# Standalone Circuit Converts Square Waves To Sine Waves

Posted on Feb 5, 2014

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...

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