Analog I/O Definition - Motorola DSP56156 Manual

The D/A converter consists of a second-order comb interpolating filter, a digital Σ∆ mod-
ulator, an analog comb decimation filter, an analog low-pass filter, a selectable attenuator,
and a differential output stage. The interpolator takes in 16-bit two's complement numbers
from the DSP core and up-samples them to a high frequency. The modulator changes
these high frequency 16-bit words into a 1-bit stream. The switched capacitor analog fil-
tering removes the out-of-band shaped modulator noise.
This Σ∆ codec block has been designed for maximum flexibility. The user can select one
of four decimation (interpolation) ratios for the A/D (D/A) converters. Operating at a nom-
inal sampling rate of 2 MHz, the A/D converter provides a 16-bit digital output with more
than 60dB S/(N+D) for input signals in a bandwidth of 0-4 KHz. The D/A converter nomi-
nally accepts a 16-bit word at 16 KHz and has a fully differential analog output which pro-
vides more than 60dB S/(N+D) in a 0-15 KHz bandwidth, for input signals in a bandwidth
of 0-4 KHz. Table 6-1 summarizes the main features of the codec block.
• 16-bit resolution
• More than 60dB S/(N+D)
• Operates at sampling clock rates between 100 KHz and 3 MHz
• No off-chip components required
• Internal voltage reference (2/5 of positive power supply)
• Low power (HCMOS)
The last decimation filtering stage of the A/D section as well as the D/A first interpolation
filtering stage section are implemented in software by the DSP core in order to reduce the
codec cell die area.
6.3

ANALOG I/O DEFINITION

This section describes the Motorola DSP56156 analog input and output characteristics
(see Figure 6-2).
There are two analog inputs MIC and AUX. Selection between MIC or AUX is made via
one control bit (INS bit) and can be changed any time as desired. The electrical specifica-
tions of the two pins are identical.
The analog output consists of a fully differential driver stage, with each output having an
operating range of Vref ± 1.0 Vp. The output op-amp is capable of driving a load of 1 k Ω in
series with 50 nF between the differential outputs.
6 - 4

ANALOG I/O DEFINITION

Table 6-1 On-chip Codec Main Features
DSP56156 ON-CHIP SIGMA/DELTA CODEC MOTOROLA