A delta-sigma ADC is usually 1 bit at its heart yet, it can achieve 16 bits or more by oversampling and digital filtering. I'm not going to go into the subtleties of how but, the same technique applies to "ordinary" ADCs - if you bump up the sample rate by 4 times and average those four samples digitally, you can get more resolution (see dithering below).

Four times over-sampling produces 1 extra bit of resolution. To get two bits of extra resolution you need to oversample 16 times.

Just think about the 4x over-sampling in practical terms. If your basic resolution was 1 volt and four consecutive samples were 1 volt, 1 volt, 2 volts and 1 volt, the average is 1.25 volts. That's an improvement. This technique will only work when there is a little noise in the system and luckily, in high bit resolution ADCs this is inevitable (see dithering below).

See also wiki - dithering: -

In ADCs, performance can usually be improved using dither. This is a very small amount of random noise (e.g. white noise), which is added to the input before conversion. Its effect is to randomize the state of the LSB based on the signal. Rather than the signal simply getting cut off altogether at low levels, it extends the effective range of signals that the ADC can convert, at the expense of a slight increase in noise. Note that dither can only increase the resolution of a sampler.

A delta-sigma ADC is usually 1 bit at its heart yet, it can achieve 16 bits or more by oversampling and digital filtering. I'm not going to go into the subtleties of how but, the same technique applies to "ordinary" ADCs - if you bump up the sample rate by 4 times and average those four samples digitally, you can get more resolution (see dithering below).

Four times over-sampling produces 1 extra bit of resolution. To get two bits of extra resolution you need to oversample 16 times.

Just think about the 4x over-sampling in practical terms. If your basic resolution was 1 volt and four consecutive samples were 1 volt, 1 volt, 2 volts and 1 volt, the average is 1.25 volts. That's an improvement. This technique will only work when there is a little noise in the system and luckily, in high bit resolution ADCs this is inevitable (see dithering below).

See also wiki - dithering: -

In ADCs, performance can usually be improved using dither. This is a very small amount of random noise (e.g. white noise), which is added to the input before conversion. Its effect is to randomize the state of the LSB based on the signal. Rather than the signal simply getting cut off altogether at low levels, it extends the effective range of signals that the ADC can convert, at the expense of a slight increase in noise. Note that dither can only increase the resolution of a sampler.

after run I an FFT, the ENOB (effective number of bits) result will achieve 12 bits.

Ask yourself, how many samples does your FFT algorithm take.

A delta-sigma ADC is usually 1 bit at its heart yet, it can achieve 16 bits or more by oversampling and digital filtering. I'm not going to go into the subtleties of how but, the same technique applies to "ordinary" ADCs - if you bump up the sample rate by 4 times and average those four samples digitally, you get more resolution.

Four times over-sampling produces 1 extra bit of resolution. To get two bits of extra resolution you need to oversample 16 times.

Just think about the 4x over-sampling in practical terms. If your basic resolution was 1 volt and four consecutive samples were 1 volt, 1 volt, 2 volts and 1 volt, the average is 1.25 volts. That's an improvement.

See also wiki - dithering: -

In ADCs, performance can usually be improved using dither. This is a very small amount of random noise (e.g. white noise), which is added to the input before conversion. Its effect is to randomize the state of the LSB based on the signal. Rather than the signal simply getting cut off altogether at low levels, it extends the effective range of signals that the ADC can convert, at the expense of a slight increase in noise. Note that dither can only increase the resolution of a sampler.

A delta-sigma ADC is usually 1 bit at its heart yet, it can achieve 16 bits or more by oversampling and digital filtering. I'm not going to go into the subtleties of how but, the same technique applies to "ordinary" ADCs - if you bump up the sample rate by 4 times and average those four samples digitally, you can get more resolution (see dithering below).

Four times over-sampling produces 1 extra bit of resolution. To get two bits of extra resolution you need to oversample 16 times.

Just think about the 4x over-sampling in practical terms. If your basic resolution was 1 volt and four consecutive samples were 1 volt, 1 volt, 2 volts and 1 volt, the average is 1.25 volts. That's an improvement. This technique will only work when there is a little noise in the system and luckily, in high bit resolution ADCs this is inevitable (see dithering below).

See also wiki - dithering: -

In ADCs, performance can usually be improved using dither. This is a very small amount of random noise (e.g. white noise), which is added to the input before conversion. Its effect is to randomize the state of the LSB based on the signal. Rather than the signal simply getting cut off altogether at low levels, it extends the effective range of signals that the ADC can convert, at the expense of a slight increase in noise. Note that dither can only increase the resolution of a sampler.