ACS Revision 2.91/February 2010

©2010 Semtech Corp.

SX8733/SX8743/SX8744

www.semtech.com

Page 11

Digital temperature sensor

ADVANCED COMMUNICATIONS & SENSING

DATASHEET

Track resistances up to 1 kOhm may be cancelled. It is important that the track resistance values in the cathode or anode

path of the external sensor are made equal. Care must be taken during PCB layout to match track resistances between the

device and the sensor. Any resistance difference will lead to cancellation errors.

In this mode, VSS is used for the current return path from the cathode. This connection must be placed close to the diode’s

cathode. One port supply the current modulation to the anode and another port supply the cancellation current to the

cathode. The resulting junction voltage modulation is measured between the 2 used ports. Figure 5 explains the method:

Assume sensor Q1 is being measured in mode 2 with 3-point

track resistance cancellation enabled. The PCB trace linking the

device to the external sensor must be matched which means the

PCB track resistance are the same. The same current is forced

out of P1 and P2.

The voltages developed on P1 and P2 traces are the same

therefore the differential voltage seen on P1-P2 is not impacted by

the track resistance.

Figure 5. 3-Point Track Resistance Cancellation

3.5.3. The Kelvin (4-wire) Track Resistance Cancellation

The sensor is connected to the chip via 4 wires. Two comes from

the current source (force : P3, P4) and two are used for the

voltage measurement (sense : P1, P2).

The Kelvin connection avoids the measurement error caused by

the voltage drop in the force path.

Sense leads are attached directly accross the external sensor.

The P1 and P2 connections must be as close as possible to

sensor anode and cathode respectively.

Figure 6. Kelvin (4-wire) Track Resistance Cancellation

3.6. Temperature Acquisition Sequence

The SX8743 monitors up to 4 sensors (3 remote and 1 internal). Theses can be scanned in sequence to make an

automatic reading of the sensors one after the other. The result of each conversion is stored in the temperature registers

(see Table 6). The register RegSensor defines which sensors are scanned in the sequence. When all the sensors are

selected, the sequence is the following: Int

→ Ext1→ Ext2→ Ext3→Int→etc.

P1

P2

VSS

V1

V2

I

I

2I

V1

P1

P2

P3

P4

I

Force (+)

Force (-)

Sense (-)

I

Sense (+)