Omega 868F, Руководство пользователя, Страница: 12 / 36

9
4.2 FOUR WIRE SIGNAL CONDITIONING
There are two phases necessary to condition the signal for
digitization. Each phase lasts for a period equal to one-half of the
back plane period of the A/D converter. These are called the zero
and signal phase.
4.2.1 Zero Phase
During the zero phase, FETs are switched to configure the circuit
shown in Figure 4-3. The fourth wire adds an additional attenuation
that slightly increases the offset voltage at the input to amplifier A
1
.
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
. The voltage charge in C
Z
is equal to IR
Z
.
3. The input of A
1
is connected through L
3
. Since, ideally, zero
current flows into A
1
the voltage at the output of A
1
is equal
to [A
1
(IL
4
+ Vos)].
4. C
C
is connected between the output of A
1
and common. The
voltage charged on C
C
is equal to the output voltage of A
1
or [A
1
(IL
4
+ Vos)].
5. C
A
is connected between the output of the A
2
network and
common. The voltage charged on C
A
is equal to the output
voltage of A
1
attentuated by A
2
or [A
1
A
2
(IL
4
+ Vos)].
Figure 4-3. Four Wire Zero Phase