Omega 868F, Руководство пользователя, Страница: 9 / 36

6
SECTION 4 THEORY OF OPERATION
This section contains a brief description of operation that should
help a technician in understanding instrument operation during a
measurement, to aid in isolating possible malfunctions. Detailed
schematics of each model are provided at the end of the manual.
4.1 THREE WIRE SIGNAL CONDITIONING
Two phases are necessary to condition the signal for digitization.
Each of these phases has a period of one-half the A/D converter back
plane (BP) period. These two phases are called the zero phase and the
signal phase, respectively. During each phase, appropriate FETs are
switched on to configure the circuit as required.
4.1.1 Zero Phase
During the zero phase, certain FETs are switched on to configure the
circuit shown in Figure 4-1. This curcuit operates similarly to a sample-
and-hold curcuit, in which voltage levels are capacitively stored for
later use. The following summarizes this function.
1. C
R
is connected across R
R
. The voltage charged on C
R
is
equal to IR
R
.
2. C
Z
is connected across R
Z
+ L through L
Z
. The voltage charge
on C
Z
is equal to I[R
Z
+ L].
Figure 4-1. Three Wire Zero Phase