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DAC8571: DAC for music audio

Daniel Hazeley
Prodigy 170 points
Part Number: DAC8571
Other Parts Discussed in Thread: DAC8811, , PCM1770, PCM1772

Hi,

I have been searching for a DAC to convert decoded mp3 file samples into an analog signal to be fed into an amplifier and speaker. I came across this DAC and am wondering if this would be a good fit for this application. I am targeting a sample rate of 44.1 kHz. I will be interfacing with the DAC using an STM32F4 micro controller (which has a plethora of communication peripherals and high internal clock speed).

I am currently using a DAC8811 along with an I/V converter Op-amp, but I would prefer to use a DAC with an I2C (or I2S) interface.

  • 0
    TI__Mastermind 46290 points
    Daniel,

    If this is a "true" audio application, why not consider an Audio DAC?

    Occasionally we see precision DACs, like the DAC8811 and DAC8571 you've mentioned, used for audio applications - but generally they are implemented with lower resolution (12-bit) DACs and are lower-performing applications. For example things like a smoke-detector or fire-alarm which includes some pre-recorded audio sample. There are exceptions, of course.

    The "hazard" with using a precision DAC for audio or AC applications is just that the products were not characterized for a comprehensive, or in some cases any, AC specifications. So if you have some THD or SNR goals in mind that are at all stringent, an audio DAC may be a better approach.

    Let us know more about exactly what your performance expectations are, beyond the sampling rate you mentioned.
  • 0 in reply to Kevin Duke
    Prodigy 170 points

    Duke,

    Thank you for your reply. I am honestly not too familiar on the differences an audio DAC would bring, but if you have some examples I would love to look into this solution.

    My performance expectations are not stringent at all. SNR over 90dB would be preferred (which is why i choose a 16 bit DAC) and THD less than 10% would be fine for now.

    For reference, the Class D Amplifier I will be using is the TPA3128D2.

  • 0 in reply to Daniel Hazeley
    TI__Mastermind 46290 points
    Daniel,

    I guess the main thing for you to consider on an interface level is that the Audio DACs are typically going to expect the input data to be provided on an I2S bus, though additional I2C or SPI bus interfaces are also often available for device configuration. Other simpler configuration options are available which we usually alias as "HW Config" which basically just use simple GPIO-style connections to logic low or high in order to configure the device, from there it is just about providing the correctly formatted I2S data to drive the audio outputs.

    Generally the Precision DACs will be very straightforward interfaces, exclusively being configured through I2C or SPI bus controls. Where things get challenging in using a Precision DAC is that they are not typically characterized for THD or SNR performance as their target use cases were just DC. As such, artifacts like code-to-code glitch, digital-feedthrough, and channel-to-channel cross-talk will have an influence. Separately, you have to consider that these resistor based topologies will have faster-than-desirable artifacts on the output which can contribute to degradation in THD / SNR. Given that you're containing yourself to a relatively narrow-band of just the audio spectrum, it's probably reasonable to contain these effects. Also 90dB is not all that crazy of a target.

    So - I would first start by considering what interfaces are available and convenient for your. Audio DACs are purpose built for this, and may serve you best, even with cost considerations in mind since you're already looking at 16-bit Precision DAC products. If for some reason those interfaces are not good for you, we can go down the road of something with a Precision DAC.
  • 0 in reply to Kevin Duke
    Prodigy 170 points

    Duke,

    Thank you for the amazing explanation. It sounds like an audio DAC is the way to go for my purposes and I have no problem interfacing with I2S.

    For control of the DAC I am also able to interface either SPI or I2C. I am also able to interface with a H/W setup if need be.

    Additionally, I will only need a max of 2 channels as I have a single speaker to drive.

  • 0 in reply to Daniel Hazeley
    TI__Mastermind 46290 points
    Daniel,

    This leaves a good deal of the Audio DAC portfolio available to you. I would encourage you to check out the parametric search tool and try to find a happy medium of the performance you're looking at, package, size, and cost.

    You might start with the PCM1770 or PCM1772. PCM1770 is includes a high power output amplifier, while PCM1770 is a line driver which would be the appropriate device for driving the Class-D amplifier you mentioned. As I said, there are other choices though.

    Let us know if you need any further help after checking the parametric search tool.

    www.ti.com/.../products.html
  • 0 in reply to Kevin Duke
    Prodigy 170 points

    Duke,

    As always, your post has been very helpful. The PCM1772 seems to be perfect for my design. I am now looking at the filter I will need on the DAC output and the datasheet seems to call for an LPF. Originally I was using a reconstruction filter design, but this seems unnecessary now.

    I'm considering a first order passive RC filter design. I want R = 1k with a targeted cutoff frequency of 100 kHz. This gives me C = 1.6 nF. Could you confirm or deny whether this filter design is appropriate?

  • 0 in reply to Daniel Hazeley
    TI__Mastermind 46290 points
    Daniel,

    In the end what is appropriate for the design is going to be left to you as you best know your requirements. Given that we've discussed audio and selected an audio part, I will offer that generally speaking 100kHz is well beyond the typical filter cut-off normally implemented for audio systems. The audible range is from 20Hz to 20kHz, so typically the filter cut-off is placed near 20kHz.

    Audio DACs generate noise that is beyond the audible band - commonly referred to as out of band noise - basically anything greater than 20kHz. Some devices control this better than others with complex internal noise shaping features. In any case, any of this out of band noise has the possibility of aliasing itself back into the low-frequency audio band and deteriorating the quality of your audio. Therefore, we generally recommend - even for devices with these noise shaping filters, at least an R/C low-pass filter in between the line-driver and the output amplifier. For higher performance systems or perhaps for devices that do not feature these noise shaping capabilities, we may suggest higher order filters.

    Last year we made a video along these lines, along with the mid-power audio amplifiers team. You can check that video out at the link below. I don't think the TINA simulations made it through to the online video posting, but the circuits are simple enough that you can probably easily recreate them and run some simulations to determine if you are satisfied with the performance.

    training.ti.com/designing-premium-audio-system
  • 0 in reply to Kevin Duke
    Prodigy 170 points

    Your response on the filter design is very helpful. Regarding the PCM1770, you mentioned this has a line driver for differential output. Does this mean that I would control the volume of the sound coming from the speaker by controlling the attenuation on the output Voltage of the DAC?
    Looking back at my design, it does seem like the PCM1770 fits better with my amplifier as you noted.