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A DAC Dilettante Module, Pulling Out all the Stops

Posted by on in Dilettante Boards and Modules
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Saturday was the hottest day of the year so far, but not hot enough to prevent me from working on a new Dilettante module. The module in question is a Digital to Analog Converter (DAC) combined with a Voltage Level Translator. 

The primary component of the DAC portion of the module consists of the MCP4728, which is a 4 channel, voltage output, device from Microchip. The module has been designed for our Digital Signal Processing using Floating Point Arithmetic and FPGAs  series, which is coming soon.

The idea behind this series is to configure different types of waveforms in the FPGA and output them using the DACs on the module. The DAC outputs will then be combined and conditioned, using operational amplifiers and passive components such as filters, before being acquired by the ADC on the DE0 NANO. 

Then, the sampled data will be analysed using algorithms implemented in the Logic Elements (LE) and embedded multipliers found in the Cyclone IV FPGA. The algorithms will be used to perform well known DSP techniques including low, high and bandpass filters and transformation algorithms like FFTs. If the series turns out well we might even get our toes wet in the world of wavelet transforms. All this should be done using floating-point arithmetic in the FPGAs. Why? I hear you ask. Stay tuned to find out.

The DAC uses 4 out of the 12 IO pins connected to the FPGA, when the Mixed Signal Dilettante board is connected to the DE0 Nano. A voltage level translator, the TXS0104E connects the DAC to the FPGA, this allows the DAC to be driven by 3.3V or 5.0V. The higher voltage allows for a higher sample output rate. The module on  the Dilettante board can be seen below.

Figure: The unpopulated DAC module on the prototyping area of the Mixed Signal Dilettante Board. 

Another voltage level translator has been used to interface the unused 7 General Purpose IO (GPIO) pins and 1 input pin signal, on the DE0 Nano, to the outside world. This is the TXS0108E, which like the previous device is manufactured by Texas Instruments. 

As can be seen, in the photographs above and  below, the components have not yet been placed on the board as I couldn't find my "No Clean" Flux pen, which is critical when working with boards like this one that consist of 0805 passives! Yes, that's right I decided to use these small Surface Mount Technology (SMT) components as I had to keep the overall size of the board small enough to fit the prototyping area on the Dilettante board as can be seen below. The DAC module measures approximately 44 mm x 29 mm, which is, agreeably, quite small.

The small size of the board means pulling out all the stops and using every trick in the book known to the DIY PCB hobbyist to make sure the module turns out successfully. This includes using a USB oscilloscope and the helping hands tool as can be seen below.

Figure: Using a USB microscope to visually inspect signal trace connectivity.

Figure: The DAC module seen through the magnifying  lens of the Helping Hands tool.

Once the design is finished and tested the method used will be written and added to the DIY PCB Development section of the website

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