adcanalogcouplingdacmixed-signal
In a mixed-signal IC, you most likely will have two ground planes, analog ground and digital ground. This seems fair enough and easily understandable but my real question is: Why would you need to insert back to back Schottky diodes between two grounds? What purpose would they serve if the ground places are already separated? No coupling should occur if the designer properly managed to split the planes.
Here is a picture of what I am trying to get information about:
Most people drive the rungs of an R2R ladder with standard CMOS output pins. The "topology" directly connected to that output pin is generally a static CMOS inverter:
where 'Q', Vdd, and Vss are physical pins you have access to, and 'A' is internal node of the chip.
Some of them use a few CMOS output pins on a microcontroller to directly drive the R2R rungs. The rest generally use a few output pins on a microcontroller to send data to some sort of latch chip or buffer chip, and then the CMOS output pins of that chip drive the R2R rungs. Often the latch chip is the 74HC595 (if their "arbitrary positive voltage" is in the range of 2.0 V to 6.0 V) or the HEF4069 / CD4069 or the HEF4094 / CD4094 (which can handle "arbitrary positive voltage" in the range of 3.0 V to 15 V).
You do not want to use 1M resistors for the virtual GND divider. Use a much lower resistor value set. For a practical working scenario the resistors should be about 10 to 50 times lower impedance than load impedance that you place on the Headphone signal from the I-Pod.
If this lower impedance divider does not suit well with your battery supplied application then you should look into using a split voltage rail divider chip or create your own. To roll your own you can use an a resistor divider like you have now, place a cap across the lower divider resistor and then buffer the divider using an opamp that is wired up as a standard voltage follower. The output of the op-amp becomes your virtual head phone ground.
Best Answer
And that's part of the problem. Many mixed-signal devices (ADC, DAC, etc.) will misbehave or become damaged if the two grounds are at too different potentials. The diodes prevent them from drifting apart too much.
Obviously if the grounds are supposed to be at completely different potentials (e.g. isolation) then the diodes should not be placed.