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參數(shù)資料
| 型號： | ADT7318ARU |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 溫度/濕度傳感器 |
| 英文描述： | DIGITAL TEMP SENSOR-SERIAL, 8BIT(s), 3Cel, RECTANGULAR, SURFACE MOUNT |
| 封裝： | QSOP-16 |
| 文件頁數(shù)： | 5/24頁 |
| 文件大?。?/td> | 279K |
| 代理商： | ADT7318ARU |
ADT7316/7317/7318
–13–
REV. PrH
Preliminary Technical Data
PRELIMINARY TECHNICAL DATA
mally, the A to D converter operates in a free-running
mode. The analog input multiplexer alternately selects
either the on-chip temperature sensor to measure its inter-
nal temperature, or a external temperature sensor. These
signals are digitized by the ADC and the results stored in
the Internal and External Temperature Value Registers.
The measured results are compared with the Internal and
External, High, Low and the temperature limits are stored
in on-chip registers. Out of limit comparisons generate
flags that are stored in the Status Register and one or
more out-of limit results will cause the
ALERT/OTI out-
put to pull low.
Theoretically, the temperature sensor and ADC can mea-
sure temperatures from -128
oC to +127oC with a resolu-
tion of 0.25
oC. However, temperatures below T
MIN and
above TMAX are outside the operating temperature range of
the device, so internal temperature measurements outside
this range are not possible. Temperature measurement
from -128
oC to +127oC is possible using an external sen-
sor.
Temperature measurement is initiated by a couple of
methods. The first method uses an internal clock count-
down of 20ms and then a conversion is preformed. The
internal oscillator is the only circuit that’s powered up
between conversions and once it times out, every 20ms, a
wake-up signal is sent to power-up the rest of the cir-
cuitry. A monostable is activated at the beginning of the
wake-up signal to ensure that sufficient time is given to
the power-up process. The monostable typically takes 4
s
to time out. It then takes typically 25s for each conver-
sion to be completed. The temperature is measured 16
times and internally averaged to reduce noise. The new
temperature value is loaded into the Temperature Value
Register and ready for reading by the I
2C or SPI interface.
The user has the option of disabling the averaging by set-
ting a bit in one of the configuration registers. The
ADT7316/7317/7318 defaults on power-up with the aver-
aging enabled.
A temperature measurement is also initiated every time the
oneshot method is used. This method requires the user to
write to the Oneshot register when a temperature measure-
ment is needed. Writing to the Oneshot register will start
a temperature conversion directly after the write operation.
The track/hold goes into hold approximately 4s
(monostable time-out) and a conversion is then initiated.
Typically 25s later the conversion is complete. As with
the previous method, the temperature is measured 16
times and internally averaged to reduce noise. The Tem-
perature Value Register is then loaded with a new tem-
perature value. If averaging is disabled for the automatic
method then it subsequently applies to the Oneshot
method also.
MEASUREMENT METHOD
INTERNAL TEMPERATURE MEASUREMENT
The ADT7316/7317/7318 contains an on-chip bandgap
temperature sensor, whose output is digitized by the on-
chip ADC. The temperature data is stored in the Internal
Temperature Value Register. As both positive and nega-
tive temperatures can be measured, the temperature data is
stored in two's complement format, as shown in Table 1.
The thermal characteristics of the measurement sensor
could change and therefore an offset is added to the mea-
sured value to enable the transfer function to match the
thermal characteristics. This offset is added before the
temperature data is stored. The offset value used is stored
in the Internal Temperature Offset Register.
EXTERNAL TEMPERATURE MEASUREMENT
The ADT7316/7317/7318 can measure the temperature of
one external diode sensor or diode-connected transistor.
The forward voltage of a diode or diode-connected tran-
sistor, operated at a constant current, exhibits a negative
temperature coefficient of about -2mV/
oC. Unfortunately,
the absolute value of Vbe, varies from device to device, and
individual calibration is required to null this out, so the
technique is unsuitable for mass-production.
The technique used in the ADT7316/7317/7318 is to
measure the change in Vbe when the device is operated at
two different currents.
This is given by:
V
be = KT/q x ln(N)
where:
K is Boltzmann’s constant
q is charge on the carrier
T is absolute temperature in Kelvins
N is ratio of the two currents
Figure 6 shows the input signal conditioning used to mea-
sure the output of an external temperature sensor. This
figure shows the external sensor as a substrate transistor,
provided for temperature monitoring on some micropro-
cessors, but it could equally well be a discrete transistor.
If a discrete transistor is used, the collector will not be
grounded, and should be linked to the base. If a PNP
transistor is used the base is connected to the D- input and
the emitter to the D+ input. If an NPN transistor is used,
the emitter is connected to the D- input and the base to
the D+ input.
We recommend that a 2N3906 be used as the external
transistor.
To prevent ground noise interfering with the measure-
ment, the more negative terminal of the sensor is not ref-
erenced to ground, but is biased above ground by an
internal diode at the D- input. As the sensor is operating
in a noisy environment, C1 is provided as a noise filter.
See the section on layout considerations for more informa-
tion on C1.
To measure
V
be, the sensor is switched between operating
currents of I and N x I. The resulting waveform is passed
through a lowpass filter to remove noise, thence to a chop-
per-stabilized amplifier that performs the functions of
amplification and rectification of the waveform to produce
a DC voltage proportional to
V
be. This voltage is mea-
sured by the ADC to give a temperature output in 8-bit
two’s complement format. To further reduce the effects of
noise, digital filtering is performed by averaging the re-
sults of 16 measurement cycles.
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