AD7817/AD7818 Data Sheet
Rev. E | Page 14 of 20
TEMPERATURE MEASUREMENT
The on-chip temperature sensor can be accessed via multiplexer
Channel 0, that is, by writing 0 0 0 to the channel address register.
The temperature is also the power on default selection. The
transfer characteristic of the temperature sensor is shown in
Figure 13. The result of the 10-bit conversion on Channel 0
can be converted to degrees centigrade by the following:
T
AMB
= −103°C + (ADC Code/4)
–55°C
+125°C
912Dec192Dec
TEMPERATURE
ADC CODE
01316-018
Figure 13. Temperature Sensor Transfer Characteristics
For example, if the result of a conversion on Channel 0 was
1000000000 (512 Dec), the ambient temperature is equal to
−103°C + (512/4) = +25°C.
Table 7 shows some ADC codes for various temperatures.
Table 7. Temperature Sensor Output
ADC Code Temperature
00 1100 0000 −55°C
01 0011 1000 −25°C
01 1001 1100 0°C
10 0000 0000 +25°C
10 0111 1000 +55°C
11 1001 0000 +125°C
TEMPERATURE MEASUREMENT ERROR DUE TO
REFERENCE ERROR
The AD7817/AD7818 are trimmed using a precision 2.5 V
reference to give the transfer function previously described. To
show the effect of the reference tolerance on a temperature reading,
the temperature sensor transfer function can be rewritten as a
function of the reference voltage and the temperature.
CODE (DEC) = ([113.3285 × K × T]/[q × V
REF
] − 0.6646) × 1024
where:
K = Boltzmann’s Constant, 1.38 × 10
−23
q = charge on an electron, 1.6 × 10
−19
T = temperature (K)
So, for example, to calculate the ADC code at 25°C,
CODE = ([113.3285 × 298 × 1.38 × 10
−23
]/[1.6 × 10
−19
× 2.5]
− 0.6646) × 1024
= 511.5 (200 Hex)
As can be seen from the expression, a reference error produces a
gain error. This means that the temperature measurement error
due to reference error will be greater at higher temperatures. For
example, with a reference error of −1%, the measurement error
at −55°C is 2.2 LSBs (+0.5°C) and 16 LSBs (+4°C) at +125°C.
SELF-HEATING CONSIDERATIONS
The AD7817/AD7818 have an analog-to-digital conversion
function capable of a throughput rate of 100 kSPS. At this
throughput rate, the AD7817/AD7818 consume between 4 mW
and 6.5 mW of power. Because a thermal impedance is associated
with the IC package, the temperature of the die rises as a result
of this power dissipation. Figure 14 to Figure 16 show the self-
heating effect in a 16-lead SOIC. Figure 14 and Figure 15 show
the self-heating effect on a two-layer and four-layer PCB. The
plots were generated by assembling a heater (resistor) and
temperature sensor (diode) in the package being evaluated. In
Figure 14, the heater (6 mW) is turned off after 30 sec. The PCB
has little influence on the self-heating over the first few seconds
after the heater is turned on. This can be more clearly seen in
Figure 15 where the heater is switched off after 2 sec. Figure 16
shows the relative effects of self-heating in air, fluid, and
thermal contact with a large heat sink.
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
–0.05
0 102030405060
TEMPERATURE (°C)
TIME (Seconds)
2-LAYER PCB
4-LAYER PCB
01316-019
Figure 14. Self-Heating Effect 2-Layer and 4-Layer PCB with the Heater
(6 mW) Turned Off After 30 sec
4-LAYER PCB
2-LAYER PCB
0.25
0.20
0.15
0.10
0.05
0
–0.05
012345
TEMPERATURE (°C)
TIME (Seconds)
01316-020
Figure 15. Self-Heating Effect 2-Layer and 4-Layer PCB with the Heater
Switched Off After 2 sec
