10

®

A typical two-wire RTD application with linearization is

cording to the desired temperature range. An equation is

given in Figure 1.

an offset canceling current to compensate for wiring resis-

are chosen such that their currents are equal. This makes the

voltage drop in the wiring resistance to the RTD a common-

mode signal which is rejected by the XTR105. The nearest

adequate for most applications. Table I provides the 1%

resistor values for a three-wire Pt100 RTD connection.

RTDs

The text and figures thus far have assumed a Pt100 RTD.

With higher resistance RTDs, the temperature range and

input voltage variation should be evaluated to ensure proper

common-mode biasing of the inputs. As mentioned earlier,

bias the inputs of the XTR105 within their common-mode

input range.

ERROR ANALYSIS

Table II shows how to calculate the effect various error sources

have on circuit accuracy. A sample error calculation for a

typical RTD measurement circuit (Pt100 RTD, 200

°C mea-

surement span) is provided. The results reveal the XTR105’s

excellent accuracy, in this case 1.1% unadjusted. Adjusting

accuracy to 0.32%. Note that these are worst case errors;

guaranteed maximum values were used in the calculations and

all errors were assumed to be positive (additive). The XTR105

achieves performance which is difficult to obtain with discrete

circuitry and requires less space.

OPEN-CIRCUIT PROTECTION

behavior with open-circuit RTD connections. It assures that

if any one of the three RTD connections is broken, the

XTR105’s output current will go to either its high current

limit (

≈27mA) or low current limit (≈2.2mA). This is easily

detected as an out-of-range condition.

FIGURE 3. Three-Wire Connection for Remotely Located RTDs.

Resistance in this line causes

a small common-mode voltage

which is rejected by XTR105 .

OPEN RTD

TERMINAL

I

O

1

2

3

≈ 2.2mA

≈27mA

≈2.2mA

RTD

(R

R

Z

(1)

R

LIN2

(1)

R

LIN1

(1)

(R

2

1

3

0.01µF

R

0.01µF

Q

2

(2)

2N2222

NOTES: (1) See Table I for resistor equations and

1% values. (2) Q

output current if any one RTD connection is

broken:

13

4

3

2

R

G

XTR105

7

6

(1)

R

G

R

G

V

IN

–

V

IN

+

V

R1

I

R2

V

REG

V+

I

RET

I

O

E

B

8

9

Q

1

I

O

I

O

14

11

12

1

10

EQUAL line resistances here

creates a small common-mode

voltage which is rejected by

XTR105.