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
[email protected]-engr.com
