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I would further ask:
- What do you mean by "pure" sine wave? What is your max harmonic distortion?
- What do you mean by good? (how good?, what noise ratio, etc...)
- What do you mean by cheap? Depending on the application, 1 Euro can be expensive and 100 Euros can be cheap.
About 1~3 Euro max
Pure sine wave means that the thd should be less then 1%
Noise ratio should be withing a acceptable range
Actually I want to build a Descent Sinewave Inverter for my house.
I will assume that your "acceptable" noise ratio is in the same range (less than 1%) as the THD.
-> On one 2ms period, around 0 V, you will have your max quantization noise. Choose the
sampling period small enough so that this noise fits your needs. Be careful with your SPI
requirement: if you find out that your sampling frequency should be, say, 2000 steps per period,
then you have to output samples at 1 MHZ. Then, since 1 sample will require at least 8~10 bits
depending on the converter, plus some possible heading data, then you will end up with a SPI
frequency around 10 MHz. One more thing: if you use a converter with a resolution greater
then 8 bits, then you will likely need to send 2 bytes per sample, which will double your SPI
frequency. Not all microcontrollers are capable of sending SPI data at 20 MHz. Check with ATMEGA.
Here is an ultra simple, high accuracy (for the price & simplicity) DDS you can make very cheaply. For a fixed frequency you can eliminate the rs-232 & other interfaces, even eliminate the crystal, and have just the uC plus 9 resistors to do the job. Some code changes will be required, but that should be easy.
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