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Issue #214 May 2008
Where Analog And Digital Collide
An Easy-To-Use LCR Meter
Third Place Microchip 2007 Design Contest
by Miguel Rusch
Start | LCR Meter | Back to Basics | The Big Picture | Creating a Wave | Analog Stages | Signal Conditioning | User Interface | Firmware | Take a Measurement | System Performance | Further Development | What's Next? | Sources & PDF
CREATING A WAVE
Creating the sinusoidal test signal easily could have been the most difficult part of this project. Luckily, I found an effective solution in the form of an AD9833 DDS signal generator chip. The device runs as a SPI slave whose 10-bit DAC outputs a square, triangle, or sine wave based on an internal 28-bit frequency-setting register and an external clock signal.
A DDS chip enables the creation of waveforms over a wide frequency range with low distortion and requires no additional filtering. The master clock signal for the DDS chip was created from the dsPIC30F4012 PWM port. The clock frequency was set at 1 MHz to deliver both good frequency resolutions from the 28-bit control register and minimize noise transfer to the measurement’s frequencies.
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