10 TEGAM WAY • GENEVA, OHIO 44041 • 440-466-6100 • FAX 440-466-6110
4-2
PRINCIPLES OF OPERATION
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 adjustable bipolar offset
voltage is added to this output by R143 before it is applied to