TLC2274 is a fantastic chip, but take care, that less than 1mA is flowing through the outputs!



Quite simple, use an additonal 9V battery for the 79L05, as I do in most of my applications.



In order to prevent digital ground return currents to flow across the analog ground routing a strict separation of analog and digital ground is necessary. Both grounds must be connected at only one place, best directly at ADC. Also, as digital and analog section both share the same positive supply voltage, low pass filters in the supply lines are helpful.
In the following example both can be seen, the star point grounding and filters in the supply lines (shown "MCP3201" is a 12bit ADC):



You can also insert a LRC filter into the supply line of your micro.
In your application you can use LRC filters consisting of a good ferrite bead (e.g. 6 hole choke), a 4R7 resistor and a parallel combination of 100n X7R and 10µ/10V tantal. This combination can be used for the micro, the PCF8591 and the TLC2274. (Each one gets his own, of course.)

One last hint: Check the idle currents of 78L05 and 79L05. No more than 3mA should flow!

Kai