I am designing a board with a ±2.5V analog domain and a 3.3V digital domain where space is at a premium. Digital and analog grounds only connect at a single point near the supplies. As such I am using/choosing among a few components that have integrated level shifting so as to directly interface with the 3.3V digital domain. Some of the choices are:
This is a low-noise design where analog signals in the 1µV range will be measured. The main A/D converters do have separate analog and digital supply connections and their Nyquist frequency is 1MHz. This problem particularly applies to the analog switches, but it is much less of a concern for the D/As as either of these will serve auxiliary purposes and will be AC-coupled into the signal paths.
However the 0.8V that the 3.3V rail is above the analog rail introduces problems, as it will bias the protection diodes on some of these components. The same can be said for digital return currents if I connect some of these component ground terminals to the analog ground.
My intended path of action is to:
- Connect all (very slow) digital signals to the switches through 10kΩ resistors, thus limiting the current through the ESD protection diodes into the analog supply to <80µA. (The stated maximum for the switch is ~20mA, power consumption in the analog rail is >10mA) With the 0.1µF bypass capacitor these will form a ~200Hz lowpass filter.
- Possibly add ferrites to the +2.5 supply rail to the analog switches (after the bypass capacitor) to further reduce any noise injection into the analog supply.
- Use digital ground and supply for the D/As. With possibly some filtering of their output.
Does this plan of action seem reasonable?
Is there anything I am not considering?