PC87591E Datasheet, PDF (187/437 Pages) National Semiconductor (TI) – LPC Mobile Embedded Controllers

Embedded Controller Modules (Continued)
AVCC
R
ADn
C_F
T^o NTC
Thermistor
AVCC
T^o
PTC
Thermistor
ADn
C_F
R
AGND
AGND
a. NTC Thermistor
b. PTC Thermistor
Figure 63. Measuring Temperature Using Thermistors
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 100Ω. 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
Manufacturer
Model
Central Semiconductor (USA)
Motorola (USA)
National Semiconductor (USA)
Rohm Semiconductor (Japan)
Samsung (Korea)
Siemens (Germany)
Zetex (England)
CMPT3904
MMBT3904
MMBT3904
SST3904
KST3904-TF
SMBT3904
FMMT3904CT-ND
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 (C_C 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.
Revision 1.07
187
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