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Operating conditions
The storage and operating temperature
range of the detectors is specied from
-40°C up to +85°C. It needs to be noted
that technical data usually reference room
temperature and may vary within the speci-
ed temperature range.
Digital Pyrodetectors –
a New Family
Pyroelectric detectors are AC type devices
and give signals upon change of received
infrared radiation. Until today, all available
detectors were analog, i.e. they provided an
analog signal output. Excelitas is the rst to
introduce a family of detectors which differ
from previous generations by offering a digi-
tal signal output.
With the DigiPyro® Family, Excelitas is
offering digital detectors for many
applications and congurations.
1.1 Integrated Electronics
The DigiPyro® series integrates the rst
stages of circuitry into the detector housing:
Amplication of the signal, then the A/D
conversion, which needs a voltage reference.
Following an internal 10 Hz electrical low
pass lter, the serial interface provides for
the “Direct Link” communication which is
a one wire bidirectional communication
feature. The whole concept runs by
its own internal oscillator, which determines
the speed of the internal process. The
“Direct Link” feature enables the user to
have the host µC request the information
and its resolution, so the host controls the
communication speed.
1.2 From Analog to Digital
The DigiPyro® series is the rst pyroelectric
detector family to display information in Bit
form, as opposed to mV signals of analogue
detectors. To give a measure for compari-
son of traditional detectors to digital ver-
sions, the rule of thumb for signal levels
versus Bit information can be used:
• Resolution: 1 LSB ≙ 6.5 µV
• Range: 0 to 16383 Count=± 53.6 mV
• DC Offset 8192 Count ≙ 53.6 mV
• Noise: 6 Count ≙ 39 µV
(with band-pass)
In typical motion electronics, the
expected signal voltages range from 100 µV
to 500 mV, so the digital signal may range
up to 100 Bit-count. In Gas Detection, the
output range of analog detectors is up to
2 mV, so the digital output may range up
to 20 Bit-count. The dynamic range of the
digital detector is wider than these men-
tioned levels and covers many other
applications.
1.3 Digital Zero Signal Line
As the pyroelectric effect generates positive
and negative signal amplitudes, the detector
circuitry needs an electrical offset to be able
to process such signals. In all analog
circuitry this value is the offset voltage,
which is usually subtracted after the rst
amplier stage.
With DigiPyro® models, the amplication
is included already, and the internal voltage
reference provides for the required offset.
For the user, this offset appears as a digital
zero line at about 8000 bit-count, and it
may vary in series from one part to the next.
To recognize the zero line of the individual
detector, the user may either use a digital
band-pass or subtract the measured offset
from the signal.
1.4 The Host Needs to Filter The Signal
The DigiPyro® does not include any
processing intelligence inside. Unlike most
analog pyrodetectors, the DigiPyro® uses
a direct communication with the hosting
microcontroller without any analog
hardware ltering (only the previously
mentioned low-pass lter). Thus, it
becomes necessary to implement all
necessary ltering by software lters within
the hosting microprocessor of the unit.
Applications
for Pyroelectric Detectors
Pyroelectric detectors had originally
been designed as single element types
for non-contact temperature measurement.
During further research, Dual-Element
types were developed with multi-facet
mirrors or Fresnel lenses, entering the
eld of motion detection, starting as passive
intrusion alarms (Burglar Alarm, PIR),
followed by automatic light switches and
security lights and lamps. The same
concept is also being applied with some
automatic door openers.
Today, the environment and its protection
is one of our most serious global concerns.
Features and instrumentation are required
to measure and monitor all kinds of gases in
our environment. One of the methods
applied is the NDIR technique, a principle
of measuring gas concentration by its
absorption properties in the infrared range.
Our detectors and sensors are a vital part
of making our environment safer.
Most PIR motion detecting devices have
been designed around Dual-Element types,
while more advanced units apply Four-
Element, “Quad” type congurations.
For Gas Sensing, single element with
narrowband lters is applied in a single
or dual-channel conguration.
NEP, D*
The NEP value is a form of signal-to-
noise ratio. The NEP value species the
minimum radiation power that can be
detected by the sensor, resulting in an
output that just exceeds the noise. NEP
refers to RMS values of signal and noise
and in addition to the electrical bandwidth.
The lower the NEP, the better the sensor is.
Sometimes also used for comparison of
sensors, the Specic Detectivity (D*) allows
the characterization of sensing materials.
It is dened as reciprocal of NEP referring
to the sensor area. Details of these
parameters as function of the electrical
frequency are given in Figure 6 below.
Responsivity, Noise vs. Frequency
Figure 6
R (kV/W) Noise (µVrms / √Hz)
f (Hz)
100
10
1
0.1
100
10
1
0.1
0.1 110
–– Responsivity –– Noise
BASICS