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 V_DD. The filter is configured for single-supply operation, with V_DD at 5 V, V_SS 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).

See associated figure

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Reader Comments Hello sir, I need a solution for the following problem: A square wave of frequency 100kHz is passed through a band pass filter and tuned to 100kHz. what is the output in time domain and frequency domain using fourier transforms or analysis.. Sir, please help me in solving this problem.. Thank you, Shanthi. Shanthi -December 29, 2008 hallo... anybody can tell me or have a example circuit for stand alone solar system..pleaseee Anonymous -September 07, 2008 VERY NICE SITE Anonymous -July 26, 2008 VERY NICE SITE Anonymous -July 26, 2008 sir I need a simple square to sinewave converter Anonymous -January 23, 2008 sir i need a circuit design of an power inverter(output sinewave 60Hz, 120watts)sir if u know pls let me know! Anthony -January 18, 2008 i need a vco (output sine and 10 hz to 100 khz) elec313 -August 26, 2007 sir i wanna make a circuit which generates sine wave to square wave in which i m not able to get the circuit of sine wave frequency divider .... sir if u know the circuit pls let me know..... Anonymous -November 12, 2006 i need to generate a sine wave whose frequency is equal to he frequency of the square wave input given to the circuit... the square wave freq may vary.. freq of sine wave should be equal to that of square wave... kindly reply soon.. need for project.. sumedh -September 30, 2006 sir, i want to generat sine wave of 40khz. so is it possibal using 16f73 pic is from that i generat square wave and the convert it in to sine . emam -July 12, 2006 i want to convert square wave genrated from PLL VCO in to Sine wave Wat to do.Plz tell,i wanna use Butterworth low pass filter ,Is it right.plz tel MR D.V.Rathour -April 24, 2006   (Article Rating: ) i need to generate sine wave of frequencies ranging from 10 to 1000Hz.. how could i do this by using pic16f77... kindly need yr help prabhu -February 14, 2006 can u suggest me phase difference circuit or IC which can give the phase difference output of two input sine signals of 50khz frequency Veejays -November 01, 2005 Where can I find RDD104 and MSFS5 IC for square wave to Sine wave conversion Durgansh -August 25, 2005   (Article Rating: ) Try the MAX292 or the MAX295 http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1370/ln/. Same type of chip. A little pricey but I have been uing it for years. These are dual voltage, and there are single voltage units too. They come in 50:1 and 100:1 clock to signal ratios. If you divide the clock frequency by 64, the 50:1 filter will give you an output 1.414 times the amplitude of the square wave input. the 100:1 will give an output slightly less than the amplitude of the square wave. Either way, the waveform will be in 64 steps. Bill Vancura -April 27, 2005 WHERE CAN I GET THE MSFS5 DEVICE? IT APPEARS NO ONE HAS IT. GET BACK A.S.A.P. PLEASE. WILLIE L. MCCAIN -March 15, 2005   (Article Rating: ) Mixed-signal integration data sheet is at: http://www.mix-sig.com/prodsh_Msi5data4y.html John Ambrose -January 26, 2005 Who makes the MFFS5? Google doesn't find anything for "MSFS5 filter." Anonymous -January 26, 2005
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