Omega 868F, User Manual

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