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ADT70GN Datasheet(PDF) 9 Page - Analog Devices

Part # ADT70GN
Description  PRTD Conditioning Circuit and Temperature Controller
Download  14 Pages
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

ADT70GN Datasheet(HTML) 9 Page - Analog Devices

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ADT70
REV. 0
–9–
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.
Table I. Copper Wire Gauge Size to Resistance Table.
Lead-wire AWG
Ohms/foot at +25ºC
12
0.0016
14
0.0026
16
0.0041
18
0.0065
20
0.0103
22
0.0162
24
0.0257
26
0.0413
28
0.0651
30
0.1027
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
Ω Platinum RTD (PF element). The
lead-wire resistance will be: R = 100 ft 3 0.0162
Ω/ft = 1.62 Ω.
Thus the total resistance you have with the PRTD will be:
R
TOTAL
.
.
=+
=
100
1 62
101 62
ΩΩ
Since the 100
Ω reference resistor is assumed to be relatively close
to the ADT70, the lead-wire resistance is negligible. This shows
1.62
Ω of inaccuracy.
From the PRTD’s data sheet, the PRTD’s sensitivity rating
(
Ω/°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
Ω PRTD is 0.385 Ω/°C
(taken from PRTD data sheet). Hence the approximate error is:
Error
C
C
=° =
°
162
0 385
421
.
/ .
/
.
ΩΩ
assuming the reference resistor is constant at 100
Ω throughout
the temperature range.
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
Ω instead of 100 Ω. 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
Ω and the element is excited with a
1 mA current source, then the artificial increase in temperature
(
ƼC) as a result of self-heating is:
∆° =
×
CI R
PACKAGE
2
0
θ
∆° =
() ×Ω× °
CmA
C W
1
1000
50
2
/
∆° =
°
CC
005
.
where:
PACKAGE = thermal resistance of package
R0 = value of RTD resistance
APPLICATION INFORMATION
As shown in Figure 27, using a 1 k
Ω PRTD, 1 kΩ reference
resistor, 49.9 k
Ω resistor between RG
A (Pin 11) and RGB (Pin
12), and shorting BIAS (Pin 4) with VREFOUT (Pin 3) together,
the output of OUTIA (Pin 14) will have a transfer function of
VmV
R
OUT
PRTD RESISTANCE
REFERENCE RESISTANCE
=Ω ×
()
1 299
.
/
RGA
RGB
+INIA
INIA
INST
AMP
SHUT-
DOWN
GND
SENSE
OUTIA AGND
DGND
VS
SHUTDOWN
+INOA
INOA
OUTOA
+VS
2.5V
REF
IOUTA
IOUTB
MATCHED
CURRENT
SOURCES
NULLA
NULLB
BIAS
2.5VREFOUT
ADT70
49.9k
VOUT @ 5mV/ C
INDEPENDENT
OP AMP
50k
+5V
1k
PRTD
1k
REF
RESISTOR
POTENTIOMETER
IS USED TO
ACHIEVE HIGHER
PRECISION OF
MATCHING
CURRENT.
1V <
VS < 5V
Figure 27. Basic Operational Diagram


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