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CDB5126

16-Bit, Stereo A/D Converter for Digital Audio

Cirrus Logic 

CS5126

+5V

+ 1 µF

Analog

Input

Anti Alias

Filter

200 Ω

Voltage

Reference

0.1 µF

+

1 µF

-5V

0.1 µF

10 Ω

VA+

1nF AINL*

AINR

CS5126

VREF

VD+

SLEEP

L/R

HOLD

SCLK

CLKIN

AGND

SDATA

0.1 µF

+ 1 µF

+5V*

fclk /256

fclk /16

f clk

REFBUF

DGND

VA-

0.1 µF

VD-

10 Ω

0.1 µF

+

1 µF

* AINR can alternatively be used

with L/R grounded

Figure 9. Monaural 2X Oversampling Connections

larly, the acquisition time for the external sam-

ple/hold amplifiers must not exceed the mini-

mum conversion time of 192 master clock cycles

(7.8µs for 48kHz stereo sampling).

Oversampling

The CS5126 can alternatively be used to over-

sample one channel (monaural) by 2X simply by

tying the L/R input high or low. This moves

much of the anti-alias burden from analog filters

to digital post-filtering. The analog filters’ cor-

ner can be pushed out in frequency with lower

roll-off, allowing lower passband ripple and

more linear phase in the audioband. Digital FIR

filtering, meanwhile, can be used to implement

high roll-off filters with ultra-low passband rip-

ple and perfectly linear phase.

Oversampling not only improves system-level

filtering performance, but it also enhances the

ADC’s dynamic range and distortion charac-

teristics. All noise energy in a sampled, digital

signal aliases into the baseband between dc and

one-half the sampling rate. For an ideal succes-

DS32F1

sive-approximation ADC the noise spectral con-

tent is white. Therefore, in a 2X oversampling

scheme such as 96kHz sampling the ADC’s

noise will be be spread uniformly from dc to

48kHz. Digital post-filtering then rejects noise

outside of the 20kHz or 22kHz bandwidth, re-

sulting in improved signal-to-noise and dynamic

range. For a white noise spectrum, a 2X reduc-

tion in bandwidth yields a 3dB improvement in

dynamic range.

Due to its on-chip self-calibration scheme, the

CS5126’s dynamic range is limited only by

white broadband noise rather than signal-depend-

ent DNL errors. Therefore, the CS5126 picks up

a full 3dB improvement in dynamic range to

95dB when implemented in 2X oversampling

schemes.

Oversampling and digital filtering also enhance

the ADC’s distortion performance. Consider for

example a full-scale 15kHz input signal to the

CS5126 sampling at 96kHz. Sampling distortion

produces THD of approximately 0.005% (86dB)

at the converter’s output. Most of the distortion

energy resides in the second and third harmonics

11

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