Revision V3.0/May 2011

©2011 Semtech Corp.

SX8733

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Page 10

ADVANCED COMMUNICATIONS & SENSING

DATASHEET

Precision Digital Temperature Sensor

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.

where T is the temperature in Celsius).

n is the pn junction ideality factor (1.00 for an ideal diode)

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

BE

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

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.

∆V

BE

V

–

n

kT

abs

q

-------------

I

2

I

1

----

 

 

ln

⋅

=

=

∆T

Rtrack

R

Track

I

2

I

1

–

200uV C

⁄

------------------------

⋅

=