Anti-electromagnetic interference sensor adjustment for home alarm systems

Feeling safe, comfortable and secure at home is a necessity for all families; today, this need is met by installing a home alarm system. With the development of home security systems, even if the owner is not at home, he can receive SMS (Short Message Service) to know if there is any unexpected event at home. But the peace of mind brought by home alarm systems also brings troubles: you know, no one wants to be disturbed by false alarms all the time. Alarm systems should meet certain anti-interference standards that electronic devices must meet. In fact, in systems with built-in GSM (Global System for Mobile Communications) modules, various sensor signals are more susceptible to interference. To help design more anti-interference systems, National Semiconductor has developed enhanced EMI (electromagnetic interference)-resistant operational amplifiers used to condition sensor signals in alarm and other systems.

Alarm System Overview

Figure 1 is a block diagram of a home alarm system, which includes sensors (which can be wired or wireless), a microcontroller, a GSM module, and a power management system. The various types of sensors that can be used include: PIR (pyroelectric infrared) sensors for detecting human movement, door and window protection sensors, gas detection sensors, temperature sensors, and smoke sensors. As shown in the block diagram, the system can be powered from a USB port or a battery, which means it has very low power consumption, thereby maximizing the battery life. Different blocks in the system require different voltage rails, which can be achieved by using the LM3668 buck-boost DC/DC converter and the ultra-low noise LDO chip LP5900, both of which are very suitable for portable applications.

To monitor the temperature, the analog temperature sensor LM94022 can be used. Its low operating current (typically 5.4 μA) and low supply voltage of 1.5V make it very suitable for low-power systems.

PIR Signal Conditioning

The signal conditioning circuit of the pyroelectric infrared sensor used to detect human body heat is described in detail below. The output voltage of the pyroelectric infrared sensor is a function of the infrared rays sensed, with a typical amplitude ranging from 0.5mV to 1mV and a frequency ranging from 0.02Hz to 16Hz. Figure 2 shows the circuit used to condition the sensor signal, and the two stages in the LPV521 provide amplification and bandpass filter functions. As shown in the Bode gain curve, its amplification factor is about 2000 at 4Hz, and the cutoff frequency at 0.6 Hz and 8 Hz is 3 dB. The LPV521 is an operational amplifier used to build active filters. It is a nanopower level 552 nW amplifier suitable for ultra-long battery life. The operating voltage range of 1.6V to 5.5V and the typical 351 nA supply current make it very suitable for remote sensor nanopower applications. The LPV521 has a precision-designed CMOS input stage that far exceeds the 50 pA maximum input bias current of similar devices over the -40°C to 125°C temperature range, and provides rail-to-rail input and output performance. The LPV521 is a member of the PowerWise® family of products with an excellent performance-to-power ratio.

R2 is the load resistor, its value is given in the PIR sensor datasheet, and it is mainly used to convert the sensor output current to voltage.

How EMI-Resistant Operational Amplifiers Work

To ensure maximum suppression of false alarms, extensive EMI filtering is usually integrated into an alarm system. The part of the signal path that is most sensitive to EMI is the interface between the sensor and the operational amplifier. In this part, the signal is analog with very low amplitude, and long cables make it more susceptible to interference. The interface between the operational amplifier and the analog-to-digital converter (ADC) is less sensitive, because the signal is amplified to a higher amplitude and the cables are usually shorter.

Since the critical point is the interface between the sensor and the operational amplifier, National Semiconductor has introduced products with integrated electromagnetic interference (EMI) filters to ensure the accuracy of analog systems by reducing radio frequency (RF) interference.

To help identify EMI-tolerant op amps, a new parameter is needed in the datasheet to quantify the EMI performance of the op amp. This quantitative indicator shows the ability of the op amp to suppress EMI, thereby comparing and ranking op amps based on EMI resistance and chip performance. Similar to CMRR, the electromagnetic interference rejection ratio (EMIRR) is the actual ratio of the change in RF signal to the change in offset voltage.

EMIRR is defined as:

Where Vrf_peak is the actual unmodulated signal amplitude and △ Vos is the resulting input referred offset voltage change.

With the widespread use of wireless data transmission, the emergence of mobile phones, Bluetooth modules and other computer peripherals, electromagnetic interference has become an increasingly concerned issue in many application areas. EMI-resistant operational amplifiers use on-chip filters to suppress unwanted signals, such as RF injection signals at the amplifier input, thereby preventing high-frequency noise from propagating in the amplifier and preventing it from entering the circuit. Integrated EMI filters provide many advantages: they help ensure signal integrity, save space on the printed circuit board, and eliminate the need for external filtering, which saves the corresponding additional cost.

Signal Conditioning for Smoke Sensors

Figure 3 shows another example of EMI-resistant sensor signal conditioning with a smoke sensor interface. An optical smoke sensor consists of an LED, lens, and photodiode. When smoke is present, the light no longer travels in a straight path, but is scattered according to the characteristics and concentration of the smoke. After the path is deviated, the light travels to the light-sensitive photodiode and is converted into a proportional current. This current is then converted to a voltage through the transimpedance amplifier configuration in the LMV831. This operational amplifier has CMOS inputs and low power consumption, and provides low input bias current, a wide temperature range of -40°C to +125°C, and excellent performance, making it a rugged general-purpose part. In addition, its EMI resistance minimizes interference, making it suitable for EMI-sensitive applications.

Figure 4 shows the EMIRR vs. frequency comparison of the LMV831 and a standard amplifier without EMI immunity. For detailed descriptions of EMI measurements, see Application Note AN1698. For products with different accuracy, speed, or power requirements, National Semiconductor offers a variety of EMI-resistant op amps that can be used to design more robust systems, including the LMP2021/22 (single/dual), LMV861/62, LMV851/52/5 (single/dual/quad), and LMV831/32/34.

Summarize

The ubiquity of cellular, Bluetooth, and Wi-Fi signals, as well as the widespread use of sensing systems with integrated radios, has made electromagnetic interference (EMI) an increasingly important consideration in the design of precision signal paths. Although the RF signal is outside the band of the op amp, the RF carrier switching can adjust the DC offset of the op amp. The increased offset voltage is amplified by the corresponding signal, thus changing the measurement value, causing false alarms to be triggered. EMI-resistant op amps use on-chip filters to suppress unwanted RF signals generated at the input and power pins, keeping the precision signal path intact.