Datasheet LTC2990 (Analog Devices) - 10

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APPLICATIONS INFORMATION
change in sensor voltage per degree temperature change
Table 1. Recommended Transistors to Be Used as Temperature
is 275µV/°C, so environmental noise must be kept to a
Sensors
minimum. Recommended shielding and PCB trace con-
MANUFACTURER PART NUMBER PACKAGE
siderations are illustrated in Figure 2. Fairchild Semiconductor MMBT3904 SOT-23 FMMT3904 SOT-23 The diode equation: Central Semiconductor CMPT3904 SOT-23 CET3904E SOT-883L k • T ⎛I ⎞ V C Diodes, Inc. MMBT3904 SOT-23 BE = η • • ln⎜ ⎟ (1) q ⎝IS ⎠ On Semiconductor MMBT3904LT1 SOT-23 NXP MMBT3904 SOT-23 can be solved for T, where T is Kelvin degrees, IS is a Infineon MMBT3904 SOT-23 process dependent factor on the order of 1E-13, η is the Rohm UMT3904 SC-70 diode ideality factor, k is Boltzmann’s constant and q is the electron charge. ideality factor of the diode sensor can be considered a temperature scaling factor. The temperature error for a V T = BE • q (2) 1% accurate ideality factor error is 1% of the Kelvin tem- ⎛I ⎞ η • k •In C ⎜ ⎟ perature. Thus, at 25°C, or 298K, a +1% accurate ideality ⎝IS ⎠ factor error yields a +2.98 degree error. At 85°C or 358K, a +1% error yields a 3.6 degree error. It is possible to scale The LTC2990 makes differential measurements of diode the measured Kelvin or Celsius temperature measured voltage to calculate temperature. Proprietary techniques using the LTC2990 with a sensor ideality factor other than allow for cancellation of error due to series resistance. 1.004, to the correct value. The scaling Equations (3) and (4) are simple, and can be implemented with sufficient 0.1µF precision using 16-bit fixed-point math in a microproces- GND SHIELD LTC2990 TRACE sor or microcontroller. V1 VCC V2 ADR1 Factory Ideality Calibration Value: V3 ADR0 470pF V4 SCL GND SDA ηCAL = 1.004 NPN SENSOR 2990 F02 Actual Sensor Ideality Value:
Figure 2. Recommended PCB Layout
ηACT Compensated Kelvin Temperature:
Ideality Factor Scaling
The LTC2990 is factory calibrated for an ideality factor of T ηCAL • T (3) 1.004, which is typical of the popular MMBT3904 NPN K _ COMP = K _MEAS ηACT transistor. The semiconductor purity and wafer-level pro- Compensated Celsius Temperature cessing limits device-to-device variation, making these devices interchangeable (typically <0.5°C) for no addi- ⎡ ⎤ T ηCAL (4) ⎢ • tional cost. Several manufacturers supply suitable transis- (T ) C _ COMP = C _MEAS + 273 ⎥ – 273 ⎣ ηACT ⎦ tors, some recommended sources are listed in Table 1. A 16-bit unsigned number is capable of representing the Discrete 2-terminal diodes are not recommended as tem- ratio η perature sensors. While an ideality factor value of 1.004 CAL/ηACT in a range of 0.00003 to 1.99997, by multiplying the fractional ratio by 215. The range of scal- is typical of target sensors, small deviations can yield ing encompasses every conceivable target sensor value. significant temperature errors. Contact LTC Marketing for The ideality factor scaling granularity yields a worst-case parts trimmed to ideality factors other than 1.004. The Rev. F 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Pin Configuration Pin Functions Functional Diagram Timing Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts