increase the ADC resolution using oversampling [duplicate]

Question

To increase the ADC resolution, oversampling technique can be used which must be followed by decimation (average) method.

Oversampling is just take several samples within time period. Unlike the normal averaging in which you might lose all the extra bits from oversampling, Decimation can remove only the lowest of low bits leaving some of the higher low bit to contribute to final result.

1. What is the lowest and higher low bit? And why the lowest low bit usually most noisy than the rest of the bits?

2. From my understanding is that the higher the extra bit can we get, a better ADC resolution can we have. In this case why can't we just use oversampling without decimation?

   For example, a summation of 16 10-bit ADC result in 14-bit ADC and when we use decimation method (divide by 4) result in 12-bit resolution.

So why can't we just use oversampling and gain 14-bit resolution instead of 12-bit resolution with decimation?

Is that to reduce the noise after the oversampling stage?

Answer 1

What is the lowest and higher low bit

Here is your typical binary number:

1001101001
^... ^ ^ ^
|    | | |______ Lowest bit
|    | |________ A "higher" low bit than the lowest bit
|    |__________ A higher bit than the two above
|________________ The highest bit

Why the lowest low bit usually most noisy than the rest of the bits

Let's take a look at say.. 3 bit number:

Decimal   Binary
      0   000
      1   001
      2   010
      3   011
      4   100
      5   101
      6   110
      7   111

Note that if we have a continuous transition between numbers A and B the lowest bit will have the most changes from 0 to 1 and vise versa. And the noise is nothing but a bunch of random (or not) continuous transitions from As to Bs.

So why can't we just use oversampling and gain 14-bit resolution instead of 12-bit resolution with decimation?

Is that to reduce the noise after the oversampling stage?

Yes. The "noisy" bits don't provide any useful information (given they are just uniformly random).

Answer 2

So why can't we just use oversampling and gain 14-bit resolution instead of 12-bit resolution with decimation?

If the noise is gaussian and there are no other noise sources, you can find a better average of the signal. However, there are other errors that prevent you from finding a better average. ADC's have DNL and INL, in short, this means all of the bits are not spaced evenly apart. This leads to error, when oversampling this error is integrated.

If you had an ideal ADC and an ideal gaussian noise source, you would be able to oversample infinitely to find a better average. With non ideal ADC's there is intrinsic error in the measuring of the signal, which prevents us from reaching a true mean by oversampling.