ADT70GR-REEL7 Analog Devices Inc, ADT70GR-REEL7 Datasheet - Page 9

• Manufacturer Part Number: ADT70GR-REEL7
• Manufacturer: Analog Devices Inc

Specifications of ADT70GR-REEL7

• Operating Temperature (min): -40C
• Operating Temperature (max): 125C
• Operating Temperature Classification: Automotive
• Mounting: Surface Mount
• Pin Count: 20
• Package Type: SOIC
• Lead Free Status / RoHS Status: Not Compliant

REV. 0
would contribute to the difference in voltage drop between the
RTD and the reference resistor. Thus, an error in reading the ac-
tual temperature could occur.
12
14
16
18
20
22
24
26
28
30
From Table I the amount of lead-wire resistance effect in the
circuit can be estimated. For example, connect 100 feet of
AWG 22 wire to a 100
lead-wire resistance will be: R = 100 ft 3 0.0162 /ft = 1.62 .
Thus the total resistance you have with the PRTD will be:
Since the 100
to the ADT70, the lead-wire resistance is negligible. This shows
1.62
From the PRTD’s data sheet, the PRTD’s sensitivity rating
( / C) can be used with the lead-wire resistance to approximate
the accuracy error in temperature degree ( C). Following the ex-
ample above, the sensitivity of the 100
(taken from PRTD data sheet). Hence the approximate error is:
assuming the reference resistor is constant at 100
the temperature range.
Lead-wire AWG
Table I. Copper Wire Gauge Size to Resistance Table.
of inaccuracy.
Error
R
TOTAL
reference resistor is assumed to be relatively close
1 62
RESISTOR
100
POTENTIOMETER
IS USED TO
ACHIEVE HIGHER
PRECISION OF
MATCHING
CURRENT.
Platinum RTD (PF element). The
1k
REF
Ohms/foot at +25ºC
0.0016
0.0026
0.0041
0.0065
0.0103
0.0162
0.0257
0.0413
0.0651
0.1027
0 385
1 62
1k
PRTD
I
I
+IN
OUTA
OUTB
IN
NULLA
C
IA
IA
PRTD is 0.385 / C
101 62
RGA
49.9k
4 21
Figure 27. Basic Operational Diagram
NULLB
50k
INST
AMP
C
MATCHED
CURRENT
SOURCES
RGB
throughout
GND
SENSE
BIAS
V
OUT
OUT
IA
@ 5mV/ C
–9–
AGND
2.5V
As shown above, this is a significant inaccuracy, especially for ap-
plications where the PRTD would be hundreds of feet away from
the ADT70. To reduce lead-wire error it is recommended to use
a larger sensitivity RTD; 1 k instead of 100 . Furthermore, in
the application circuit section, Figure 28 illustrates how to elimi-
nate such error by using the part’s general purpose op amp.
Self-Heating Effect
Another contributor to measurement error is the self-heating ef-
fect on the RTD. As with any resistive element, power is dissi-
pated in an amount equal to the square of the excitation current
times the resistance of the element. The error contribution of the
heat generated by this power dissipation can easily be calculated.
For example, if the package thermal resistance is 50 C/W, the
RTD nominal resistance is 1 k and the element is excited with a
1 mA current source, then the artificial increase in temperature
( ºC) as a result of self-heating is:
where:
R
APPLICATION INFORMATION
As shown in Figure 27, using a 1 k PRTD, 1 k reference
resistor, 49.9 k resistor between RG
12), and shorting BIAS (Pin 4) with V
the output of OUT
V
2.5V
REF
REFOUT
PACKAGE
0
OUT
= value of RTD resistance
+5V
1V <
1 299
= thermal resistance of package
V
V
ADT70
S
S
DOWN
SHUT-
<
C
C
C
DGND
mV
5V
0 05
I R
1
2
mA
IA
0
(Pin 14) will have a transfer function of
C
+V
OUT
+IN
2
SHUTDOWN
IN
S
OA
PACKAGE
R
OA
OA
1000
PRTD RESISTANCE REFERENCE RESISTANCE
INDEPENDENT
OP AMP
50
A
REFOUT
C W
(Pin 11) and RG
(Pin 3) together,
ADT70
B
(Pin