Decoupling of voltage references for audio sigma delta ADC

Hi,

For a 16-bit audio performance switched-cap sigma delta ADC, do I have to off-chip decuple the positive and negative voltage references? Is it ok that I just on-chip buffer these references instead to save two pins (and if no other options)?

Thanx!

Re: Decoupling of voltage references for audio sigma delta A

It is OK when using internal references

Re: Decoupling of voltage references for audio sigma delta A

carl_chao said:

Hi,

For a 16-bit audio performance switched-cap sigma delta ADC, do I have to off-chip decuple the positive and negative voltage references? Is it ok that I just on-chip buffer these references instead to save two pins (and if no other options)?

Thanx!

Click to expand...

To achieve 16bit, the noise floor should be at least below -126dB, and to achieve this low noise floor, ur reference should provide "quiet" DC voltage. For example, if the 3 order (-60dB/decade noise shaping ) SDM are used, then u have to attenuate at least -66dB (-(126-60)dB) noise in ur reference. The main noise from SDM is the switching noise generated from switches, which is operated at fclk= fs*OSR, to attenuate this noise with -66dB, u will need a 1st order LPF with cut off frequency fc=fclk/2000 which is usually quite small, e.g. 3kHz.
if the output resistance of your buffer is 50 ohm, then u will need a capacitor ~=1uF !!! It's impossible to do this cap inside ur chip.
In summary, the external decoupling cap. is needed if you want to achieve 16bit resolution.

Re: Decoupling of voltage references for audio sigma delta A

why the noise floor is -126db? 16-bit resolution requires dynamic range 98db, you mean the signal is 98-126=-28db?

Re: Decoupling of voltage references for audio sigma delta A

gingerjiang said:

why the noise floor is -126db? 16-bit resolution requires dynamic range 98db, you mean the signal is 98-126=-28db?

Click to expand...

98dB looks correct