ADT7518
Rev. A | Page 24 of 40
2 register and set C4 to 1 (if not done so already), then write all
0s to Bits C0:C2. All subsequent conversions will be done on the
VDD channel only. To change the channel selection to the inter-
nal temperature channel, write to the Control Configuration 2
register and set C0 = 1. When measuring in single-channel
mode, conversions on the channel selected occur directly after
each other. Any communication to the ADT7518 stops the
conversions, but they are restarted once the read or write
operation is completed.
Internal Temperature Measurement
The ADT7518 contains an on-chip band gap temperature
sensor whose output is digitized by the on-chip ADC. The
temperature data is stored in the Internal Temperature Value
register. Because both positive and negative temperatures can be
measured, the temperature data is stored in twos complement
format, as shown in Table 9. The thermal characteristics of the
measurement sensor could change and, therefore, an offset is
added to the measured 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 ADT7518 can measure the temperature of one external
diode sensor or diode-connected transistor.
The forward voltage of a diode or diode-connected transistor,
operated at a constant current, exhibits a negative temperature
coefficient of about 2 mV/癈. Unfortunately, because the
absolute value of VBE varies from device to device, and indivi-
dual calibration is required to null this out, the technique is
unsuitable for mass production.
The technique used in the ADT7518 is to measure the change in
V
BE
when the device is operated at two different currents. This is
given by
( )
N
n
q
KT
V
BE
1
/ ?/DIV>
=
where:
K is Boltzmanns constant.
q is the charge on the carrier.
T is the absolute temperature in kelvins.
N is the ratio of the two currents.
Figure 41 shows the input signal conditioning used to measure
the output of an external temperature sensor. This figure shows
the external sensor as a substrate transistor, provided for temp-
erature monitoring on some microprocessors, but it could
equally well be a discrete transistor.
If a discrete transistor is used, the collector is not 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. A 2N3906 is recommended
as the external transistor.
To prevent ground noise interfering with the measurement, the
more negative terminal of the sensor is not referenced 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 Layout Considerations
section for more information on C1.
To measure V
BE
, the sensor is switched between operating cur-
rents of I and N ?I. The resulting waveform is passed through a
low-pass filter to remove noise, then to a chopper-stabilized
amplifier that performs the functions of amplification and recti-
fication of the waveform to produce a dc voltage proportional
to V
BE
. This voltage is measured by the ADC to give a temper-
ature output in 10-bit twos complement format. To further
reduce the effects of noise, digital filtering is performed by
averaging the results of 16 measurement cycles.
Layout Considerations
Digital boards can be electrically noisy environments, and care
must be taken to protect the analog inputs from noise, particu-
larly when measuring the very small voltages from a remote
diode sensor. The following precautions should be taken:
1. Place the ADT7518 as close as possible to the remote
sensing diode. Provided that the worst noise sources such
as clock generators, data/address buses, and CRTs are
avoided, this distance can be 4 inches to 8 inches.
2. Route the D+ and D tracks close together, in parallel, with
grounded guard tracks on each side. Provide a ground
plane under the tracks, if possible.
3. Use wide tracks to minimize inductance and reduce noise
pickup. A 10 mil track minimum width and spacing is
recommended.
GND
D+
D
GND
10 MIL
10 MIL
10 MIL
10 MIL
10 MIL
10 MIL
10 MIL
Figure 50. Arrangement of Signal Tracks
4. Try to minimize the number of copper/solder joints, which
can cause thermocouple effects. Where copper/solder
joints are used, make sure that they are in both the D+ and
D path and at the same temperature. Thermocouple
effects should not be a major problem because 1癈 corres-
ponds to about 240 礦, and thermocouple voltages are
about 3 礦/癈 of temperature difference. Unless there are
two thermocouples with a big temperature differential
between them, thermocouple voltages should be much less
than 200 mV.
发布紧急采购，3分钟左右您将得到回复。
相关PDF资料
AT30TS00-MAH-T
SENSOR DGTL TEMP I2C/SMBUS 8WDFN
AT30TSE002B-MAH-T
SENSOR DGTL TEMP I2C/SMBUS 8WDFN
BD3504FVM-TR
IC REG CTRLR SGL POS ADJ 8MSOP
BD3521FVM-TR
IC REG CTRLR SGL 1.5V MSOP8
BD9153MUV-E2
IC REG TRPL BCK/LINEAR 24VQFN
CAT2300VP2-GT3
IC SENSE FET CONTROLLER 8TDFN
CAT34TS02VP2GT4A
IC TEMP SENSOR 2K MEMORY 8TDFN
CAT6095VP2-GT4
IC TEMP SENSOR STAND ALONE 8TDFN
相关代理商/技术参数
ADT7518ARQZ-REEL
制造商:Analog Devices 功能描述:Temp Sensor Analog Serial (4-Wire, SPI, I2C) 16-Pin QSOP T/R
ADT7518ARQZ-REEL7
制造商:Analog Devices 功能描述:Temp Sensor Analog Serial (4-Wire, SPI, I2C) 16-Pin QSOP T/R
ADT7519
制造商:AD 制造商全称:Analog Devices 功能描述:SPI-/I2C-Compatible, Temperature Sensor,4-Channel ADC and Quad Voltage Output
ADT7519ARQ
制造商:Analog Devices 功能描述:Temp Sensor Digital Serial (4-Wire, SPI, I2C) 16-Pin QSOP
ADT7519ARQ-REEL
制造商:Analog Devices 功能描述:Temp Sensor Digital Serial (4-Wire, SPI, I2C) 16-Pin QSOP T/R
ADT7519ARQ-REEL7
制造商:Analog Devices 功能描述:Temp Sensor Digital Serial (4-Wire, SPI, I2C) 16-Pin QSOP T/R 制造商:Analog Devices 功能描述:TEMP SENSOR DGTL SERL (4-WIRE, SPI) 16QSOP - Tape and Reel
ADT7519ARQZ
功能描述:IC TEMP SNSR QUAD DAC 16-QSOP RoHS:是 类别:集成电路 (IC) >> PMIC - 热管理 系列:- 标准包装:3,000 系列:- 功能:温度开关 传感器类型:内部 感应温度:85°C 分界点 精确度:±6°C（最小值） 拓扑:ADC（三角积分型），比较器，寄存器库 输出类型:开路漏极 输出警报:是 输出风扇:是 电源电压:2.7 V ~ 5.5 V 工作温度:-55°C ~ 125°C 安装类型:表面贴装 封装/外壳:SC-74A，SOT-753 供应商设备封装:SOT-23-5 包装:带卷 (TR) 其它名称:ADT6501SRJZP085RL7-ND
ADT7519ARQZ1
制造商:AD 制造商全称:Analog Devices 功能描述:SPI/I2C Compatible, Temperature Sensor, Four Channel ADC and Quad Voltage Output DAC