Atmel AT32UC3A3128, Manual

Page: 913 / 1021

913
32072H–AVR32–10/2012
AT32UC3A3
34. Audio Bitstream DAC (ABDAC)
Rev: 1.0.1.1
34.1 Features
•
Digital Stereo DAC
•
Oversampled D/A conversion architecture
– Oversampling ratio fixed 128x
– FIR equalization filter
– Digital interpolation filter: Comb4
– 3rd Order Sigma-Delta D/A converters
•
Digital bitstream outputs
•
Parallel interface
•
Connected to DMA Controller for background transfer without CPU intervention
34.2 Overview
The Audio Bitstream DAC converts a 16-bit sample value to a digital bitstream with an average
value proportional to the sample value. Two channels are supported, making the Audio Bit-
stream DAC particularly suitable for stereo audio. Each channel has a pair of complementary
digital outputs, DATAn and DATANn, which can be connected to an external high input imped-
ance amplifier.
The output DATAn and DATANn should be as ideal as possible before filtering, to achieve the
best SNR and THD quality. The outputs can be connected to a class D amplifier output stage to
drive a speaker directly, or it can be low pass filtered and connected to a high input impedance
amplifier. A simple 1st order low pass filter that filters all the frequencies above 50kHz should be
adequate when applying the signal to a speaker or a bandlimited amplifier, as the speaker or
amplifier will act as a filter and remove high frequency components from the signal. In some
cases high frequency components might be folded down into the audible range, and in that case
a higher order filter is required. For performance measurements on digital equipment a minimum
of 4th order low pass filter should be used. This is to prevent aliasing in the measurements.
For the best performance when not using a class D amplifier approach, the two outputs DATAn
and DATANn, should be applied to a differential stage amplifier, as this will increase the SNR
and THD.