1°C Triple SMBus Temperature Sensor

Datasheet

SMSC EMCT03

13

Revision 2.3 (04-19-05)

DATASHEET

Chapter 4 Application Information

This chapter provides information on maintaining accuracy when using diodes as remote sensors with

SMSC Environmental Monitoring and Control devices. It is assumed that the users have some

familiarity with hardware design and transistor characteristics.

SMSC supplies a family Environmental Monitoring and Control (EMC) devices that are capable of

accurately measuring temperatures. Most devices include an internal temperature sensor along with

the ability to measure one or more external sensors. The characteristics of an appropriate diode for

use as the external sensor are listed in this chapter. Recommendations for the printed circuit board

layout are provided to help reduce error caused by electical noise or trace resistance.

4.1

Maintaining Accuracy

4.1.1

Physical Factors

Temperature measurement is performed by measuring the change in forward bias voltage of a diode

when two different currents are forced through the junction. The circuit board itself can impact the

ability to accurately measure these small changes in voltage. For example, an excessive amount of

series resistance can introduce error in the measurement.

4.1.1.1

Layout

Apply the following guidelines when designing the printed circuit board:

1. Route the remote diode traces on the top layer.

2. Place a ground guard signal on both sides of the differential pair. This guard band should be

connected to the ground plane at least every 0.25 inches.

3. Place a ground plane on the layer immediately below the diode traces.

4. Keep the diode traces as short as possible.

5. Keep the diode traces parallel, and the length of the two traces identical within 0.3 inches.

6. Use a trace width of 0.01 inches with a 0.01 inch guard band on each side.

7. Keep the diode traces away from sources of high frequency noise such as power supply filtering

or high speed digital signals.

8. When the diode traces must cross high speed digital signals, make them cross at a 90 degree

angle.

9. Avoid joints of copper to solder that can introduce thermocouple effects.

These recommendations are illustrated in Figure 4.1 Routing the Diode Traces on page 14.