TM 11-6625-3071-14
3478A
Figure 7-F-1. 3478A Simplified Block Diagram
c. Ohms Function. If the Ohms Function is
selected, the Ohms Current Source supplies a known dc
current to the unknown resistance. A voltage drop
proportional to the unknown resistance and the current is
applied to the Input Terminals. A regular dc
measurement is then made (see step b) and the ohms
reading is calculated by the Main Controller circuitry
using the zero measure- ment (see step f) and
calibration constant(s).
d. AC Volts Function. If the AC Volts Function is
selected, the input is connected to the AC to DC Con-
verter. The converter changes the voltage to its
equivalent (RMS) dc voltage and applies it to the A/D
Converter. The A/D Converter changes the voltage to
digital information and applies it to the A/D Con- troller.
The A/D Controller passes it on to the main controller
which calculates the correct reading using the
zero measurement (see step f) and calibration
constant(s).
e. AC Current or DC Current Function. If the AC
Current or DC Current Functions are selected, the input
current is applied to resistor R107 (.1 ohm). The result-
ant voltage drop across the resistor is then measured.
An ac or dc volts measurement is made, dependent on
the function selected (AC Current or DC Current,
respectively).
f. Autozero Function. If the 3478A’s Autozero
Function is enabled, an offset measurement (known as a
zero measurement) is made before an input
measurement. The zero measurement is made by
connecting the input of the Input Amplifier (HI INPUT
Terminal is open) to ground (LO INPUT Terminal). The
resultant offset of
7-F-2
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