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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4-2
PRINCIPLES OF OPERATION
R11 are twice the value of R8 and R9. The reference voltage
of 3 V is present between the VREF signal at one end of R9
and the +3.6 V power supply present at one end of R11. The
voltage at one end of the sensing resistor string is present at
U11A pin 1 which feeds R8. The voltage at the other end of
the sensing resistor string is present at U11B pin 7 which
feeds R10. U11A and U11B are very high input impedance
buffer amplifiers which sense the voltage across the current
carrying portion of the sensing resistor string without
causing errors in the test current. R127, R129 and CR1
provide overvoltage input protection for pin 3 of U11A, and
similarly R46, R128 and CR2 for U11B. U2B will shut off the
current source if the voltage on the I+ binding post becomes
less than about -3 V.
Test Current Adjustment
The test current on the 2 milliohm range is 1.4 amps. It is
very stable, but not adjustable. The 1.4 A current stability is
determined by the stability of R54, the 3 V reference
developed by U4 and to a minor extent other circuit
components. The test currents on the four other ranges are
adjusted to approximately 1/10, 1/100, 1/1000 and 1/10000
of the actual value of the 2 milliohm range test current
during the calibration/ adjustment process. Trimpots R53,
R51, R49 and R114 are used for this adjustment, and are
adjusted sequentially in that order. Adjusting the current
affects the readings, so current adjustment INDEPENDENT of
the readings adjustment is not possible.
Voltage Measurement
The voltage across the resistance being measured is
amplified by U10A, U10B and U9B. The E+ and E- test jacks
are the input to this differential amplifier, and the output is
connected to the voltmeter IC U4 pins 38 and 39. R130,
R131, R141, R142, CR3 and CR4 provide overvoltage input
protection. U10A, U10B, R132, R133 and R134 provide a
differential gain of 5: the voltage between U10A pin 1 and
U10B pin 7 is 5 times the voltage between E- and E+. U9B
provides voltage gain of 10, and develops its output between
VREF at pin 40 of U4 and VIN+ at pin 39 of U4. An
