PC87591L datasheet(187/437 Pages) NSC

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Remote Diode Selection

Temperature accuracy depends on a good quality transistor used as a diode-connected, small-signal transistor. Accuracy

has been experimentally verified for the devices, among them Motorola, listed in Table 25. A temperature sensor can directly

measure the die temperature of CPUs with on-board temperature-sensing diodes.

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

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

µA for the highest expected temperature. The forward

voltage must be less than 0.95V at 100

µA for the lowest expected temperature. Large power transistors do not work. Also,

ensure 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.

Table 25. Remote Sensor Transistor Manufacturers

Filtering the Noise on Temperature Input Signals

Noise may be coupled to the input signal for various reasons, including close proximity to digital circuits. The ADC conver-

sion technology has good noise rejection especially for low-frequency signals, such as power supply hum. Rejection of high-

frequency noise can be improved using an external filter capacitor and PC board layout.

High-frequency noise is best filtered by placing a capacitor (CC of 2.2 nF) between pins DP and DN (see Figure 64), in prox-

imity to the PC87591x pins. The connection to the diode should use a twisted wire pair or a PCB that is read out using the

guidelines below.

It is most important to prevent any leak to ground, especially from the DP pin and serial resistance on the current loop to the

diode.

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

NTC

ADn

AVCC

AGND

Figure 63. Measuring Temperature Using Thermistors

a. NTC Thermistor

b. PTC Thermistor

AVCC

PTC

ADn

AGND

Thermistor

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