Standalone Circuit Converts Square Waves To Sine Waves

Description: Many microcontrollers or PICs have uncommitted digital-to-analog converter (DAC) outputs that can generate sine waves. However, these outputs typically have low resolution (8 to 10 bits), resulting in total harmonic distortion (THD) in the range of 1%. Alternatively, using a fifth- or seventh-order switched-capacitor filter with a square-wave output requires the use of two I/O pins on the microcontroller unit (MCU). One pin is designated for the filter input, while the other is used for the filter clock. Both outputs must produce square waves that maintain a 100:1 ratio. Due to the additional processes the MCU must manage beyond sine wave generation, dedicating two timers or one timer and firmware often imposes excessive system overhead. Consequently, designers may need to opt for a faster or more expensive MCU. A more efficient solution involves utilizing an RDD104 selectable four-decade CMOS divider and an MSFS5 switched-capacitor filter to create a two-chip sine wave source with 0.2% distortion. The RDD104 features two pins that allow selection among four dividers: divide-by-10, divide-by-100, divide-by-1000, and divide-by-10k. This device can operate with either an external clock on pin 5 or with a crystal. Its maximum frequency range is 1.5 MHz at 5 V DC. The schematic illustrates the square-wave-to-sine-wave converter. A crystal and a 10 MΩ resistor connect across pins 5 and 6 of the RDD104, with a 100 pF capacitor (C5) connected to pin 5. The input capacitance of the MSFS5, along with the connection between pin 6 of the RDD104 and pin 4 of the MSFS5, ensures equal capacitance on pin 2 of the crystal. With DIV_SEL_1 set low and DIV_SEL_2 set high, the 100:1 divider is selected. The MSFS5 is a pin-selectable, seventh-order, low-pass/six-pole bandpass switched-capacitor filter capable of being configured as Butterworth, Bessel, or elliptic low-pass filters, as well as full, 1/3-, and 1/6-octave bandpass filters. The Clock_Out of the RDD104 is AC-coupled to the Clock input of the MSFS5, which is configured for 1/6-octave bandpass operation to maximize attenuation of square-wave harmonics without affecting the fundamental frequency. Bandpass and 1/6-octave configuration is achieved by connecting FSEL and TYPE to VDD. The filter operates on a single supply, with VDD at 5 V, VSS at 0 V, and GND connected to mid-supply using two resistors (R4 and R5). A 0.1 µF capacitor decouples the input. The output from the RDD104 is attenuated using two 10 kΩ resistors and AC coupled to the filter input of the MSFS5. This configuration produces a 10 kHz, 1 V RMS sine-wave output. The total current consumption is less than 2 mA at 5 V DC, making this solution suitable for portable applications. The total harmonic distortion is measured at 0.2% over a bandwidth from 400 Hz to 300 kHz using an AP Portable One Plus Access.

The described circuit employs a combination of a CMOS divider and a switched-capacitor filter to effectively generate high-quality sine waves from a microcontroller. By utilizing the RDD104, the circuit can achieve precise frequency division, allowing for flexible sine wave generation while minimizing the load on the MCU. The MSFS5 filter further enhances the output by selectively filtering out unwanted harmonics, ensuring that the fundamental frequency remains intact. This design is particularly advantageous in applications where low power consumption and compact size are critical, such as in portable electronic devices. The careful selection of components and configuration parameters allows for a robust and efficient sine wave generation system that meets the demands of modern electronic applications.Many microcontrollers or PICs will have uncommitted digital-to-analog converter (DAC) outputs that can be used to generate sine waves. But these are generally low resolution (8 to 10 bits), yielding a total harmonic distortion (THD) in the 1% range.

Or, using a fifth- or seventh-order switched-capacitor filter with a square-wave output ties up two I/O pins on the MCU. One output is used for the filter input and one for the filter clock. Also, the two outputs must be square waves and track at a 100:1 ratio. Because the MCU will have more processes to handle than just generating a sine wave, tying up two timers or one timer and firmware usually requires too much system overhead. Therefore, the system designer is forced to use a faster or more expensive MCU. Here`s a better approach: Use an RDD104 selectable four-decade CMOS divider and an MSFS5 switched-capacitor filter to create a two-chip, 0.

2%-distortion sine-wave source. The RDD104 has two pins that select one of four dividers: divide-by-10, divide-by-100, divide-by-1000, and divide-by-10k. The device can be used either with an external clock on pin 5 or with a crystal. The maximum frequency range is 1. 5 MHz at 5 V dc. The figure shows the schematic for the square-wave-to-sine-wave converter. A crystal and a 10-MO resistor are connected across pins 5 and 6 of the RDD104. A 100-pF capacitor (C5) is tied to pin 5. The input capacitance of the MSFS5 and the connection between pin 6 of the RDD104 and pin 4 of the MSFS5 provides equal capacitance on pin 2 of the crystal.

With DIV_SEL_1 tied low and DIV_SEL_2 tied high, the 100:1 divider is selected. The MSFS5 is a pin-selectable, seventh-order, low-pass/six-pole bandpass switched-capacitor filter. The eight-pin IC can set for Butterworth, Bessel, or elliptic low-pass filters; or for full, 1/3- and 1/6-octave bandpass filters. Clock_Out of the RDD104 is ac-coupled to the Clock input of the MSFS5. The MSFS5 is set for 1/6-octave bandpass operation for maximum attenuation of square-wave harmonics without attenuating the fundamental.

Bandpass and 1/6-octave configuration is obtained by tying FSEL and TYPE to VDD. The filter is configured for single-supply operation, with VDD at 5 V, VSS at 0 V, and GND tied to mid-supply with two resistors (R4 and R5). A 0. 1- µF capacitor decouples the input. The output of the RDD104 is attenuated with two 10-kO resistors and ac coupled to the filter input of the MSFS5.

With this configuration, a 10-kHz, 1-V rms sine-wave output is achieved. Total current consumption is less than 2 mA at 5 V dc, making this solution ideal for portable applications. THD is at 0. 2% for a bandwidth from 400 Hz to 300 kHz (measured with an AP Portable One Plus Access).