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TMP03GBC bảng dữ liệu(PDF) 4 Page - Analog Devices |
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4 / 16 page TMP03/TMP04 REV. 0 –4– (continued from page 1) The TMP03/TMP04 is a powerful, complete temperature measurement system with digital output, on a single chip. The onboard temperature sensor follows in the footsteps of the TMP01 low power programmable temperature controller, offering excellent accuracy and linearity over the entire rated temperature range without correction or calibration by the user. The sensor output is digitized by a first-order sigma-delta modulator, also known as the “charge balance” type analog-to- digital converter. (See Figure 1.) This type of converter utilizes time-domain oversampling and a high accuracy comparator to deliver 12 bits of effective accuracy in an extremely compact circuit. ∫ INTEGRATOR COMPARATOR 1-BIT DAC ∑∆ MODULATOR VOLTAGE REF & VPTAT DIGITAL FILTER CLOCK GENERATOR TMP03/04 OUT (SINGLE-BIT) Figure 1. TMP03/TMP04 Block Diagram Showing First-Order Sigma-Delta Modulator Basically, the sigma-delta modulator consists of an input sampler, a summing network, an integrator, a comparator, and a 1-bit DAC. Similar to the voltage-to-frequency converter, this architecture creates in effect a negative feedback loop whose intent is to minimize the integrator output by changing the duty cycle of the comparator output in response to input voltage changes. The comparator samples the output of the integrator at a much higher rate than the input sampling frequency, called oversampling. This spreads the quantization noise over a much wider band than that of the input signal, improving overall noise performance and increasing accuracy. The modulated output of the comparator is encoded using a circuit technique (patent pending) which results in a serial digital signal with a mark-space ratio format that is easily decoded by any microprocessor into either degrees centigrade or degrees Fahrenheit values, and readily transmitted or modulated over a single wire. Most importantly, this encoding method neatly avoids major error sources common to other modulation techniques, as it is clock-independent. Output Encoding Accurate sampling of an analog signal requires precise spacing of the sampling interval in order to maintain an accurate representation of the signal in the time domain. This dictates a master clock between the digitizer and the signal processor. In the case of compact, cost-effective data acquisition systems, the addition of a buffered, high speed clock line can represent a significant burden on the overall system design. Alternatively, the addition of an onboard clock circuit with the appropriate accuracy and drift performance to an integrated circuit can add significant cost. The modulation and encoding techniques utilized in the TMP03/TMP04 avoid this problem and allow the overall circuit to fit into a compact, three-pin package. To achieve this, a simple, compact onboard clock and an oversampling digitizer that is insensitive to sampling rate variations are used. Most importantly, the digitized signal is encoded into a ratiometric format in which the exact frequency of the TMP03/TMP04’s clock is irrelevant, and the effects of clock variations are effectively canceled upon decoding by the digital filter. The output of the TMP03/TMP04 is a square wave with a nominal frequency of 35 Hz ( ±20%) at +25°C. The output format is readily decoded by the user as follows: T1 T2 Figure 2. TMP03/TMP04 Output Format Temperature ( °C) = 235 − 400 × T1 T 2 Temperature ( °F) = 455 − 720 × T1 T 2 The time periods T1 (high period) and T2 (low period) are values easily read by a microprocessor timer/counter port, with the above calculations performed in software. Since both periods are obtained consecutively, using the same clock, performing the division indicated in the above formulas results in a ratiometric value that is independent of the exact frequency of, or drift in, either the originating clock of the TMP03/TMP04 or the user’s counting clock. |
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