TMP04 AD [Analog Devices], TMP04 Datasheet - Page 4

• Manufacturer Part Number: TMP04
• Description: Serial Digital Output Thermometers
• Manufacturer: AD [Analog Devices]

TMP03/TMP04
(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.
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
Figure 1. TMP03/TMP04 Block Diagram Showing
First-Order Sigma-Delta Modulator
VOLTAGE REF
GENERATOR
CLOCK
VPTAT
&
INTEGRATOR
MODULATOR
1-BIT
DAC
COMPARATOR
DIGITAL
FILTER
TMP03/04
OUT
(SINGLE-BIT)
–4–
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:
Temperature ( C) = 235
Temperature ( F) = 455
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.
Figure 2. TMP03/TMP04 Output Format
T1
720 T1
400 T1
T 2
T 2
T2
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