8 Analog Interface
8 – 6
8.3 D/A CONVERSION
The D/A conversion circuitry of the ADSP-21msp58/59’s analog
interface consists of a sigma-delta digital-to-analog converter (DAC),
an analog smoothing filter, a programmable gain amplifier, and a
differential output amplifier.
8.3.1 DAC
The analog interface’s DAC implements digital filters and a sigma-
delta modulator with the same characteristics as the filters and
modulator of the ADC. The DAC consists of a digital high pass filter,
an anti-imaging interpolation filter, and a digital sigma-delta
modulator.
The DAC receives 16-bit data values from the ADSP-21msp58/59’s
DAC Transmit data register (which is memory-mapped at address
0x3FEC in data memory). The data stream is filtered first by the DAC’s
high pass filter and then by the anti-imaging interpolation filter. These
filters have the same characteristics as the ADC’s anti-aliasing
decimation filter and digital high pass filter.
The output of the interpolation filter is fed to the DAC’s digital sigma-
delta modulator, which converts the 16-bit data to 1-bit samples at a
1.0 MHz rate. The modulator noise-shapes the signal such that errors
inherent to the process are minimized in the passband of the converter.
The bit stream output of the sigma-delta modulator is fed to the DAC’s
analog smoothing filter where it is converted to an analog voltage.
8.3.1.1 High Pass Filter
The DAC’s digital high pass filter has the same characteristics as the
high pass filter of the ADC. The high pass filter removes frequency
components at the low end of the spectrum; it attenuates signal energy
below the passband of the converter. The DAC’s high pass filter can be
bypassed by setting bit 8 (DABY) of the ADSP-21msp58/59’s analog
control register.
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