PC87366 datasheet(207/240 Pages) NSC

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12.0 Temperature Sensor (TMS) (Continued)

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The transistor must be a small-signal type with a relatively high forward voltage; otherwise, the A/D input voltage range can

be violated. The forward voltage must be greater than 0.25V at 10

µA at the highest expected temperature. The forward volt-

age must be less than 0.95V at 100

µA at the lowest expected temperature. Large power transistors do not work. Also, en-

sure that the base resistance is less than 100

Ω. Tight specifications for forward-current gain (+50 to +150, for example)

indicate devices with consistent VBE characteristics.

Thermal mass can seriously degrade the temperature sensor’s effective accuracy. The use of smaller packages for remote

sensors, such as SOT23s, improves the situation. The delay effect should be expected when measuring temperature using

the internal diode.

Table 56. Remote Sensor Transistor Manufacturers

12.5.2

ADC Noise Filtering

The ADC is an integrating type with inherently good noise rejection, especially of low-frequency signals such as power sup-

ply hum. Micropower operation places constraints on high frequency noise rejection; therefore, careful PC board layout and

proper external noise filtering are required for high accuracy remote measurements in electrically noisy environments.

High frequency EMI is best filtered at DXP and DXN with an external 2200 pF capacitor. This value can be increased to

about 3300pF (max), including cable capacitance. Capacitance higher than 3300 pF introduces errors, due to the rise time

of the switched current source.

Nearly all noise sources tested cause the ADC measurements to be higher than the actual temperature, depending on the

frequency and amplitude.

12.5.3

PC Board Layout

1. Place the temperature sensor as close as practical to the remote diode. In a noisy environment, such as a computer

motherboard, this distance can be 4 in. to 8 in. (typical) or more, as long as the worst noise sources (such as CRTs, clock

generators, memory buses, and ISA/PCI buses) are avoided.

2. Do not route the DXP DXN lines next to high inductance signals. Also, do not route the traces across a fast memory bus,

which can easily introduce +30˚C error, even with good filtering. Otherwise, most noise sources are fairly benign.

3. Route the DXP and DXN traces in parallel and in close proximity to each other, away from any high voltage traces such

as +12VDC. Beware of leakage currents from PC board contamination; e.g., a 20M leakage path from DXP to ground

causes about +1˚C error.

4. Connect guard traces to GND on either side of the DXP DXN traces (Figure 45). With guard traces in place, routing near

high-voltage traces is not a problem.

5. Route through as few vias and crossunders as possible to minimize copper/solder thermocouple effects.

6. When introducing a thermocouple, make sure that both the DXP and the DXN paths have matching thermocouples. In

general, PC board-induced thermocouples are not a serious problem. A copper-solder thermocouple exhibits 3V/˚C, and

it takes about 200V of voltage error at DXP DXN to cause a +1˚C measurement error. So, most parasitic thermocouple

errors are swamped out.

7. Use wide traces. Narrow ones are more inductive and tend to pick up radiated noise. The 10 mil widths and spacings

recommended in Figure 45 are not absolutely necessary (as they offer only a minor improvement in leakage and noise),

but try to use them where practical.

Manufacturer

Model

Central Semiconductor (USA)

CMPT3904

Motorola (USA)

MMBT3904

National Semiconductor (USA)

MMBT3904

Rohm Semiconductor (Japan)

SST3904

Samsung (Korea)

KST3904-TF

Siemens (Germany)

SMBT3904

Zetex (England)

FMMT3904CT-ND

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