Intermittent A/D converter

To save power in a remote device, I want to turn on a circuit containing an A/D converter once every few seconds only. However, I can't find specs for how soon after powering up an A/D its outputs would be valid. I am interested in a low-power device (naturally), 8 bits out, 4 MSPS or better, with max input voltage range about 2-3 volts. Any suggestions?

Reply to
Richard Henry
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The very first part I looked at in your ball park was specd.

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One microsecond from power down. What where you looking at?

Best, Dan.

Reply to
Dan Bloomquist

Good find, except the serial interface means I will have to run some of the connected circuitry about 8 times faster than I want.

Reply to
Richard Henry

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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Reply to
tkirk

It's a remote battery-powered device that powers up, listens for a signal, then responds if addressed or goes back to sleep if nothing is there. The A/D is not needed if it just going back to sleep, only if it has to read a message and respond.

I realized there would be some stabilization time after turning on the A/D. My difficulty is very few parts have a declared specification for it.

Reply to
Richard Henry

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