aaron_do wrote on Jan 12th, 2014, 7:32am:Hi carlgrace,
I guess my question wasn't worded very well...definitely if you're noise-limited, DEM would aggravate the problem.
My doubt is that for whilte noise, you have a normal distribution in the voltage levels. When the voltage noise exceeds a certain level, you may have a bit-error. However, that only happens once in 10,000 samples (for example).
For DEM, the problem is linearity, not noise. So if you design for 9b linearity and then use DEM to improve the linearity to 12b, each individual sample still only meets the 9b linearity requirement. My concern is that if your modulation scheme has a large number of amplitude levels, then each individual sample could potentially be wrong.
Or is my analysis wrong. Perhaps, as you suggest, the net effect is simply an increase in the noise level.
thanks for the help,
Aaron
Hi Aaron,
It's hard to interpret your statement "each individual sample only meets the 9b linearity requirement". If you're taking a sample out of context the idea of linearity doesn't make sense because linearity is defined as a measurement involving an ensemble of measurements.
Remember, with DEM if you could magically fix the input EXACTLY, you would still get various voltage levels due to the DEM. On average they would be more accurate.
Here's an intuitive way to see what is going on.
If you have linearity problems then you will have strong peaks in the output spectrum of your converter. DEM chops off the tops of those peaks and distributes the power among all the other bins. Since the overall power is the same, it has to raise the noise floor to do it. This isn't exactly correct mathematically, but it gets the intuition of the process.
I agree with Frank that DEM may not be super helpful if BER is your #1 concern. BER depends a lot on the noise and DEM increases noise (in exchange for improved linearity)