10 TEGAM WAY • GENEVA, OHIO 44041 • 440
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466
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6100 • FAX 440
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466
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6110
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4-3
PRINCIPLES OF OPERATION
adjustable bipolar offset voltage is added to this output by
R143 before it is applied to VIN- at pin 38 of U4. C18 and
140 are a low pass filter to reduce noise.
The voltmeter IC U4 converts the amplified voltage across
the resistance being measured into a displayed reading.
Because the test current is precisely regulated along with the
various gains chosen in the circuit, the displayed value is
directly proportional to the resistance being measured. U4
develops a reference voltage of 3 V between pins 8 and 40.
The reference is divided by the resistor string R31, R32,
R146 and R143. R146 provides an adjustable voltage used
as the analog to digital conversion reference to pin 44. R143
provides an adjustable zero reading.
Display
U4 directly drives the LCD, except for the decimal points and
the LOBAT indicator. The decimal point associated with the
range selected is displayed when the range switch SW1
provides a +3.6 V signal to one of the inputs of U7. The
LOBAT signal is displayed when U2A detects the voltage at
pin 40 of U4 falls below ground. Since this pin is maintained
by U4 at the reference voltage of 3 V below the battery
voltage, LOBAT will be displayed when the battery is less
than approximately 3 V.
