ADT7483A(2012) Ver la hoja de datos (PDF) - ON Semiconductor
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ADT7483A Datasheet PDF : 20 Pages
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ADT7483A
Theory of Operation
The ADT7483A is a local and 2 remote temperature
sensor and over/under temperature alarm. When the
ADT7483A is operating normally, the on-board ADC
operates in a freerunning mode. The analog input
multiplexer alternately selects either the on-chip
temperature sensor or one of the remote temperature sensors
to measure its local temperature. The ADC digitizes these
signals, and the results are stored in the local, Remote 1, and
Remote 2 temperature value registers.
The local and remote measurement results are compared
with the corresponding high, low, and THERM temperature
limits stored in on-chip registers. Out-of-limit comparisons
generate flags that are stored in the status register. A result
that exceeds the high temperature limit, the low temperature
limit, or a remote diode open circuit causes the ALERT
output to assert low. Likewise, exceeding THERM
temperature limits causes the THERM output to assert low.
The ALERT output can be reprogrammed as a second
THERM output.
The limit registers can be programmed, and the device
controlled and configured, via the serial SMBus. The
contents of any register can also be read back via the SMBus.
Control and configuration functions consist of:
Switching the Device between Normal Operation and
Standby Mode
Selecting the Temperature Measurement Scale
Masking or Enabling the ALERT Output
Switching Pin 13 between ALERT and THERM2
Selecting the Conversion Rate
Temperature Measurement Method
A simple method of measuring temperature is to exploit
the negative temperature coefficient of a diode, measuring
the baseemitter 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 ADT7483A is to measure the change in VBE when the
device is operated at two different currents.
Figure 14 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, but
it could equally 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 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. C1 can be optionally
added as a noise filter (recommended maximum value
1,000 pF).
To measure DVBE, the operating current through the sensor
is switched among two related currents, I and N I. The
currents through the temperature diode are switched
between I and N I, giving DVBE. The temperature is then
calculated using the DVBE measurement.
The resulting DVBE waveforms pass through a 65 kHz
low-pass filter to remove noise and then to a
chopper-stabilized amplifier. This amplifies and rectifies the
waveform to produce a dc voltage proportional to DVBE.
The ADC digitizes this voltage and produces a temperature
measurement. To reduce the effects of noise, digital filtering
is performed by averaging the results of 16 measurement
cycles for low conversion rates. At rates of 16, 32, and
64 conversions/second, no digital averaging takes place.
Signal conditioning and measurement of the local
temperature sensor is performed in the same manner.
VDD
I
N I IBIAS
D+
REMOTE
C1*
SENSING
TRANSISTOR
D−
BIAS
DIODE
LOW-PASS FILTER
fC = 65 kHz
VOUT+
To ADC
VOUT−
*CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS. C1 = 1,000 pF MAX
Figure 14. Input Signal Conditioning
Temperature Measurement Results
The results of the local and remote temperature
measurements are stored in the local and remote temperature
value registers and are compared with limits programmed
into the local and remote high and low limit registers.
The local temperature measurement is an 8-bit
measurement with 1C resolution. The remote temperature
measurements are 10-bit measurements, with eight MSBs
stored in one register and two LSBs stored in another
register. Table 6 lists the temperature measurement registers.
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