DAC Dynamic Range, SNR

Behnam,

1)  Digital attenuation does not automatically mean dynamic range will change with the value of attenuation.  As long as the attenuation is not severe enough that the LSB's are now smaller than the noise floor, the dynamic range will remain constant.  However as the attenuation becomes more severe, LSB's will eventually become smaller than the noise floor, which will indeed impact the DNR.

2) Yes, if the signal is only represented in 5 bits then it will be dominated by the noise floor. 

Thanks Patrick

I am sorry, but I still have some difficulties regarding the first question. I ask it in another way:

When the digital volume gain is 0dB, I use the full scale input range of the DAC (24bits) and feed a pure sinusoidal wave into the DAC. Since the dynamic range of the IC is 112dB,  I would expect an output analog signal which has a peak in the frequency domain (FFT). The maximum amplitude of the peak is at 0dB level and its basal level is at  -112 dB which is the level of the floor noise. (see figure A)

Now, I use the same digital code at the input of the DAC and set the digital volume gain to -60dB. How does the output signal power spectrum change? I know that the maximum peak will be at -60dB, but I do not know the level of the output noise. Will it be again at -112 (Figure B1) or it will be at -112-60 (figure B2) or in between (-112-X) ? (the value of X is important for me)

Is there any graph showing the difference between the maximum peak and its basal value (i.e, X value) as a function of DAC digital gain?

Thanks,

Behnam

> In another word, does the level of floor noise remain constant and we will have a dynamic range equal to 112-100=12dB?

That's how I would interpret it. The digital volume control is just a multiplier in a digital domain. Based on my understanding of the miniDSP enabled codecs, the digital volume control is just a piece of firmware doing the multiplication. The datasheet is muddy in this regard though.

> When the volume is 0 the DR is said to be equal 112dB. The DAC is 24bit thus in theory the dynamic range is 144dB.

> Does is mean that the first 5 LSB of the DAC {(144-112)/6.02=5.3} are not usable? and if a signal is only represented by 5 bits the output analog signal is not distinguishable from the floor noise?

That is how I would interpret it as well.

The digital volume control makes sense if the dynamic range of your audio signal is lower than the dynamic range of your DAC. For example, 112 bits DR equals to around 18.5 bits (without signum bit). 16bit signed audio data has dynamic range of 15*6=90dB. Therefore there is 112-90=22dB headroom for the digital volume control before the 16bit signal dynamic range is hurt.

I am not an employee of TI and I am not an audio expert. This is how I understand the topic.

Vojtech

- According to "Vojtech" comment my answer will be "figure B1". Right?

- What about analog gain controllers like PGA4311 or PGA2311? Does the level of floor noise reduces by the reduction of the gain?

I am asking this question because I need high-quality signals between 0dB and 120dB (a really wide range for a medical application). If the level of the output noise does not change with the gain, for my application I need to have at least 160dB dynamic range In order to get a signal which amplitude is 0dB with 40dB SNR.  If the noise level reduces by the reduction of the gain, then I will be sure that cascading a DAC and a PGA will reach me to my solution.

Behnam