There should be plenty of them on the market: the best candidate will be SAR-type ADC or sigma-delta ADC. You can find them at www.maxim-ic.com. One of the candidate will be MAX1400.
I don't see the point of using flash ADC to convert 10~20Hz signal -- it is not efficient at all. Integration type or sigma-delta type ADC should be the natural choice.
Hi!
Please specify what do you mean by “low power”?
Power = ?,
Supply voltage = ?, etc.
There are good old 8-bits SAR ADC as ADC0801/2/3/4/5 which are really not very "low power" but may be will do the job?