Powering multiple ADCs from a single power supply in bipolar mode
First of all, you need to convert 9V to 5V. The easiest way is to find a linear voltage regulator or LDO (low dropout regulator, essentially the same thing). Its advantage is low noise. And for your low-current application it would be the best choice. Besides, if you look for fixed voltage output versions, you can find just a 3-pin linear voltage regulator that will step down 9V to 5V for you.
Input pin (9v), ground pin, output pin (5v), that's it. Watch out: most linear regulators have minimal load current. They MUST output some minimum current to keep output voltage spable, otherwise they could overshoot and burn ADC. ADCs need VERY little current, so you may want to put a resistor between Linear V. Regulator's output and ground just to have leaking current that will keep it stable (at 5V you can take 500Ohm (standrad value 470R) resistor and you'll have 10ma current leakage, which will work with pretty much any regulator (most need 2ma-10ma), but if your regulator needs less, you can reduce the current by increasing resistance, 5V/R = Iminimum, if such resistor doesn't exist, take next lower standard value - current will be just over minimum then and you'll be within the specs). If you find some regulator without minimum load current required or with very low requirements, that would be a huge win, especially for a battery-powered device you're making. So you may need to look through a dozen datasheets, some will mention &quot;no minimum load current&quot;, saw one like that recently, but they're not common, you may want to go with some microamp-range minimum load regulators, which are easier to find.
So you connect all your ADCs in parallel to the same 5V regulator output. A wire from 5V to ADC1, a wire from the same 5V to ADC2 and so on. And connects all grounds together as well of course. You may want to put a 0.1uF capacitor next to EACH ADC's power pin to reduce noise, but if you want to test the circuit, you don't really need them at first, everything will function without them, they're for stability and precision, not needed at this stage of development.
And DON'T use 2 batteries in series, you don't want to convert 18V to 5V! It's a lot of wasted energy and a capacity of only 1 battery. One battery at a time is better. Or you'll need to put batteries in parallel for 9V and double capacity. Google how to parallel the batteries, it's not so simple to make it safe, you can't just connect them with each other directly (it will even work, but it's unhealthy for batteries long-term and potentially dangerous).
Advantage of this circuit: small, simple, few basic components, easy to understand, stable, low noise
I don't recommend switching regulator here. Current requirements are low enough for linear regulator to make sense, besides, linear has a lot less noise, which is good for ADC.
EDIT
The solution you found will work fine. As for connecting them all to the same supply - YES. All to the same +-2.5V. Later add bypass cap for every of them in final design, but it will already work. ADCs take microscopic currents, the chips can easily power some 1000 of ADCs at a time