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ADS7142 Datasheet(PDF) 65 Page - Texas Instruments |
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ADS7142 Datasheet(HTML) 65 Page - Texas Instruments |
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65 / 74 page  LPV812 R C ADS7142 SCL SDA ALERT 1.8V to 3.3V HOST MCU PIR Sensor Copyright © 2017, Texas Instruments Incorporated 65 ADS7142 www.ti.com SBAS773A – SEPTEMBER 2017 – REVISED DECEMBER 2017 Product Folder Links: ADS7142 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated 8.2.2 Event-triggered PIR sensing with ADS7142 Figure 119. PIR Sensor with ADS7142 8.2.2.1 Design Requirements A passive infrared (PIR) sensor is a commonly used sensor to detect motion by measuring infrared light emitted from any object that generates heat. PIR sensors are small, inexpensive, low-power, rugged, have a wide lens range, and are easy to use. PIR sensors are commonly used in security lighting and alarm systems used in indoor environments. When there is no motion or heat-emitting object in the vicinity of the sensor, the PIR sensor output is a DC voltage which is typically specified in the PIR datasheet. When a source of heat, such as a person or animal, comes into the sensor field of view, then the PIR sensor output changes. The amplitude of this signal is proportional to the speed and distance of the object relative to the sensor and is in the range of millivolts peak- to-peak. PIR sensors are often followed by a signal conditioning stage which amplifies the IR sensor output. A PIR sensor can be interfaced with the ADS7142 to make an ultra-low-power, autonomous PIR motion detector. The Autonomous Modes of the ADS7142 with threshold monitoring enables the system to put the host MCU into a low-power sleep mode and wake up the MCU only when motion is detected by the PIR sensor. Figure 119 shows a typical block diagram for an autonomous PIR motion detector using the ADS7142. 8.2.2.2 Detailed Design Procedure The analog signal conditioning circuit is shown in the schematic in Figure 120. The first stage of the amplifier filter acts as a bandpass filter while the second stage applies an inverting gain. Components R10 and C5 serve as a low-pass filter to stabilize the supply voltage at the input to the sensor. Resistor R5 sets the bias current in the JFET output transistor of the PIR motion sensor. To save power, R5 is larger than recommended and essentially current starves the sensor. This comes at the expense of decreased sensitivity and higher output noise at the sensor output, which is a fair tradeoff for increased battery lifetime. Some of the loss in sensitivity at the sensor output can be compensated by a gain increase in the filter stages. Stage 1 of Figure 120 is arranged as a non-inverting gain filter stage. This provides a high-impedance load to the sensor so its bias point remains fixed. Because this stage has an effective DC gain of one due to C2, the sensor output bias voltage provides the DC bias for the first filter stage. Feedback diodes D1 and D2 provide clamping so that the op amps in both filter stages stay out of saturation for motion events which are close to the sensor. Stage 1 has a low and high cutoff frequency of 0.7 Hz and 10.6 Hz respectively and a gain of 220. Stage 2 is arranged as an inverting summer gain stage and is AC-coupled to Stage 1. A DC bias of VCC/2 is connected to the non-inverting input of the amplifier in this stage. Due to the higher gain in the filter stages and higher output noise from the sensor, care must be taken to optimize the placement of the high-frequency filter pole and the window comparator thresholds to avoid false detection. |
Similar Part No. - ADS7142_17 |
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Similar Description - ADS7142_17 |
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