Ac Volts Measurements; Resistance Measurements; Ohms Connections - HP 3478A Technical Manual

all full scale inputs (e.g. 10V input to the A/D for 30mV,
300mV, 3V, etc. inputs to the 3478A).
3-21. When the DC Volts Function is selected, the
right side of the display shows "MVDC" or "VDC",
dependent on the range selected. The reading can be
displayed either as a 5 1/2, 4 1/2, or 3 1/2 digit reading,
dependent on the selected Number Of Digits Displayed
(see paragraph 3-54). Refer to Table 1-1 or Table 4-1 for
the DC Volts Function's accuracy specifications.
3-22.

AC Volts Measurements

3-23. The 3478A uses a True RMS AC to DC
Converter to measure ac voltages from 1uV to 300V in
four ranges: 300mV, 3V, 30V, and 300V. The response
of the converter is from 20Hz to 100KHz on all ranges (to
300KHz on the 30V Range only). All ranges are
protected from input voltages up to 450V peak. Select
the AC Volts Function for ac volts measurements by
pressing the - v button.
3-24. The AC to DC Converter changes the ac input
voltage to dc volts, which is then measured by the
3478A's A/D Converter. All ranging is done in the AC to
DC Converter, applying the same voltage value to the
A/D Converter for all full scale inputs.
3-25. When the AC Volts Function is selected, the
right side of the display shows "MVAC" or "VAC",
dependent on the range selected. The reading can be
displayed either as a 51/2, 41/2, or 31/2 digit reading,
dependent on the selected Number of Digits Displayed
(see paragraph 3-54.) A .6 second delay is also applied
in the ACV Function before a reading is taken (also,
during a range change). Refer to Table 1-1 or Table 4-1
for the AC Volts Function's accuracy specifications.
3-26.

Resistance Measurements

3-27. The 3478A is
measurements from .0001 ohms to 30M ohms in seven
ranges. The ranges extend from the 30 ohm to the 30M
ohm range. All ranges are protected from input voltages
up to 350V peak. Resistance measurements can be
made using either the 2-Wire or 4-Wire ohms
configuration, which are selected by the 2 WIRE Ω and 4
WIRE Ω buttons, respectively. Refer to Figure 3-4 for the
correct ohms connections.
3-28. Resistance measurements are made by applying
a known current (generated by the 3478A) to the
unknown resistance. The resultant voltage drop is then
measured by the 3478A's dc circuitry (Input Circuitry and
A/D Converter). In the 2-Wire Ohms Function, the
voltage drop is measured across the HI and LO INPUT
Terminals. In the 4-Wire Ohms Function, the voltage is
measured across the HI and LO Ω SENSE Terminals.
able to make resistance
The Ohms Current Source generates the known current
Figure 3-4. Ohms Connections
which is applied to the unknown resistance (in both ohms
functions) from the HI INPUT Terminal. The current
values are as follows:
Range
30 ohm
300 ohm
3K ohm
30K ohm
300K ohm
3M ohm
30M ohm
3-29. When an Ohms Function is selected, the right
side of the display shows either "OHM", "KOHM", or
"MOHM", dependent on the range selected. In addition,
the "2 Ω" annunciator is on for the 2-Wire Ohmns
3-4
TM 11-6625-3071-14
Current
1mA
1mA
1mA
100uA
10uA
1uA
1uA