dac
I was reading an article on how to this guy was generating a sine wave digitally. He had a DAC made out of resistors. What I'm unsure about is what type of dac it is? To me it doesn't look like an r-2r dac.
I've noticed the resistors R20-R22 are the mirrior image of R23-R25, am assuming there is a particular reason for this?
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 – 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.
All DACs require a voltage or current reference. The reference can be supplied by an external chip, or be built-in to the DAC.
Here is one thing you should check: Is your reference oscillating?
Causes of reference oscillation include:
not using a filter capacitor that a manufacturer recommends on the reference output of DACs with built-in references
1.1 not using a filter capacitor that a manufacturer recommends on a external reference output
oscillation in an op-amp circuit which "buffers" the reference due to capacitive loading or design error.
3.1 Not using an op-amp "buffer" circuit when one is required. Are you drawing too much current from the reference?
Excessive noise on any power supply (try powering up your circuit from the appropriate combination of batteries)
It does not look like a digital interference problem because the disturbance seems to be very steady, and there are no "spikes" or other discontinuities in the waveform. To be sure, please measure the frequency of the disturbance and find interference that varies at the same frequency or an i/o port read or write that occurs at the same frequency
Best Answer
I can't say for certain without simulating the circuit, but it looks as though the circuit is designed to produce a certain waveform if the Q signals are switched through the sequence 000000 100000 110000 111000 111100 111110 111111 or the reverse. The symmetry of the resistors on the left and right side will cause the resulting waveform to likewise be symmetric. This approach may be used as the first step to generating a sine-ish waveform. It may also be used to reduce the level of high-frequency components in a logic-level waveform without causing timing distortion. For example, suppose one had a 115,200bps serial data stream that one wished to send out over a wire while minimizing radiated emissions. If one used an analog filter to remove the high-frequency components, a rising edge which is preceded by a long low time could be delayed by a different amount from a rising edge one which follows a short low pulse. By contrast, if one fed the signal into a shift register whose outputs drove a resistor network as shown here, and the shift register was clocked at e.g. 9.216MHz (baud rate times 8), such a circuit would reduce the quantity of signal components in the range from 115.3KHz to 4.6Mhz. It's much easier to design an analog circuit that will filter out stuff over 4.6Mhz while leaving 115,200-baud data undisturbed, than to design one which will has filter out stuff at 130Khz but leave the data undisturbed.