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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
22
SPECIFICATIONS continued
MODEL 869
TEMPERATURE SENSOR TYPE:
Three wire or four wire 100 Ω
platinum RTD (alpha = .00385)
4 WIRE
ACCURACY*
SETTING
RANGE
RESOLUTION
(18
°C
TO 28 °C: 1 Year)
200 °C
-100.0 °C to 199.9 °C
0.1
±
0.3 °C
-199.9 °C to -100.1 °C
0.1
±
1.5 °C
630 °C
-100 °C to 630 °C
1
±
1 °C
-220 °C to -101 °C
1
±
2 °C
*ACCURACY:
Three wire accuracy is the same if contact
resistance errors are removed by calibration
of instrument plus probe at 0°C. Includes DIN
43760 (ITS-90) conformity, repeatability,
temperature coefficient (18° to 28°C), time
stability (one year) and errors with up to 50 Ω
of lead resistance (each lead). Excludes probe
errors; however, probe errors around 0°C may
be compensated by an internal adjustment.
REPEATABILITY:
0.1 °C
ambient temperature.
TEMPERATURE COEFFICIENT:
18° to 28°C; included in
accuracy specification. From
-10° to 18°C, and 28° to
50°C: less than ±0.015°C/°C.
MAXI
MUM LEAD RESISTANCE:
(each lead):
Four wire: 50 Ω
Three wire: 10 Ω
SENSOR CURRENT:
500
µ
A max.
MODELS 868 AND 869
GENERAL SPECIFICATIONS:
DISPLAY:
3
1
/
2
digit LCD, 0.5" (13 mm) height. Polarity and deci-
mal point indication
.
CONVERSION RATE:
1.5 readings per second.
OVERRANGE AND OPEN
SENSOR INDICATION:
Typical for one week at constant
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