ADT7485AARMZ-R View Datasheet(PDF) - ON Semiconductor
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ADT7485AARMZ-R Datasheet PDF : 12 Pages
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ADT7485A
Table 6. Maximum Reported Input Voltages
Voltage Channel
12 V
5.0 V
VCC
2.5 V
VCCP
Full−Scale Voltage
16 V
8.0 V
4.0 V
4.0 V
4.0 V
Input Circuitry
The internal structure for the analog inputs is shown in
Figure 14. The input circuit consists of an input protection
diode and an attenuator, plus a capacitor that forms a
first−order, low−pass filter to provide input immunity to
high frequency noise.
12VIN
120k W
20k W
30pF
5VIN
93k W
47k W
30pF
3.3VIN
68k W
71k W
30pF MUX
2.5VIN
45k W
94k W
30pF
VCCP
17.5k W
52.5k W
35pF
Figure 14. Internal Structure of Analog Inputs
Voltage Measurement Command Codes
The voltage measurement command codes are detailed in
Table 7. Each voltage measurement has a read length of two
bytes in little endian format (LSB followed by MSB). All
voltages can be read together by addressing Command Code
0x10 with a read length of 0x10. The data is retrieved in the
order listed in Table 7.
Table 7. Voltage Measurement Command Code
Voltage Channel
12 V
5.0 V
VCC
2.5 V
VCCP
Command Code
0x10
0x11
0x12
0x13
0x14
Returned Data
LSB, MSB
LSB, MSB
LSB, MSB
LSB, MSB
LSB, MSB
Voltage Data Format
The returned voltage value is in twos complement, 16−bit,
binary format. The format is structured so that voltages in
the range of ±32 V can be reported. In this way, the reported
value represents the number of 1/1024 V in the actual
reading, allowing a resolution of approximately 1 mV.
Table 8. Analog−to−Digital Output vs. VIN
Voltage
Twos Complement
MSB
LSB
12
0011 0000
0000 0000
5.0
0001 0100
0000 0000
3.3
0000 1101
0011 0011
3.0
0000 1100
0000 0000
2.5
0000 1010
0000 0000
1.0
0000 0100
0000 0000
0
0000 0000
0000 0000
Temperature Measurement
The ADT7485A 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 ADT7485A 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 9.
Table 9. Temperature Monitoring Sequence
Channel
Number
0
1
Measurement
Local temperature
Remote 1 temperature
Conversion
Time (ms)
12
38
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 ADT7485A 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.
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