AD7816AR Analog Devices Inc, AD7816AR Datasheet - Page 13

• Manufacturer Part Number: AD7816AR
• Description: IC ADC 10BIT W/TEMP SNSR 8-SOIC
• Manufacturer: Analog Devices Inc

Specifications of AD7816AR

• Rohs Status: RoHS non-compliant
• Function: Temp Monitoring System (Sensor)
• Topology: ADC, Multiplexer, Register Bank
• Sensor Type: Internal
• Sensing Temperature: -40°C ~ 85°C
• Output Type: MICROWIRE™, QSPI™, SPI™
• Output Alarm: Yes
• Output Fan: Yes
• Voltage - Supply: 2.7 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• For Use With: EVAL-AD7816/7/8EB - BOARD EVALUATION FOR AD7816/7/8

REV. C
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 II below shows some ADC codes for various temperatures.
TEMPERATURE MEASUREMENT ERROR DUE TO
REFERENCE ERROR
The AD7816, AD7817, and AD7818 are trimmed using a pre-
cision 2.5 V reference to give the transfer function described
previously. 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.
where:
So, for example, to calculate the ADC code at 25°C
CODE = ([113.3285 × 298 × 1.38 × 10
As can be seen from the expression, a reference error will pro-
duce a gain error. This means that the temperature measure-
ment 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 would be 2.2 LSBs (0.5°C) and
16 LSBs (4°C) at 125°C.
SELF-HEATING CONSIDERATIONS
The AD7817 and AD7818 have an analog-to-digital conversion
function capable of a throughput rate of 100 kSPS. At this
throughput rate the AD7817 and AD7818 will 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 will
rise as a result of this power dissipation. The graphs below show
the self-heating effect in a 16-lead SOIC package. Figures 12
and 13 show the self-heating effect on a two-layer and four-layer
PCB. The plots were generated by assembling a heater (resistor)
CODE (Dec) = ([113.3285 × K × T]/[q × V
= 511.5 (200 Hex)
K = Boltzmann’s Constant, 1.38 × 10
q = Charge on an electron, 1.6 × 10
T = Temperature (K)
– 0.6646) × 1024
ADC Code
00 1100 0000
01 0011 1000
01 1001 1100
10 0000 0000
10 0111 1000
11 1001 0000
Table II. Temperature Sensor Output
Temperature
–55°C
–25°C
0°C
+25°C
+55°C
+125°C
–23
REF
]/[1.6 × 10
–19
] – 0.6646) × 1024
–23
–19
× 2.5]
–13–
and temperature sensor (diode) in the package being evaluated.
In Figure 12, 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 13 where the heater is switched off after 2 sec-
onds. Figure 14 shows the relative effects of self-heating in air,
fluid and in thermal contact with a large heat sink.
These diagrams represent the worst-case effects of self-heating.
The heater delivered 6 mW to the interior of the package in all
cases. This power level is equivalent to the ADC continuously
converting at 100 kSPS. The effects of the self-heating can be
reduced at lower ADC throughput rates by operating on Mode
2 (see Operating Modes section). When operating in this mode,
the on-chip power dissipation reduces dramatically and, as a
consequence, the self-heating effects.
–0.05
–0.05
Figure 12. Self-Heating Effect Two-Layer and
Four-Layer PCB
Figure 13. Self-Heating Effect Two-Layer and
Four-Layer PCB
0.25
0.20
0.15
0.10
0.05
0.00
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
0
10
1
AD7816/AD7817/AD7818
4-LAYER PCB
4-LAYER PCB
20
2
TIME – secs
TIME – secs
30
2-LAYER PCB
3
40
2-LAYER PCB
4
50
60
5