Why Your Audio DAC or Digital Amp Should Spec Dynamic Range Instead of SNR
May 17, 2018
|By: Christine Young
Blogger, Maxim Integrated
One of the most commonly used specifications to compare signal quality in an audio analog-to-digital converter (ADC), digital-to-analog converter (DAC), or digital-input amplifier is the standardized AES17 dynamic range measurement. This test checks the noise floor in the presence of a -60dBFS level signal. A similar, alternative measurement is the signal-to-noise (SNR) measurement that compares the full-scale signal to the idle or zero-input noise floor. At first glance, these two specifications might seem to provide a similar measurement, but in fact the SNR measurement can be quite misleading.
Many low-noise DAC and digital-input amplifiers include a mute or squelch feature that will allow exceptionally low noise at the output when the input signal is idle (zero). While this is a nice feature, it only improves the noise floor when there is no signal present. In some amplifiers the noise floor can increase very audibly by > 3dB when the input signal changes from idle to the lowest level of audible signal. The dynamic range specification avoids this problem by comparing the full-scale signal to the noise floor with an active (unmuted) signal. The mute feature for low idle noise has another undesirable side effect in that it often causes an audible click-pop during the transition into and out of mute caused by a shift in the output DC voltage.
If a DAC or digital-input amplifier does not include the standard AES17 dynamic range measurement specification in the electrical characteristics table, this is often done purposefully by the supplier to hide a particularly unfavorable active noise floor. A discerning customer for these products will insist that the supplier provide the dynamic range performance of their part before considering its use in their application. See Table 1 for a comparison of the MAX98357 digital pulse-code modulation (PCM) input Class D power amplifier with its closest competitor.
Tablets are among the portable electronic devices that can benefit from the MAX98357, which provides Class AB audio performance with Class D efficiency.
Table 1. Comparing MAX98357 with Competitive Product
|Data Sheet Noise Floor||22.8µV||24.0µV|
|Measured Noise Floor at Power On||22.8µV||22.8µV|
|Measured Noise Floor After Signal Applied||22.8µV||35.6µV|
|Data Sheet Dynamic Range||103.5dB||N/A|
|Measured Dynamic Range||103.5dB||100.3dB|