Datasheet

© 2007-2012 Microchip Technology Inc. DS70292G-page 277
dsPIC33FJ32GP302/304, dsPIC33FJ64GPX02/X04, AND dsPIC33FJ128GPX02/X04
22.0 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 dsPIC33FJ64GP804 and
dsPIC33FJ128GP804 devices. The
dsPIC33FJ64GP802 and dsPIC33FJ128GP802
devices provide positive DAC output and negative DAC
output voltages.
22.1 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 Recon-
struction 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
22.2 DAC Module Operation
The functional block diagram of the Audio DAC module
is shown in Figure 22-1. 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.
22.3 DAC Output Format
The DAC output data stream can be in a two’s comple-
ment 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” then
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” then 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 dsPIC33FJ32GP302/304,
dsPIC33FJ64GPX02/X04, and
dsPIC33FJ128GPX02/X04 families of
devices. It is not intended to be a compre-
hensive reference source. To comple-
ment 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 website (www.microchip.com).
2: Some registers and associated bits
described in this section may not be avail-
able 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.