The 74HC595 has long been one of my favourite logic ICs. It takes a serial bitstream and converts it into a parallel output with 3-state drivers. They can be cascaded almost infinitely, allowing you to create extremely wide parallel output stages for driving large LED displays or just about anything else that needs an on/off signal. This 8-Bit Serial DAC uses the 74HC595 shift register to drive an R2R resistor ladder, creating a basic DAC (digital to analog converter) that’s not only inexpensive but opens up the internals of a DAC for you to probe with test equipment.
The downsides of using an R2R ladder are typically decreased accuracy and higher integral and differential nonlinearity. Don’t know what those are? Well, this kit is a great way to learn! Because discrete resistors are never quite the exact same value, you end up with slight differences between the ideal voltage output at any particular value and what is actually output. Even in DACs that are part of integrated circuits, getting the resistors laser-trimmed can’t quite remove all this nonlinearity. Instead, it can just be reduced to an acceptable level for any particular application.
Because the 74HC595 is limited to roughly 30MHz (individual devices vary, and you can test a few to find some that can handle higher frequencies), an 8-Bit DAC like this is limited to at most 3.75Msps. This is more than adequate for even very high-quality audio output, and even high enough to do some basic RF experimentation in the AM radio band.
The project has been released under the MIT license, so you can study the implementation and make changes or implement a similar circuit yourself on a breadboard! But for the price, it’s nice to have all the components on a PCB with a convenient pin header for attaching to a microcontroller.
