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 is a powerful, complete temperature measurement
system with digital output, on a single chip. The onboard tem-
perature 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 digi-
tal 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
VOLTAGE REF
GENERATOR
AND VPTAT
CLOCK
INTEGRATOR
MODULATOR
1-BIT
DAC
COMPARATOR
DIGITAL
FILTER
TMP03/04
OUT
(SINGLE-BIT)
avoids major error sources common to other modulation tech-
niques, 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 repre-
sentation 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 uti-
lized in the TMP03 avoid this problem and allow the overall
circuit to fit into a compact, 3-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’s clock is
irrelevant, and the effects of clock variations are effectively can-
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
readily decoded by the user as follows:
Temperature (°C) =
Temperature (°F) =
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 peri-
ods 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 or the
user’s counting clock.
T1
455 −
235 −






720 × T1
400 × T1
T 2
T 2






T2