4-Bit DAC Demo

Here we'll increase the resolution to 4 bits, giving 16 discrete voltage steps. This uses an old (but superior) topology known as R-2R, since it is a ladder made up of two sets of resistors, one set double in value of the other. It's been around forever, but I first saw it in Don Lancaster's CMOS Cookbook, (Indianapolis: Howard W. Sams & Company, 1977), pp. 288, 289. That, by the way, is still one of my favorite reads; I'm always getting something clever from it.

I've added a buffer/amplifier/shifter to transform the waveform to 10Vpp, a common music synthesizer standard. In the demo software, the frequency is about 1 kHz. The resulting waveform looks like this:

I found that I needed a 10pF feedback cap on the op-amp to eliminate an undershoot at the start of the signal. This may have been a result of the breadboard.

Click to get the source code.
Click to get the schematic PDF.

Next Project: 4-Bit Internal DAC Demo