1°C Triple SMBus Sensor with Resistance Error Correction
Datasheet
5.1.1.2
Bypass Capacitors
Accurate temperature measurements require a clean, stable power supply. Locate a 0.1µF capacitor
as close as possible to the power pin with a good ground. A low ESR capacitor (such as a 10µF
ceramic) should be placed across the power source. Add additional power supply filtering in systems
that have a noisy power supply.
A capacitor may be placed across the DP/DN pair at the remote sensor in noisy environments. Do not
exceed a value of 2.2nF if this capacitor is installed.
5.1.1.3
Manufacturing
Circuit board assembly processes may leave a residue on the board. This residue can result in
unexpected leakage currents that may introduce errors if the circuit board is not clean. For example,
processes that use water-soluble soldering fluxes have been known to cause problems if the board is
not kept clean.
5.1.1.4
5.1.1.5
Thermal Considerations
Keep the sensor in good thermal contact with the component to be measured. The temperature of the
leads of a discrete diode will greatly impact the temperature of the diode junction. Make use of the
printed circuit board to disperse any self-heating that may occur.
Remote Sensors Connected by Cables
When connecting remote diodes with a cable (instead of traces on the PCB) use shielded twisted pair
cable. The shield should be attached to ground near the EMC1033, and should be left unconnected
at the sensor end. Belden 8451 cable is a good choice for this application.
5.1.2
Sensor Characteristics
The characteristics of the diode junction used for temperature sensing will affect the accuracy of the
measurement.
5.1.2.1
Selecting a Sensor
A diode connected small signal transistor is recommended. Silicon diodes are not a good choice for
remote sensors. Small signal transistors such as the 2N3904 or the 2N3906 are recommended. Select
a transistor with a constant value of hFE in the range of 2.5 to 220 microamps. The magnitude of hFE
is not critical, because the variation in hFE from one device to another cancels out of the temperature
equations.
5.1.2.2
5.1.2.3
Compensating for Ideality of the diode
The remote diode may have an ideality factor based on the manufacturing process. Inaccuracy in the
temperature measurement resulting from this ideality factor may be eliminated by configuring the
ideality factor register. The EMC1033 is trimmed to an ideality factor of 1.008.
Circuit Connections
The more negative terminal for the remote temperature diode, DN, is internally biased with a forward
diode voltage. Terminal DN is not referenced to ground. Remote temperature diodes can be
constructed as shown in Figure 5.2 Remote Temperature Diode Examples on page 23.
Revision 1.1 (01-19-07)
SMSC EMC1033
DATA2S2HEET
SMSC [ SMSC CORPORATION ]
