Why my DAC works without GND and VCC?
All in- and outputs on most digital devices today have clamp diodes to the power rail. These diodes are there for protection of the device, to prevent a pin from having a higher or lower voltage than the supply rail. What you are experiencing here is that you actually power the DAC through its data pins and the respective protection diodes. Although the device seems to work, it is not designed for this mode of operation. For example you may easily exceed maximum pin current or maximum protection diode current. It is often unspecified if and how a device fails when doing so.







simulate this circuit &ndash; Schematic created using CircuitLab

And below is what happens when you connect two or more input pins, remove the regular power supply (and add the decoupling cap). If you look carefully, you'll notice that the four diode clamps form a diode bridge rectifier (Graetz circuit). According to the article linked in the @PeteKirkham comment below, the external cap isn't even necessary. Probably the parasitic capacity in the device is in that case already enough to power the device.







simulate this circuit