ADC specs

percy

Member
2004-04-25 5:47 pm
CA
Take a look at these two ADCs. Which one do you think is better suited for audio ADC ?

What I am trying to understand is that even though the DNL/INL of the 18bit ADC is much worse than the 16bit ADC, why is the SNR, SINAD and SFDR better by atleast 3db than the 16bit ADC and the THD still the same as the 16bit ? How is that possible ? What am I missing about interpreting or understand ADC specs ?!
I notice that they have conveniently 'forgotten' to give ENOBs for the 18bit dac.
 

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poobah

Member
2005-11-15 7:24 pm
Higher SINAD and SNR etc... are BETTER numbers.

In particular: SNR is Signal to Noise Ratio: The higher the signal, or the lower the noise... the higher this ratio will be.

SINAD is similar...

Now... DNL and INL are bit more complex... lower is better in this case. Keep in mind that each additional bit of resolution implies 6
dB of range... So comparing INL and DNL directly between 16 & 18 bit units is not apples to apples. The 18 bit converter has a 12 dB head start. You could say as well the 16 bit converter has a 12 dB handicap.

So while the 16 bit unit has .5 lsb xNL... and the 18 bit has 2 lsb xNL; the 18 bit unit effectively has nearly the same noise level as the 16 bit unit...

When all is said and done a difference of 1.5 LSB's corresponds to a "hit" of 9 dB on a 12 dB advantage... why the SNL and SINAD are only about 3 dB better on the 18 bit unit.

Confused yet?

SINAD would be a good factor to compare for audio use...

:)
 

poobah

Member
2005-11-15 7:24 pm
There is some truth to what you say. The fact of matter is... it can be difficult to get a 12 bit ADC to behave.

That doesn't mean that the extra resolution is a farce. It simply means that sample averaging (filtering), and dithering (usually happens for you automatically) are required to extract the finer measurements.

One of the central issues with ADC design lies in the fact that the same piece of silicon doing the conversion is also an inherently noisy place to be... it is digital after all.

Look at some of the other ADC TI & BB have to offer... the PCM series come to mind. These have much higher SNR's etc...

Be prepared to stand on your head and "arf like a seal" if you wish to achieve the full capabilities of these devices... it ain't easy. You won't do it with alligator leads or a radio shack proto board.

In the case of a 24 bit converter with a 5 volt differential input: 1 count equals 0.6 uV.

:)
 

poobah

Member
2005-11-15 7:24 pm
I had a few of the ADCs back then... BURR BROWN was still the big kid on the block.

They were laser trimmed hybrid construction... and about $1K...

The new ADC techniques... shoving little packets of current around allows much better noise performance.