CSP1027 datasheet(13/64 Pages) AGERE

13 / 64 page

Lucent Technologies Inc.

Data Sheet

CSP1027 Voice Band Codec for

December 1999

Cellular Handset and Modem Applications

4 Architectural Information (continued)

4.2.7 Digital-to-Analog Path

Starting at the bottom right of Figure 3 on page 5, the

∆-Σ D/A conversion process begins with a 16-bit two's

complement PCM signal read from the DI serial input.

The PCM is interpolated up to 1 MHz in two stages and

low-pass filtered at each stage to attenuate 8 kHz

images.

The PCM input is latched into the cdx(D/A) register at

a nominal word rate of 8 kHz. The signal is then option-

ally high-pass filtered. This filter has the same transfer

function as the A/D high-pass filter.

A digital sample-and-hold increases the word rate by a

factor of 5 from 8 kHz to 40 kHz. The seventh-order IIR

digital low-pass filter then removes the spectral images

between 4 kHz and 20 kHz and predistorts the pass-

band to compensate for the filtering done during the

interpolation up to the 1 MHz word rate. The transfer

function of this low-pass filter is the same as the one

employed in the A/D converter.

The output of the low-pass filter feeds a programmable

gain adjustment block that serves as a volume control.

The gain can be changed in 3 dB increments from

0 dB to –45 dB. The attenuation level is set by writing

the OGSEL field in the cioc0 register (see Table 7 on

page 26).

The digital modulator block further increases the word

rate by a factor of 25 from 40 kHz to 1 MHz. Through

quantization and noise shaping, the digital

∆-Σ modula-

tor creates 1-bit output words at 1 MHz.

The modulator 1-bit output drives a structure combining

a 1-bit D/A converter and a second-order switched-

capacitor filter having a cutoff frequency of 8 kHz

(based on a 1 MHz clock). This is all shown as the D/A

block in Figure 3 on page 5.

This is followed by a second-order active Chebychev fil-

ter having a cutoff frequency of 35 kHz.

The passband ripple of the analog filters is small

enough such that they have virtually no effect on the

passband response.

The output amplifier buffers the analog filter output.

The frequency responses of the A/D and D/A paths are

essentially the same. See Figures 6 through 15 for the

magnitude and delay responses versus frequency.

4.3 Programmable Features

4.3.1 Active/Inactive Modes

The CSP1027 has active and inactive modes of opera-

tion which are selected by the ACTIVE field in the

cioc0 register (see Table 7 on page 26). The default

value upon reset and powerup is ACTIVE = 0 (i.e.,

inactive). In the inactive mode, the codec clocks are

disabled, data transfers by the codec are disabled, and

analog bias currents are shut off. This state is useful in

battery-powered applications when prolonged periods

of inactivity are expected. It takes approximately

600 ms for the codec to reach full steady-state perfor-

mance in going from inactive to active. This is primarily

due to the charging of the large external capacitors,

CREF and CREG. However, the codec is functionally

useful after 100 ms.

4.3.2 Input Select

When the A/D preamplifier is selected (EIGS = 0), the

INSEL field of cioc0 (see Table 7 on page 26) switches

the preamp input between the MICIN and AUXIN

inputs. When external gain select is used (EIGS = 1),

the INSEL field has no effect.

4.3.3 A/D Input Ranges

When the preamplifier is used (EIGS = 0), the IRSEL

field of the cioc0 register (see Table 7 on page 26)

selects the 500 mVp range when IRSEL = 0 and the

160 mVp range when IRSEL = 1. IRSEL has no effect

when the external gain select mode is used (EIGS = 1).

When EIGS = 1, the inverting amplifier of Figure 5 on

page 7 replaces the preamplifier. The input range in

this mode is the following:

VFULL-SCALE =

4.3.4 Output Mute Function

The D/A converter output can be selectively muted with

the MUTE field in the cioc0 register (see Table 7 on

page 26). The default value upon reset is muted

(MUTE = 0). The mute function is implemented (Figure

3 on page 5) internally by a MUX following the D/A

input. Placing the mute function here causes the signal

at the analog output to gradually decay/rise over

approximately 1 ms upon muting/unmuting. This effect

is due to the impulse response and group delay of the

digital filters. This implementation will reduce any

potentially undesirable transient effects such as pops,

when the D/A is muted.

Rin

Rfb

----------

1.578 Vp

CSP1027 Datasheet(PDF) 13 Page - Agere Systems

CSP1027 Datasheet(HTML) 13 Page - Agere Systems

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

Datasheet Download

Link URL