sx8744emstrt Semtech Corporation, sx8744emstrt Datasheet - Page 10

• Manufacturer Part Number: sx8744emstrt
• Description: Precision Diode Digital Temperature For Up To 3 External Sensors
• Manufacturer: Semtech Corporation

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 I
by the ADC. The measurement is then repeated using the excitation current divided by a fixed value. This current I
value of 10uA. The following equation relates the V
For a current ratio I
fixed relationship between V
voltage is then converted to digital with an ADC.
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 V
around 200uV/ºC and the I=I
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.
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 T
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.
ADVANCED COMMUNICATIONS & SENSING
k is the Boltzmann’s constant (1.381 x 10
q is the charge on the electron (1.602 x 10
T
where T is the temperature in Celsius).
n is the pn junction ideality factor (1.00 for an ideal diode)
abs
is the absolute temperature in Kelvins. (T
2
/I
1
=10 and an ideality factor n=1.010, this gives a
2
of 100uA is passed through the diode first. The base-emitter voltage of the transistor is measured
2
BE
-I
1
=90uA, 1 ohm of parasitic resistance
and temperature of 200uV/ºC. The
-23
-19
J/K)
Cb).
abs
BE
=273.15 + T
difference voltage with current and temperature T where:
Page 10
SX8733/SX8743/SX8744
Precision Diode Digital Temperature
BE
For Up To 3 External Sensors
V
BE
T
=
Rtrack
D
V
range 20ºC to 70ºC.
BE2
=
–
V
R
BE1
Track
=
www.semtech.com
----------------------- -
200uV C
n
DATASHEET
I
2
kT
------------ -
–
q
abs
I
1
ln
1
 
 
has a
I
--- -
I
2
1