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ADT7490
switched between I and N1 ? I, giving D V BE1 , and then
between I and N2 ? I, giving D V BE2 . The temperature can
then be calculated using the two D V BE measurements. This
method can also cancel the effect of any series resistance on
the temperature measurement.
The resulting D V BE waveforms are passed 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 D V BE .
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.
The results of remote temperature measurements are
stored in 10-bit, twos complement format, as listed in
Table 17. The extra resolution for the temperature
measurements is held in the Extended Resolution Register 2
(0x77). This gives temperature readings with a resolution of
0.25 ? C.
I
N2 ? I
N1 ? I
I BIAS
V CC
D+
V OUT+
REMOTE
SENSING
TRANSISTOR
D ?
LOW-PASS FILTER
f C = 65 kHz
Figure 27. Signal Conditioning for Remote Diode Temperature Sensors
To ADC
V OUT ?
Series Resistance Cancellation
Parasitic resistance to the ADT7490 D+ and D ? inputs
(seen in series with the remote diode) is caused by a variety
of factors, including PCB track resistance and track length.
This series resistance appears as a temperature offset in the
remote sensor ’s temperature measurement. This error
typically causes a 0.5 ? C offset per ohm of parasitic
resistance in series with the remote diode.
The ADT7490 automatically cancels out the effect of this
series resistance on the temperature reading, giving a more
the remote sensor is automatically canceled from the
temperature result.
The construction of a filter allows the ADT7490 and the
remote temperature sensor to operate in noisy environments.
Figure 28 shows a low-pass RC filter with the following
values:
R + 100 W , C + 1 nF (eq. 3)
This filtering reduces both common-mode noise and
differential noise.
accurate result without the need for user characterization of
this resistance. The ADT7490 is designed to automatically
cancel, typically up to 1.5 k W of resistance. By using an
advanced temperature measurement method, this is
transparent to the user. This feature allows resistances to be
REMOTE
TEMPERATURE
SENSOR
100 W
100 W
1 nF
D+
D ?
added to the sensor path to produce a filter, allowing the part
to be used in noisy environments.
Noise Filtering
For temperature sensors operating in noisy environments,
previous practice was to place a capacitor across the D+ pin
and the D ? pin to help combat the effects of noise. However,
large capacitance affect the accuracy of the temperature
measurement, leading to a recommended maximum
capacitor value of 1,000 pF. This capacitor reduces the
noise, but does not eliminate it, which makes using the
sensor difficult in a very noisy environment.
The ADT7490 has a major advantage over other devices
for eliminating the effects of noise on the external sensor.
Using the series resistance cancellation feature, a filter can
be constructed between the external temperature sensor and
the part. The effect of any filter resistance seen in series with
Figure 28. Filter between Remote Sensor and ADT7490
Factors Affecting Diode Accuracy
Remote Sensing Diode
The ADT7490 is designed to work with either substrate
transistors built into processors or discrete transistors.
Substrate transistors are generally PNP types with the
collector connected to the substrate. Discrete types can be
either PNP or NPN transistors connected as a diode
(base-shorted to the collector). To reduce the error due to
variations in both substrate and discrete transistors, a
number of factors should be taken into consideration:
? The ideality factor, n f , of the transistor is a measure of
the deviation of the thermal diode from ideal behavior.
The ADT7490 is trimmed for an n f value of 1.008. Use
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