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參數(shù)資料
| 型號： | AD7414ARM-2REEL7 |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 溫度/濕度傳感器 |
| 英文描述： | DIGITAL TEMP SENSOR-SERIAL, 10BIT(s), 2Cel, SQUARE, SURFACE MOUNT |
| 封裝： | MICRO, SOIC-8 |
| 文件頁數(shù)： | 8/12頁 |
| 文件大?。?/td> | 182K |
| 代理商： | AD7414ARM-2REEL7 |
AD7414
–5–
REV. PrB
Preliminary Technical Data
PRELIMINAR
Y
TECHNICAL
DA
TA
CIRCUIT INFORMATION
The AD7414 is a stand alone digital temperature sensor.
The on-chip temperature sensor allows an accurate
mea-
surement of the ambient device temperature to be made.
The 10-bit A/D converter converts the temperature mea-
sured into a two’s complement format for storage in the
Temperature Register. The A/D converter is made up of a
conventional succesive-approximation converter based
around a capacitior DAC. The serial interface is I
2C and
SMBus compatible. The AD7414 requires a 2.7 V to 5.5
V power supply. The temperature sensor has a working
measurement range of -55
oC to +125 oC for version A
and -55 oC to +135 oC for version S.
FUNCTIONAL DESCRIPTION
Temperature measurement is initiated by a couple of
methods. The first method uses an internal clock count-
down of 400s and then a conversion is preformed. It
takes typically 25s for each conversion to be completed.
The new temperature value is loaded into the Tempera-
ture Value Register and ready for reading by the I
2C inter-
face.
A temperature measurement is also initiated every time a
read or write operation to the AD7414 takes place. The
track/hold goes into hold appromimaely 3s after the
STOP contition and a conversion is then initiated. Typi-
cally 25s later the conversion is complete and the Tem-
perature Value Register is loaded with a new temperature
value.
The measurement modes are compared with high and low
temperature limits, stored in two 8-bit read/write registers.
If the measurement is greater than the high limit, the
ALERT pin is activated (if it has already beeen enabled in
the Configuration Register). The ALERT pin is deacti-
vated again when the temperature measured is less then
the low limit. This ALERT pin is compatible with the
SMBus SMBALERT option.
Configuration functions consist of:
- switching between normal operation and full power-
down.
- enabling or disabling the SCL and SDA filters.
- enabling or disabling the ALERT function.
- setting ALERT pin polarity.
Figure 2. Typical Connection Diagram
MEASUREMENT TECHNIQUE
A common method of measuring temperature is to exploit
the negative temperature coefficient of a diode, or the
base-emitter voltage of a transistor, operated at constant
current. Unfortunately, this technique requires calibration
to null out the effect of the absolute value of VBE, which
varies from device to device.
The technique used in the AD7414 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 electron (1.6 x 10
-19 Coulombs).
T is absolute temperature in Kelvins.
N is the ratio of the two currents.
Figure 3. Temperature Measurement Technique
Figure 3 shows the method the AD7414 uses to measure
the ambient device temperature. To measure
V
BE, the
sensor (substrate transistor) is switched between operating
currents of I and N x I. The resulting waveform is passed
through a chopper-stabilized amplifier that performs the
functions of amplification and rectification of the wave-
form to produce a dc voltage proportional to
V
BE. This
voltage is measured by the ADC to give a temperature
output in 10-bit twos complement format.
TEMPERATURE DATA FORMAT
The temperature resolution of the ADC is 0.25
oC which
corresponds to one LSB of the ADC. The ADC can theo-
retically measure a temperature span of 255
oC; the practi-
cal lowest value is limited to -55
oC due to device
maximum ratings. The S grade can measure a tempera-
ture range of -55
oC to +135oC (temperature data format is
shown in Table 2); the A grade can measure a temperature
range of -55
oC to +125oC (temperature data format is
shown in Table 3).
SDA
SC L
AL E R T
AS
SUP P LY
2.7 V t o
5.5 V
GN D
A D 74 14
VDD
C /P
10F
0 .1 F
TO AD C
V OU T+
V OUT-
SENSIN G
T RA NSIST O R
V
DD
I
N x I
SENSIN G
T RA NSIST O R
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