SBAA322A January 2019 – September 2024 OPA1677 , OPA1678 , PCM1753-Q1 , PCM1754-Q1 , PCM5102A
Filter Input | Cutoff Frequency | Filter Gain |
---|---|---|
4VPP, 1.42VRMS | –3dB at 23kHz | –1V/V, 0dB |
This circuit shows the implementation of a second-order active-filter for audio digital-to-analog converter (DAC) applications. In applications such as automotive head units, home theater soundbars, and AV receivers, it is critical to minimum undesirable noise in the audible range, approximately 20Hz to 22kHz. For this reason, many delta-sigma type audio DACs implement noise shaping techniques that force the noise generated by the over-sampling functionality of the DAC outputs side of the audible range, this is process is called 'noise-shaping,' while the actual noise is called 'out-of-band noise.' Many common audio DACs, such as the PCM1753-Q1, have noise-shaping that forces the out-of-band noise to approximately 50% the sampling rate, fS, of the digital source. While this noise is not generally considered audible, it can have detrimental effects on the amplifier circuits that are found on the output of audio DACs. For example, this out-of-band noise may be aliased by class-D amplifiers that operate at higher frequencies back into the audible range. In addition, this noise would also experience the same analog gain of the output amplifiers if no filtering is implemented. The second-order active-filter design allows a higher level of noise attenuation closer to the audible band than a simple, first-order RC filter. In addition, it allows the audio output of the system to have a lower impedance and current drive than the audio DAC can provide due to the output drive capabilities of the op amp featured in the filter.
where
where
The following graph shows the simulated filter response of the second-order active filter as well as the response of a simple, first order RC filter with approximately the same fC. Note that the filter roll-off for the first order filter is –20dB/decade, while the roll-off for the active filter is –40dB/decade.
The following graph shows the simulated total noise contribution of the circuit, with the exception of the DAC. The was simulated using the OPA1678, which is specified as having 4.5nV/√ Hz at 1kHz, and the OPA1612, which is specified as having 1.1nV/√ Hz at 1kHz. The results show that the contribution of the current noise in the system result in a greater total noise in the OPA1612 versus the OPA1678, even though the OPA1612 has lower voltage noise.
The output of the DAC was measured in the frequency domain with the second-order, active-filter as well as the first-order RC filter. The output of the DAC was set to –60dB full-scale amplitude at 1kHz frequency with a sample rate of 48kHz.
The graph shows that the out-of-band noise begins to increase at around 24kHz, which is expected given the noise-shaping of the PCM1753-Q1. The second-order filter has approximately a 20dB lower output at 100kHz when compared to the RC filter.
Device | Key Features | Link | Other Possible Devices |
---|---|---|---|
PCM1753-Q1, PCM1754-Q1 (1) |
24-bit resolution, 106dB typical SNR, 0.002% typical THD+N, single-ended, voltage-output, audio DAC | Automotive 106-dB SNR stereo digital-to-analog converter (DAC) (software control) | Audio DACs |
OPA1678 | Low distortion, low noise, low input current, dual amplifier for audio applications | Single Channel 450nA Precision Nanopower Operational Amplifier | Audio Op-Amps Overview |
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