EVAL-ADT7467EB Analog Devices Inc, EVAL-ADT7467EB Datasheet - Page 17

• Manufacturer Part Number: EVAL-ADT7467EB
• Description: BOARD EVALUATION FOR ADT7467
• Manufacturer: Analog Devices Inc
• Series: dBCool®r

Specifications of EVAL-ADT7467EB

• Sensor Type: Temperature
• Sensing Range: -40°C ~ 120°C
• Interface: SMBus (2-Wire/I²C)
• Sensitivity: ±1.5°C
• Voltage - Supply: 3 V ~ 5.5 V
• Embedded: No
• Utilized Ic / Part: ADT7467
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Remote Temperature Measurement
The ADT7467 can measure the temperature of two remote
diode sensors or diode-connected transistors connected to
Pins 10 and 11 or to Pins 12 and 13.
The forward voltage of a diode or diode-connected transistor
operated at a constant current exhibits a negative temperature
coefficient of about –2 mV/°C. Unfortunately, the absolute
value of V
individual calibration; therefore, the technique is unsuitable for
mass production. The technique used in the ADT7467 is to
measure the change in V
currents. This is given by
where:
K is Boltzmann’s constant.
q is the charge on the carrier.
T is the absolute temperature in Kelvin.
N is the ratio of the two currents.
Figure 23 shows the input signal conditioning used to measure
the output of a remote temperature sensor. This figure shows
the external sensor as a substrate transistor provided for
temperature monitoring on some microprocessors. It could also
be a discrete transistor such as a 2N3904/2N3906.
TRANSISTOR
If a discrete transistor is used, the collector is not grounded and
should be linked to the base. If a PNP transistor is used, the base
is connected to the D– input and the emitter is connected to the
D+ input. If an NPN transistor is used, the emitter is connected
to the D– input and the base is connected to the D+ input.
Figure 25 and Figure 26 show how to connect the ADT7467 to
an NPN or PNP transistor for temperature measurement. 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
D– input.
Figure 23. Signal Conditioning for Remote Diode Temperature Sensors
SENSING
REMOTE
Δ
V
D+
D–
BE
BE
=
varies from each device and thus requires
KT
I
/ ×
q
N2 × I
1
n
( )
N1 × I I
BE
N
when the device is operated at three
BIAS
f
C
= 65kHz
LPF
V
DD
TO ADC
V
V
OUT+
OUT–
Rev. A| Page 17 of 80
To measure ΔV
switched among three related currents. Shown in Figure 23,
N1 × I and N2 × I are different multiples of the current I. The
currents through the temperature diode are switched between
I and N1 × I, resulting in ΔV
I and N2 × I, resulting in ΔV
calculated using the two ΔV
also cancel the effect of series resistance on the temperature
measurement.
The resulting ΔV
low-pass filter to remove noise and then sent to a chopper-
stabilized amplifier that amplifies and rectifies the waveform to
produce a dc voltage proportional to ΔV
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 7. The extra
resolution for the temperature measurements is held in the
Extended Resolution Register 2 (Reg. 0x77). This produces
temperature readings with a resolution of 0.25°C.
Noise Filtering
For temperature sensors operating in noisy environments,
previous practice involved placing a capacitor across the D+ and
D− pins to help combat the effects of noise. However, large capaci-
tances affect the accuracy of the temperature measurement,
leading to a recommended maximum capacitor value of 1,000 pF.
A capacitor of this value reduces the noise but does not eliminate
it, making use of the sensor difficult in a very noisy environment.
The ADT7467 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 device. The
effect of filter resistance seen in series with the remote sensor is
automatically canceled from the temperature result.
The construction of a filter allows the ADT7467 and the remote
temperature sensor to operate in noisy environments. Figure 24
shows a low-pass R-C-R filter with the following values:
This filtering reduces both common-mode noise and
differential noise.
R = 100 Ω, C = 1 nF
TEMPERATURE
Figure 24. Filter Between Remote Sensor and ADT7467
REMOTE
SENSOR
BE
, the operating current through the sensor is
BE
waveforms are passed through a 65 kHz
BE
BE1
BE2
measurements. This method can
; then they are switched between
. The temperature can then be
100Ω
100Ω
BE
. The ADC digitizes
1nF
D+
D–
ADT7467