Fundamental Questions: ADCs, DACs, I2S

[rsbonini]

I have a question which google (despite my trying) has not been able to answer. I hope someone here with more knowledge than I would be gracious enough as to correct my understanding on the following point:

I2S data is converted and output by a DAC relative to that specific device's own 0dBFS level and as nothing to do with the voltage the signal was originally sampled at.

example: If an ADC (0dBFS = 2.5Vpp) samples a 1Vpp signal and outputs this signal via I2S to a DAC (0dBFS = 5Vpp), the DAC would output an analog signal at 2Vpp.

Is this correct?

[kevinkr]

Yes..

[rsbonini]

Thanks for the speedy reply, greatly appreciated. One simple follow up: does anyone know which DACs have the highest 0dBFS level available?

[analog_sa]

Some dacs have current rather than voltage outputs which makes answering your question more complicated.

[rsbonini]

Good point, sorry about that.

The current setup is designed around a voltage output DAC, however I can not get get the final level required for the project without resorting to large amounts of analog gain (40 dB) after the DAC. This raises the noise floor beyond what is acceptable at lower listening levels. I was hoping to overcome this by utilizing a DAC with a higher output in the first place.

If a current output DAC can offer a significantly higher output and lower noise, I would be happy to consider it.

[rsbonini]

I should note that this is a unique project with unusually high output level specifications, and that a minimum full volume output of about 10Vpp is required

[stratus46]

Originally Posted by rsbonini

I should note that this is a unique project with unusually high output level specifications, and that a minimum full volume output of about 10Vpp is required

So worst case you add an opamp at the DAC output to get whatever level you want.

G²

[Zero Cool]

I have s somewhat related question...is there a standard or a reference voltage level for 0dbfs??? IE should 0dbfs = say 2vrms? or is there a standard for what 0dbfs is voltage wise?


Zc

[jcx]

monolithic audio DAC, ADC use mixed signal CMOS IC processes - it is handy to use "standard" digital supply V of 5 V, anaolg reference can be close to that in some chips trying for maximum dynamic range, further trying for maximum dynamic range usually involves differential in/out


V output CS4398 DAC is specd as giving 120 dB dynamic range with 6.65 Vpp differential output

you should put a buffer/filter op amp after any audio DAC, differential to SE conversion can include gain



a pretty common desktop consumer digital audio analog output reference level is 2.0 Vrms = 0 dB fs

a technical oddity is that some bit patterns can produce a "over 0 dBfs" on the filtered/reconstructed analog output - a guard band of 3-6 dB would be necessary to avoid this potential problem

[abraxalito]

Originally Posted by rsbonini

The current setup is designed around a voltage output DAC, however I can not get get the final level required for the project without resorting to large amounts of analog gain (40 dB) after the DAC. This raises the noise floor beyond what is acceptable at lower listening levels. I was hoping to overcome this by utilizing a DAC with a higher output in the first place.

From what you say about too much noise at lower listening levels, you have the additional gain after the volume control. Moving it prior to the volume control would mean at lower listening levels, the noise also would be lower.