SX8744 Просмотр технического описания (PDF) - Semtech Corporation
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SX8744 Datasheet PDF : 24 Pages
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SX8733/SX8743/SX8744
Digital temperature sensor
ADVANCED COMMUNICATIONS & SENSING
DATASHEET
3.3. Differential and Single-Ended External Sensor
Table 4 indicates whether the external sensors are connected differentially or in single-ended configuration with grounded
cathode.
Differential modes have better common-mode rejection of external noise pickup. The external noise pickup is present
equally on the anode and the cathode therefore differential noise is minimized.
Single-ended modes allow more sensors to be connected to the chip but they are more prone to noise pickup since the
cathode is connected to a common VSS and, therefore, any external noise pickup tends to be developed mainly across the
anode which will be measured as temperature noise.
A filtering capacitor is recommended to decrease measurement noise especially if the external sensor is connected to the
chip by a long trace. A capacitor with a value of 100 pF should be placed as close as possible to the chip pins.
3.4. Measurement Principle
The circuit uses the intrinsic thermal property of a diode to measure temperature. Temperature is calculated by measuring
the base-emitter voltage of a transistor. Two different currents are sourced to the diodes. The base-emitter voltage is
measured in each case. With a fixed current ratio, temperature is accurately calculated by measuring the difference in the
base-emitter voltage at the two currents.
The excitation current I2 of 100uA is passed through the diode first. The base-emitter voltage of the transistor is measured
by the ADC. The measurement is then repeated using the excitation current divided by a fixed value. This current I1 has a
value of 10uA. The following equation relates the VBE difference voltage with current and temperature T where:
k is the Boltzmann’s constant (1.381 x 10-23 J/K)
q is the charge on the electron (1.602 x 10-19 Cb).
Tabs is the absolute temperature in Kelvins. (Tabs=273.15 + T
where T is the temperature in Celsius).
n is the pn junction ideality factor (1.00 for an ideal diode)
For a current ratio I2/I1=10 and an ideality factor n=1.010, this gives a
fixed relationship between ∆VBE and temperature of 200uV/ºC. The
voltage is then converted to digital with an ADC.
∆VBE
=
VBE2–VBE1
=
n
⋅
k---T----a---b--s
q
ln
I-I--21
3.5. Parasitic Track Resistance Cancellation
The temperature measurement method described previously assumes
a very low series impedance in the sensor path.With a typical ∆VBE
around 200uV/ºC and the ∆I=I2-I1=90uA, 1 ohm of parasitic resistance
in the sensor path gives approximately 0.45ºC of temperature error.
This may result in a significant error if the external sensor is located
some distance away from the chip. Track resistance cancellation
schemes decrease temperature error due to high resistance in the
tracks from the device to the sensors.
∆TRtrack
=
RTra
ck
⋅
-----I--2----–-----I--1-----
200uV ⁄ C
3.5.1. Algorithmic Track Resistance Cancellation
The use of the algorithmic track resistance cancellation allows automatic cancellation of resistances in series with the
temperature diode by using current modulation to bias the external diode. This is done transparently to the user. This
method may cancel up to 1 kOhm of series resistance. The best accuracy is in the TD range 20ºC to 70ºC.
3.5.2. 3-Point Track Resistance Cancellation
The 3-point track resistance cancellation requires one additional connection to the external PN junction so that an
additional current source can be used to cancel out the error voltage due to the track resistance.
ACS Revision 2.91/February 2010
©2010 Semtech Corp.
Page 10
www.semtech.com
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