AD7818ARMZ 查看數據表（PDF） - Analog Devices

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AD7818ARMZ

Dual, Low Power CMOS, Analog Front End with DSP Microcomputer

Analog Devices 

AD7817/AD7818

Data Sheet

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:

TAMB = −103°C + (ADC Code/4)

+125°C

–55°C

192Dec

ADC CODE

912Dec

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.

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

2-LAYER PCB

0.45

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

0.40

0.35

0.30

0.25

0.20

4-LAYER PCB

0.15

0.10

0.05

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 × VREF] − 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,

0

–0.05

0

10

20

30

40

50

60

TIME (Seconds)

Figure 14. Self-Heating Effect 2-Layer and 4-Layer PCB with the Heater

(6 mW) Turned Off After 30 sec

0.25

0.20

0.15

0.10

2-LAYER PCB

4-LAYER PCB

0.05

CODE = ([113.3285 × 298 × 1.38 × 10−23]/[1.6 × 10−19 × 2.5]

− 0.6646) × 1024

= 511.5 (200 Hex)

0

–0.05

0

1

2

3

4

5

TIME (Seconds)

Figure 15. Self-Heating Effect 2-Layer and 4-Layer PCB with the Heater

Switched Off After 2 sec

Rev. D | Page 14 of 20

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