System-Level Optimization of a DAC for Hearing-Aid Audio Class D Output Stage

Abstract

This paper deals with system-level optimization of a digital-to-analog converter (DAC) for hearing-aid audio Class D output stage. We discuss the ΣΔ modulator system-level design parameters-the order, the oversampling ratio (OSR) and the number of bits in the quantizer. We show that combining a reduction of the OSR with an increase of the order results in considerable power savings while the audio quality is kept. For further savings in the ΣΔ modulator, overdesign and subsequent coarse coefficient quantization are used. A figure of merit (FOM) is introduced to confirm this optimization approach by comparing two ΣΔ modulator designs. The proposed optimization has impact on the whole hearing-aid audio back-end system including less hardware in the interpolation filter and half the switching rate in the digital-pulse-width-modulation (DPWM) block and Class D output stage.

System-Level Power Optimization of Digital Audio Back End for Hearing Aids

“…Hence, a figure of merit (FOM) developed for digital filters [2, 28,35] can be applied. In this work, the FOM is used to judge the complexity of the IF and the  modulator by counting the number and complexity of adders [11,[20][21][22][23]. This leads to…”

“…For the case of the 'Initial design', the peak SQNR = 106 dB. Since peak SQNR = 98 dB is sufficient according to the specification, the margin left is used for coarse coefficient quantization to reduce the FOM [20]. The resulting peak SQNR of the modulator is then 98 dB.…”

“…NTF gain = 1.5) is to decrease the OSR of the modulator from 64 to 32 and increase its order from 3 to 6. This keeps the ideal peak SQNR = 106 dB so that coefficients can again be coarsely quantized and give resulting SQNR = 98 dB after coefficient quantization with low FOM [20]. By performing these changes in the  modulator, the aim is to reduce the switching frequency of the Class D output stage [3] and the DPWM block by 50% as this frequency is the same as the operating frequency of the  modulator (see Fig.…”

“…With savings in the PA and the IF, the only block of the back end system that remains to be investigated to see whether or not this optimization approach is reasonable is the  modulator. This investigation was performed in [20]. Taking the Matlab fixed-point models and calculating the FOM according to (1) gives the data and FOM in Table 2 …”

System-level power optimization for a ΣΔ D/A converter for hearing-aid application

“…A thorough discussion on hearing-aid audio back-end system specification and the Σ∆ modulator is provided in [4]. We assume ideal 16 bit quantization of the system input signal that has band-width (BW) of 10 kHz.…”