Design Goals

Design Description

Some MSP430™ microcontrollers (MCUs) contain configurable integrated signal chain elements such as op-amps, DACs, and programmable gain stages. These elements make up a peripheral called the Smart Analog Combo (SAC). For information on the different types of SACs and how to leverage configurable analog signal chain capabilities, visit MSP430 MCUs Smart Analog Combo Training. To get started with your design, download the Low-Noise Long-Range PIR Sensor Conditioner Circuit Design Files.

This design leverages two of the four integrated op-amp blocks (SACs) in the MSP430FR2355 MCU. Two SAC_L3 peripherals are configured as cascaded op-amps in general-purpose mode to amplify and filter the signal from a passive infrared (PIR) sensor. The circuit includes multiple low-pass and high-pass filters to reduce noise at the output of the circuit to be able to detect motion at long distances and reduce false triggers. The output of the second-stage op-amp in this circuit can be internally or externally connected to other integrated peripherals in the MSP430FR2355 MCU. For example, the analog-to-digital converter (ADC) window comparator can sample this output periodically (with no CPU intervention) and trigger an interrupt when the signal crosses a threshold, indicating motion or an alert.

Design Notes

• The common-mode voltage and output-bias voltage are set using the resistor dividers between R₂ and R₃ (and R₇ and R₈).
• Two or more amplifier stages must be used to allow for sufficient loop gain.
• Additional low-pass and high-pass filters can be added to further reduce noise.
• Capacitors C₄ and C₈ filter noise by decreasing the bandwidth of the circuit and help stabilize the amplifiers.
• RC filters on the output of the amplifiers (for example, R₆ and C₅) are required to reduce the total integrated noise of the amplifier.
• The maximum gain of the circuit can be affected by the cut-off frequencies of the filters. Adjust the cut-off frequencies to achieve the desired gain.
• For this design, two SAC_L3 peripherals in the MSP430FR2355 MCU are configured as cascaded op-amps in general-purpose mode.
• This design can also be implemented by using the transimpedance amplifier (TIA) and SAC_L1 peripheral in the MSP430FR2311 MCU for the cascaded op-amps, but since the maximum input voltage of the TIA is limited to VCC/2, limit the common-mode voltage and gain accordingly.
• The Low-Noise Long-Range PIR Sensor Conditioner Circuit Design Files include a code example demonstrating how to properly configure the SAC_L3 and ADC window comparator peripherals in the MSP430FR2355 MCU.

Design Steps

1. Choose large-valued capacitors C₁, C₅, and C₉ for the low-pass filters. Select these capacitors first because large-valued capacitors have limited standard values to select from compared to standard resistor values.
   ${\mathrm{C}}_1={\mathrm{C}}_5={\mathrm{C}}_9=10\mathrm{\mu F}$
2. Calculate resistor values for R₁, R₆, and R₁₁ to form the low-pass filters.
   $$\begin{gathered} {\mathrm{R}}_1={\mathrm{R}}_6={\mathrm{R}}_{11}=\frac{1}{2\mathrm{\pi }\times {\mathrm{f}}_{\mathrm{H}}\times {\mathrm{C}}_1}=\frac{1}{2\mathrm{\pi }\times 10\mathrm{Hz}\times 10\mathrm{\mu F}}=1.592\mathrm{k\Omega } \\ \mathrm{Choose} {\mathrm{R}}_1={\mathrm{R}}_6={\mathrm{R}}_{11}=1.6\mathrm{k\Omega } (\mathrm{Standard} \mathrm{value}) \end{gathered}$$
3. Select capacitor values for C₂, C₃, C₆, and C₇ for the high-pass filters.
   ${\mathrm{C}}_2={\mathrm{C}}_3={\mathrm{C}}_6={\mathrm{C}}_7=33\mathrm{\mu F}$
4. Calculate the resistor values for R₄ and R₉ for the high-pass filters.
   $$\begin{gathered} {\mathrm{R}}_4={\mathrm{R}}_9=\frac{1}{2\mathrm{\pi }\times {\mathrm{f}}_{\mathrm{L}}\times {\mathrm{C}}_2}=\frac{1}{2\mathrm{\pi }\times 0.7\mathrm{Hz}\times 33\mathrm{\mu F}}=6.89\mathrm{k\Omega } \\ \mathrm{Choose} {\mathrm{R}}_4={\mathrm{R}}_9=6.9\mathrm{k\Omega } (\mathrm{Standard} \mathrm{value}) \end{gathered}$$
5. Set the common-mode voltage of the amplifier to mid-supply using a voltage divider. The equivalent resistance of the voltage divider should be equal to R₄ to properly set the corner frequency of the high-pass filter.
   $$\begin{gathered} {\mathrm{R}}_2={\mathrm{R}}_3={\mathrm{R}}_7={\mathrm{R}}_8=2\times {\mathrm{R}}_4=2\times 6.9\mathrm{k\Omega }=13.8\mathrm{k\Omega } \\ \mathrm{Choose} {\mathrm{R}}_2={\mathrm{R}}_3={\mathrm{R}}_7={\mathrm{R}}_8=13.8\mathrm{k\Omega } (\mathrm{Standard} \mathrm{value}) \end{gathered}$$
6. Calculate the gain required by each gain stage to achieve the total gain requirement. Distribute the total gain target of the circuit evenly between both gain stages.
   $\mathrm{Gain}=\frac{90\mathrm{dB}}{2}=45\mathrm{dB}=177.828\frac{\mathrm{V}}{\mathrm{V}}$
7. Calculate R₅ to set the gain of the first stage.
   $$\begin{gathered} {\mathrm{R}}_5=\left(\mathrm{Gain}-1\right)\times {\mathrm{R}}_4=\left(177.828\frac{\mathrm{V}}{\mathrm{V}}-1\right)\times 6.9\mathrm{k\Omega }=1.22\mathrm{M\Omega } \\ \mathrm{Choose} {\text{R}}_5=1.23\mathrm{M\Omega } (\mathrm{Standard} \mathrm{value}) \end{gathered}$$
8. Calculate C₄ to set the low-pass filter cut-off frequency.
   $$\begin{gathered} {\mathrm{C}}_4=\frac{1}{2\mathrm{\pi }\times {\mathrm{f}}_{\mathrm{H}}\times {\mathrm{R}}_5}=\frac{1}{2\mathrm{\pi }\times 10\mathrm{Hz}\times 1.23\mathrm{M\Omega }}=12.939\mathrm{nF} \\ \mathrm{Choose} {\mathrm{C}}_4=13\mathrm{nF} (\mathrm{Standard} \mathrm{value}) \end{gathered}$$
9. Since the gain and cut-off frequency of the first gain stage is equal to the second gain stage, set all component values of both stages equal to each other.
   $$\begin{gathered} {\mathrm{R}}_1={\mathrm{R}}_6=1.6\mathrm{k\Omega } \\ {\mathrm{R}}_7={\mathrm{R}}_8=13.8\mathrm{k\Omega } \\ {\mathrm{R}}_9={\mathrm{R}}_4=6.9\mathrm{k\Omega } \\ {\mathrm{R}}_{10}={\mathrm{R}}_5=1.23\mathrm{M\Omega } \\ {\mathrm{C}}_8={\mathrm{C}}_4=13\mathrm{nF} \end{gathered}$$
10. Calculate R₁₁ to set the cut-off frequency of the low-pass filter at the output of the circuit.
    $$\begin{gathered} {\mathrm{R}}_{11}=\frac{1}{2\mathrm{\pi }\times {\mathrm{f}}_{\mathrm{H}}\times {\mathrm{C}}_9}=\frac{1}{2\mathrm{\pi }\times 10\mathrm{Hz}\times 10\mathrm{\mu F}}=1.592\mathrm{k\Omega } \\ \mathrm{Choose} {\mathrm{R}}_{11}=1.6\mathrm{k\Omega } (\mathrm{Standard} \mathrm{value}) \end{gathered}$$

Design Simulations

AC Simulation Results

Noise Simulation Results

Target Applications

• Motion detector
• Occupancy detection (people tracking and people counting)
• Building automation
• IP network camera
• Lighting sensor
• Thermostat
• Video doorbell

References

1. Texas Instruments, Low-Noise Long-Range PIR Sensor Conditioner Circuit, design files
2. Texas Instruments, 16MHz integrated analog microcontroller with 3.75-KB FRAM, Op Amp, TIA, comparator with DAC, 10-bit ADC, product page
3. Texas Instruments, How to Use the Smart Analog Combo in MSP430^TM MCUs, application report
4. Texas Instruments, MSP430 MCUs Smart Analog Combo, training video

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