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ADT7488A Datasheet PDF : 12 Pages
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ADT7488A
voltage and therefore has adequate headroom to cope with
overvoltage. The full−scale voltage that can be recorded for
each channel is shown in Table 6.
Table 6. Maximum Reported Input Voltages
Voltage Channel
VCC
2.5 V
VCCP
Full−Scale Voltage
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.
3.3VIN
68kΩ
71kΩ
30pF
2.5VIN
45kΩ
94kΩ
30pF
MUX
VCCP
17.5kΩ
52.5kΩ
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
0x12 with a read length of 0x06. The data is retrieved in the
order listed in Table 7.
Table 7. Voltage Measurement Command Code
Voltage Channel
VCC
2.5 V
VCCP
Command Code
0x12
0x13
0x14
Returned Data
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
LSB
MSB
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 ADT7488A has three dedicated temperature
measurement channels: one for measuring the temperature
of an on−chip band gap temperature sensor, and two for
measuring the temperature of a remote diode, usually
located in the CPU or GPU.
The ADT7488A monitors one local and two remote
temperature channels. 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
2
Measurement
Local temperature
Remote 1 temperature
Remote 2 temperature
Conversion
Time (ms)
12
38
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 ADT7488A 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 DVBE, the operating current through the
sensor is switched between three related currents. Figure 15
shows N1 x I and N2 x I as different multiples of the
current I. The currents through the temperature diode are
switched between I and N1 x I, giving DVBE1, and then
between I and N2 x I, giving DVBE2. The temperature can
then be calculated using the two DVBE measurements. This
method can also cancel the effect of series resistance on the
temperature measurement. The resulting DVBE 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 DVBE. 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
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