AD7816-7818 Analog Devices, AD7816-7818 Datasheet - Page 12

• Manufacturer Part Number: AD7816-7818
• Description: Single- and 4-Channel/ 9 us/ 10-Bit ADCs with On-Chip Temperature Sensor
• Manufacturer: Analog Devices

AD7816/AD7817/AD7818
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 preci-
sion +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 K = Boltzmann’s Constant, 1.38
So, for example, to calculate the ADC code at 25 C
CODE = ([113.3285 298
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 be-
tween 4 mW and 6.5 mW of power. Because a thermal imped-
ance 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) and temperature sensor (diode) in the package
being evaluated. In Figure 12, the heater (6 mW) is turned off
CODE (Dec) = ([113.3285 K
= 511.5 (200 Hex)
q = Charge on an electron, 1.6 10
T = Temperature (K)
– 0.6646)
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
1024
1.38
T]/[q V
10
Temperature
–55 C
–25 C
0 C
+25 C
+55 C
+125 C
–23
REF
]/[1.6 10
10
–19
] – 0.6646) 1024
–23
–19
2.5]
–12–
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 seconds. 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
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
REV. A