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參數(shù)資料
| 型號： | ADT7485ARMZ |
| 廠商： | ON SEMICONDUCTOR |
| 元件分類： | 溫度/濕度傳感器 |
| 英文描述： | DIGITAL TEMP SENSOR-SERIAL, 16BIT(s), 1Cel, SQUARE, SURFACE MOUNT |
| 封裝： | LEAD FREE, PLASTIC, MO-187-BA, MSOP-10 |
| 文件頁數(shù)： | 5/16頁 |
| 文件大小： | 235K |
| 代理商： | ADT7485ARMZ |
Preliminary Technical Data
ADT7485
Rev. PrA | Page 13 of 16
TEMPERATURE MEASUREMENT
The ADT7485 has two dedicated temperature measurement
channels: one for measuring the temperature of an on-chip band
gap temperature sensor, and one for measuring the temperature
of a remote diode, usually located in the CPU or GPU.
The ADT7485 monitors one local and one remote temperature
channel. Monitoring of each of the channels is done in a round-
robin sequence. The monitoring sequence is in the order shown
in Table 13.
Table 13. Temperature Monitoring Sequence
Channel
Number
Measurement
Conversion Time (ms)
0
Local temperature
52
1
Remote 1 temperature
52
TEMPERATURE MEASUREMENT METHOD
A simple method for measuring temperature is to exploit the
negative temperature coefficient of a diode by measuring the
base-emitter voltage (VBE) of a transistor operated at constant
current. Unfortunately, this technique requires calibration to
null the effect of the absolute value of VBE, which varies from
device to device.
The technique used in the ADT7485 measures the change in
VBE when the device is operated at three different currents.
Figure 15 shows the input signal conditioning used to measure
the output of a remote temperature sensor. This figure shows
the remote sensor as a substrate transistor, which is provided for
temperature monitoring on some microprocessors, but it could
also be a discrete transistor. If a discrete transistor is used, the
collector is not grounded and should be linked to the base. To
prevent ground noise from 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
D1 input. If the sensor is operating in an extremely noisy
environment, C1 can be added as a noise filter. Its value should
not exceed 1000 pF.
To measure ΔVBE, the operating current through the sensor is
switched between three related currents. Figure 15 shows N1 × I
and N2 × I as different multiples of the current I. The currents
through the temperature diode are switched between I and
N1 × I, giving ΔVBE1, and then between I and N2 × I, giving
ΔVBE2. The temperature can then be calculated using the two
ΔVBE measurements. This method can also cancel the effect of
series resistance on the temperature measurement. The
resulting ΔVBE waveforms are passed through a 65 kHz low-pass
filter to remove noise and then through a chopper-stabilized
amplifier to amplify and rectify the waveform, producing a dc
voltage proportional to ΔVBE. The ADC digitizes this voltage,
and a temperature measurement is produced. To reduce the
effects of noise, digital filtering is performed by averaging the
results of 16 measurement cycles for low conversion rates.
Signal conditioning and measurement of the internal
temperature sensor is performed in the same manner.
C11
D1+
BIAS
DIODE
1CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS.
VDD
TO ADC
VOUT+
VOUT–
REMOTE
SENSING
TRANSISTOR
D1–
I
N1 × I
N2 × I
IBIAS
LOW-PASS FILTER
fC =65kHz
05197-
004
Figure 15. Signal Conditioning for Remote Diode Temperature Sensors
READING TEMPERATURE MEASUREMENTS
The temperature data returned is two bytes in little endian
format, that is, LSB before MSB. All temperatures can be read
together by using Command Code 0x00 with a read length of
0x04. The command codes and returned data are described in
Table 14.
Table 14. Temperature Channel Command Codes
Temp Channel
Command Code
Returned data
Internal
0x00
LSB, MSB
External
0x01
LSB, MSB
All Temps
0x00
Internal LSB, Internal MSB;
External LSB, External MSB
SST TEMPERATURE SENSOR DATA FORMAT
The data for temperature is structured to allow values in the
range of ±512°C to be reported. Thus, the temperature sensor
format uses a twos complement, 16-bit binary value to represent
values in this range. This format allows temperatures to be
represented with approximately a 0.016°C resolution.
Table 15. SST Temperature Data Format
Twos Compliment
Temperature (°C)
MSB
LSB
125
1110 0000
1100 0000
80
1110 1100
0000 0000
40
1111 0110
0000 0000
20
1111 1011
0011 1110
5
1111 1110
1100 0000
1
1111 1111
1100 0000
0
0000 0000
+1
0000 0000
0100 0000
+5
0000 0001
0100 0000
+20
0000 0100
1100 0010
+40
0000 1010
0000 0000
+80
0001 0100
0000 0000
+125
0001 1111
0100 0000
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