Shhh, keep it down (out-of-band noise, that is)


 Nowadays, engineers designing audio systems for cars face many competing demands. Besides offering a superb audio experience, the design needs to abide by specific automotive system constraints, such as:

Weight and space: A vehicle that weighs less can hit mandated fuel economy targets more easily. Because every ounce in the car counts, audio designers place a premium on highly efficient systems (where no bulky heat sink is required) and solutions with few components. This is even more valuable in space-constrained head units and instrument clusters.   

Reliability: This is a key consideration for any equipment in the car, including the audio system.  It needs to withstand stringent automotive conditions and keep electromagnetic interference below industry-mandated limits, such as the CISPR25 L5 standard for electromagnetic compatibility (EMC).

This means designers have to juggle multiple tradeoffs across all the devices in the audio signal chain, like those in the very simplified block diagram below. 

Figure 1. Typical audio signal chain

In the DAC and audio amplifier portions of the audio signal chain, using a low out-of-band noise DAC can improve audio system design. Typical audio systems use a low-pass filter between the DAC and the audio amplifier to filter out-of-band noise.

What is a DAC’s out-of-band noise and why does it matter?

Any DAC will produce noise that extends well above audio frequencies, and this effect is strongest in delta-sigma DACs. These converters use noise-shaping techniques to improve in-band signal-to-noise ratio (SNR) performance at the expense of generating increased out-of-band noise.

Figure 2. Delta-sigma DAC noise shaping

It seems as if this would not be an issue in an audio system. Wouldn’t this out-of-band noise be inaudible as it is outside the audio band?  It can actually be a significant issue in automotive audio design.

As a sampled-data system, the Class D amplifier aliases the input signal. This means the DAC out-of-band noise is “folded back” in the audio band, increasing distortion and noise. And, as this out-of-band noise extends to higher frequencies, it can interfere with other devices. Most importantly, in the specific case of automotive audio, this can cause failure of mandatory and stringent automotive EMC tests, such as CISPR 25 class 5. To provide the optimal converter performance, the out-of-band noise must be low-pass filtered.

What kind of LPF does my application need?

Sometimes, the most basic low pass filter (LPF), the RC filter, will do the job. But sometimes automotive audio designers find that an RC filter stage is not enough on its own. As the out-of-band noise depends on the DAC architecture, even active filtering may be required for the most common DACs. Active filtering adds substantial complexity, cost and additional components to the system.  And more components can increase the weight of the system as well. For example, some high-performance DAC applications require half a dozen passives, an operational amplifier and, potentially, a new power management stage to supply bipolar rails.

                                                   

Figure 3. Suggested low-pass filter for a high performance DAC

Next-generation DACs like the PCM5102A-Q1 continue the legacy of TI/Burr-Brown innovation by using advanced the current segment architecture to greatly reduce out-of-band noise, making a traditional 20kHz low pass filter a thing of the past.

Figure 4. PCM5102A-Q1

Improved mid-band out-of-band noise performance from the DAC translates to lower system cost and component count, since it requires no filter stage to minimize audio noise and distortion. In addition to the cost and component count reductions that eliminating the filter provides, PCM5102A-Q1’s low out-of-band noise at higher frequencies can make it easier to pass EMC tests, which can reduce development cost and time. This is particularly important when using electrically long traces and wires in systems capable of audio line-out.

By reducing out-of-band noise, audio design engineers can deliver a stellar in-vehicle audio performance, all while helping reduce costs and promote reliability for the system.