Since you have a switched capacitor,  it may be possible to charge only half of it directly from dc input to get the same effect as charging the full cap. with half the voltage.

Thanks both of you for the suggestions.


Aaron

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I was thinking maybe a capacitor divider C1/(C1+C2) and then switch the positions of the capacitors to get C2/(C1+C2), and somehow average out. My main concern is the accuracy. I'm not certain I can get 12b accurate capacitor matching. Also, this divider is supposed to be part of an ADC, so ideally I want it's calibration to run in the background. Still kinda piecing it together....


thanks,
Aaron

Aaron - you ought to be able to this with two matched caps and two steps:
1) charge one cap to Vdd and the other to 0V
2) connect both caps together.

Assuming the caps are matched, the voltage across the capacitors will be Vdd/2. It isn't going to be hard to get caps to match to 12 bits in a modern process.


Another way that has a larger coolness factor is to alternately connect two caps in series with the supply, then in parallel with eachother. In the steady state there is no charge transfer, therefore the capacitors must be charged to the same voltage in each configuration. That is, the voltage on each will be Vdd/2. This is true regardless of the cap matching. Pretty cool, but the parasitics of the flying cap must be the same on top and bottom or you will get an error. (You accomplish this by making the flying cap out of two caps with their terminals swapped.)

I'd just use the first method. If you really can't match the caps well enough maybe try a little of both with 3 or more steps (caveat - I haven't actually tried this one):
1) discharge both caps;
2) connected them in series with the supply (each will be approximately Vdd/2);
3) connect them in parallel (even closer to Vdd/2).

Each time you repeat 2&3 the voltage will get closer to Vdd/2, again keeping in mind the parasitic on both plates of the flying cap must match.