EMCT03_05 Datasheet, PDF (13/16 Pages) SMSC Corporation – 1°C Triple SMBus Temperature Sensor

1°C Triple SMBus Temperature Sensor
Datasheet
Chapter 4 Application Information
4.1
4.1.1
4.1.1.1
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.
Maintaining Accuracy
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.
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.
SMSC EMCT03
13
DATASHEET
Revision 2.3 (04-19-05)