Datasheet

© 2007-2012 Microchip Technology Inc. DS70291G-page 297
dsPIC33FJ32MC302/304, dsPIC33FJ64MCX02/X04 AND dsPIC33FJ128MCX02/X04
23.0 AUDIO DIGITAL-TO-ANALOG
CONVERTER (DAC)
The Audio Digital-to-Analog Converter (DAC) module
is a 16-bit Delta-Sigma signal converter designed for
audio applications. It has two output channels, left and
right to support stereo applications. Each DAC output
channel provides three voltage outputs, positive DAC
output, negative DAC output, and the midpoint voltage
output for the dsPIC33FJ64MC804 and
dsPIC33FJ128MC804 devices.
23.1 KEY FEATURES
• 16-bit resolution (14-bit accuracy)
• Second-Order Digital Delta-Sigma Modulator
• 256 X Over-Sampling Ratio
• 128-Tap FIR Current-Steering Analog
Reconstruction Filter
• 100 KSPS Maximum Sampling Rate
• User controllable Sample Clock
• Input Frequency 45 kHz max
• Differential Analog Outputs
• Signal-To-Noise: 90 dB
• 4-deep input Buffer
• 16-bit Processor I/O, and DMA interfaces
23.2 DAC Module Operation
The functional block diagram of the Audio DAC module
is shown in Figure 23-1. The Audio DAC module
provides a 4-deep data input FIFO buffer for each
output channel. If the DMA module and/or the
processor cannot provide output data in a timely
manner, and the FIFO becomes empty, the DAC
accepts data from the DAC Default Data register
(DACDFLT). This safety feature is useful for industrial
control applications where the DAC output controls an
important processor or machinery. The DACDFLT
register should be initialized with a safe output value.
Often the safe output value is either the midpoint value
(0x8000) or a zero value (0x0000).
The digital interpolator up-samples the input signals,
where the over-sampling ratio is 256x which creates
data points between the user supplied data points. The
interpolator also includes processing by digital filters to
provide noise shaping to move the converter noise
above 20 kHz (upper limit of the pass band). The output
of the interpolator drives the Sigma-Delta modulator.
The serial data bit stream from the Sigma-Delta
modulator is processed by the reconstruction filter. The
differential outputs of the reconstruction filter are
amplified by Op Amps to provide the required
peak-to-peak voltage swing.
23.3 DAC Output Format
The DAC output data stream can be in a two’s
complement signed number format or as an unsigned
number format.
The Audio DAC module features the ability to accept
the 16-bit input data in a two’s complement signed
number format or as an unsigned number format.
The data formatting is controlled by the Data Format
Control bit (FORM<8>) in the DAC1CON register.
The supported formats are:
• 1 = Signed (two’s complement)
• 0 = Unsigned
If the FORM bit is configured for Unsigned data the
user input data yields the following behavior:
• 0xFFFF = most positive output voltage
• 0x8000 = mid point output voltage
• 0x7FFF = a value just below the midpoint
• 0x0000 = minimum output voltage
If the FORM bit is configured for signed data the user
input data yields the following behavior:
• 0x7FFF = most positive output voltage
• 0x0000 = mid point output voltage
• 0xFFFF = value just below the midpoint
• 0x8000 = minimum output voltage
The Audio DAC provides an analog output proportional
to the digital input value. The maximum 100,000
samples per second (100 ksps) update rate provides
good quality audio reproduction.
Note 1: This data sheet summarizes the features
of the dsPIC33FJ32MC302/304,
dsPIC33FJ64MCX02/X04 and
dsPIC33FJ128MCX02/X04 family of
devices. It is not intended to be a
comprehensive reference source. To
complement the information in this data
sheet, refer to Section 33. “Audio
Digital-to-Analog Converter (DAC)”
(DS70211) of the dsPIC33F/PIC24H
Family Reference Manual, which is
available from the Microchip web site
(www.microchip.com).
2: Some registers and associated bits
described in this section may not be
available on all devices. Refer to
Section 4.0 “Memory Organization” in
this data sheet for device-specific register
and bit information.
Note: The DAC module is designed
specifically for audio applications and is
not recommended for control type
applications.