Datasheet TMP03, TMP04 (Analog Devices) - 4

TMP03/TMP04
(continued from page 1) avoids major error sources common to other modulation tech- The TMP03 is a powerful, complete temperature measurement niques, as it is clock-independent. system with digital output, on a single chip. The onboard tem-
Output Encoding
perature sensor follows in the footsteps of the TMP01 low Accurate sampling of an analog signal requires precise spacing power programmable temperature controller, offering excellent of the sampling interval in order to maintain an accurate repre- accuracy and linearity over the entire rated temperature range sentation of the signal in the time domain. This dictates a without correction or calibration by the user. master clock between the digitizer and the signal processor. In The sensor output is digitized by a first-order sigma-delta the case of compact, cost-effective data acquisition systems, the modulator, also known as the “charge balance” type analog-to- addition of a buffered, high speed clock line can represent a digital converter. (See Figure 1.) This type of converter utilizes significant burden on the overall system design. Alternatively, time-domain oversampling and a high accuracy comparator to the addition of an onboard clock circuit with the appropriate deliver 12 bits of effective accuracy in an extremely compact accuracy and drift performance to an integrated circuit can add circuit. significant cost. The modulation and encoding techniques uti- lized in the TMP03 avoid this problem and allow the overall ⌺⌬
MODULATOR
circuit to fit into a compact, 3-pin package. To achieve this, a
INTEGRATOR
simple, compact onboard clock and an oversampling digitizer
COMPARATOR
that is insensitive to sampling rate variations are used. Most
VOLTAGE REF AND VPTAT
importantly, the digitized signal is encoded into a ratiometric format in which the exact frequency of the TMP03’s clock is irrelevant, and the effects of clock variations are effectively can-
1-BIT DAC
celed upon decoding by the digital filter. The output of the TMP03 is a square wave with a nominal frequency of 35 Hz (± 20%) at 25°C. The output format is
TMP03/04 CLOCK DIGITAL OUT GENERATOR FILTER
readily decoded by the user as follows:
(SINGLE-BIT)
Figure 1. TMP03 Block Diagram Showing First-Order
T1 T2
Sigma-Delta Modulator Basically, the sigma-delta modulator consists of an input sampler, a summing network, an integrator, a comparator, and a 1-bit Figure 2. TMP03 Output Format 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 Temperature (°C) = 235 − 400 × T1  T 2  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 Temperature (°F) = 455 − 720 × T1 oversampling. This spreads the quantization noise over a much  T 2  wider band than that of the input signal, improving overall noise The time periods T1 (high period) and T2 (low period) are performance and increasing accuracy. values easily read by a microprocessor timer/counter port, with The modulated output of the comparator is encoded using a the above calculations performed in software. Since both peri- circuit technique which results in a serial digi- ods are obtained consecutively, using the same clock, tal signal with a mark-space ratio format that is easily decoded performing the division indicated in the above formulas results by any microprocessor into either degrees centigrade or degrees in a ratiometric value that is independent of the exact frequency Fahrenheit values, and readily transmitted or modulated over a of, or drift in, either the originating clock of the TMP03 or the single wire. Most importantly, this encoding method neatly user’s counting clock. –4– REV. B Document Outline RevHistory_TMP03_04.pdf REVISION HISTORY