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Texas Instruments Reduce False Triggers and Extend Battery Life in PIR Sensing Applications Application notes
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Reduce False Triggers and Extend Battery Life in PIR
Sensing Applications
Peggy Liska
Passive Infrared (PIR) sensors are widely used in
building automation applications to detect motion from
a person or object. An amplifier and window
comparator circuit is often used to gain and filter the
output of analog-output PIR sensors and to generate
microprocessor interrupts during motion events. This
circuit can be implemented using a quad op-amp or a
dual op-amp plus two comparators. Alternatively, a
digital-output PIR sensor with integrated buffers and
comparators can be used. Both the analog-output and
digital-output PIR sensors have distinct advantages,
but also come with trade-offs. Figure 1 illustrates an
innovative PIR sensing circuit architecture consisting
of an analog-output PIR sensor, a nanopower system
monitoring ADC, and a dual-channel signal
conditioning amplifier. The ADC includes some unique
features that can be used to intelligently monitor the
sensor without having to wake up the host MCU.
Vdd
Alert
MCU
TLV8542a
Non-inverting
Bandpass
Filter
TLV8542b
Inverting
Gain
I2C
Programmable
Threshold
ADS7142
Other Sensor
i.e. Thermistor,
PIR, etc
Figure 1. PIR Sensing System Block Diagram
When comparing these different system architectures,
it is important to consider the key trade-offs. Table 1
summarizes these common care-abouts and trade-offs
assuming cost comparability.
In systems where robustness is important to help
avoid false triggers and thereby save overall power
consumption and increase overall battery life, the
“Dual Op-Amp + ADC + Analog PIR” solution can offer
better performance compared to the other solutions.
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Circuit
Battery Life
Robustness
No.
of
ICs
Digital PIR
Sensor
Highest power
Fixed sensitivity and
filtering of the input
signal
1 IC
Quad OpAmp +
Analog PIR
Lower power than
Digital PIR
Fixed sensitivity and
filtering of the input
signal
2 ICs
Dual OpAmp +
Comparator
+ Analog
PIR
Lowest power
Fixed sensitivity and
filtering of the input
signal
3 ICs
Programmable
thresholds offer flexible
sensitivity and filtering
of the input signal
3 ICs
Dual OpLower power than
Amp + ADC the Quad Op-Amp
+ Analog
+ Analog PIR and
PIR
Digital PIR Sensor
Reduce False Triggers with Programmable
Thresholds
PIR
+
Table 1. PIR Sensing System Solutions
The Dual Op-Amp + ADC + Analog PIR solution offers
programmable comparator thresholds to help fine-tune
the sensor trigger window and thereby reduce false
triggers. The ADS7142 is a nanopower sensor monitor
with an integrated window comparator that uses
programmable high and low thresholds and an alert
output to wake up the main controller from its lowest
power state when motion is detected. These channelindependent thresholds are more flexible than a fixedvalue resistor divider Dual Op-Amp + Comparator +
Analog PIR approach and therefore enable easier
device-to-device calibration at the time of production
as well as in-the-field calibration. This can help
improve the overall accuracy of the PIR sensing
circuitry by individually adjusting for the manufacturing
variances between sensors. The ADS7142 also
incorporates a pre-alert event counter that can help
filter out unwanted input noise. This feature ensures
that the input signal remains above (or below) the
programmed threshold level for a specified number of
samples before the alert signal is generated. This
helps reduce false triggers due to transients or noise
on the PIR sensor output signal. Figure 2 show an
example of the event counter programmed to a count
of 8 and the effect this has on the Alert output signal
based on the PIR sensor output (ADC input) signal.
Reduce False Triggers and Extend Battery Life in PIR Sensing Applications
Copyright © 2018, Texas Instruments Incorporated
Peggy Liska
1
www.ti.com
NOTE: PRE_ALT_MAX_EVENT_COUNT = 70h
(waits for 8 counts to set alert)
2
High Threshold
1
3
8
2
4
1
5
High Threshold - Hysteresis
7
3
4 5
6
6
Counter Reset because the high-side-comparator reset before 8.
most integrated, digital-output PIR sensors. Table 2
summarizes this comparison with several different
amplifier options. The ADC and Op-Amp power
consumption numbers are taken from the datasheet
while the PIR sensor power consumption was taken
from a measured system. The TLV8802 amplifier
consumes 320 nA per channel in a leaded VSSOP
package while the TLV8542 consumes 550 nA per
channel in a 1.5 mm by 1.5 mm leadless QFN
package. The main difference between the TLV8802
and TLV8542 is power versus size with the TLV8802
being lower power and the TLV8542 being smaller.
Table 2. PIR Sensor Solution Comparison
ALERT
ADS7142
+ LPV8802
ADS7142
+ TLV8542
Digital Output
PIR Sensor
(PYD1588/89)
ADC
500 nA
500 nA
N/A
Op-Amp
640 nA
1.1 µA
N/A
Figure 2. ADS7142 Pre-Alert Counter for Filtering
The ability of the ADS7142 to validate the input signal
prior to sending out the alert helps improve the overall
robustness of the measurement by avoiding false
triggers.
Extend Battery Life with Autonomous Monitoring
PIR
Sensor
600 nA
600 nA
3 µA (typ),
5 µA (max)
Total
1.74 µA
2.2 µA
3 to 5 µA
In addition to reducing false triggers, the
programmable thresholds also enable the system to
be more power-efficient. The ADS7142 and op-amp
circuit autonomously monitors the input signal which
allows the MCU to stay in its lowest power mode
longer. The integrated oscillator on the ADS7142
allows the device to operate completely on its own
without intervention from the MCU.
In addition to being low power, the solution in Figure 1
is also optimized for small size. The QFN package of
the ADS7142 measures only 1.5 mm by 2.0 mm while
the QFN package of the TLV8542 measures only 1.5
mm by 1.5 mm. With the required passive
components, the design can get down to less than 10
x 14 mm. This solution is also competitively priced to
be attractive for cost-sensitive designs.
This fully-autonomous monitoring is also done at
industry-leading low-power levels. At 1.8V, the
ADS7142 consumes < 1 µW of power (500 nA at 1.8
V) and the TLV8542 conditions the PIR signal with
minimal impact to the power budget, consuming only
550 nA per channel. In a real PIR measurement
system, the total combined current consumption of the
ADC, op-amp, and PIR sensor was measured to be
2.2 µA. This combined power consumption is less than
In conclusion, there are several different approaches
for conditioning PIR sensor output data in motion
detection systems. The Dual Op-Amp + ADC + Analog
PIR solution offers flexibility for false trigger avoidance
and lower power consumption at a competitive price
and size compared to a digital-output PIR sensor
solution.
2
Reduce False Triggers and Extend Battery Life in PIR Sensing Applications
Peggy Liska
Copyright © 2018, Texas Instruments Incorporated
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