Based on your comment about the micro, do you actually need 4 MSPS? Are you powering up, taking a lot of samples, and then powering down, or are you just taking a few samples each time you power up? TI and Microchip both make some great little low power A/Ds if you only need
10 bits or so and only a few KSPS.
Often times, if you are also powering down your analog circuitry (op- amps and the like), they will take far longer to stabilize than the A/ D itself. At the company I work for, we do extremely low power embedded designs, and have used this trick ourselves, completely turning off power to all of the analog circuitry including the A/D when not in use. It sometimes takes several tens of milliseconds for everything to re-stabilize after reapplying power due to the capacitances in the analog circuitry and the op-amp chips. The A/D usually has been powered up and ready long before the power to it has stopped ringing and fully stabilized enough to take accurate readings. Your best bet is to put an oscilloscope on the power pin and reference of the A/D and see first of all how long it takes that to come up and stabilize. Chances are that will actually be the deciding time for you, more so than the A/D spec or how long it takes to do a few samples before powering back down.
Of note, if all you are powering down is the A/D, you might want to look into getting an A/D with a built-in sleep mode rather than completely pulling power. Several manufacturers such as Linear Tech are now making A/Ds with sleep mode currents less than 2 uA, and exiting sleep mode will almost always be faster than waiting for power to stabilize after turning it on.
Good luck!
Tim snipped-for-privacy@rogue-engr.com
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