1 code that is easier to work with. This circuitry is shown on drawing number 2053-076 at
the back of this manual.
The other interfaces (IEEE, RS232C, and printer) are made possible by Valhalla's 1020 Smart Interface
Board. The 1020 takes range information from contacts of the range switches and digital data from the
voltmeter. It decodes this information and makes it available at any of four ports. A complete
description of the operation of this interface is beyond the scope of this manual. Please contact Valhalla
Scientific if problems are encountered.
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