Cirrus Logic AN241 - Manual (Page 13)

AN241
13
4.8.3 Anti-Aliasing Capacitor
Figure 6 implements a common mode capacitor between the analog input and the associated reference
voltage pin on each channel of the converter. This capacitor is commonly referred to as the anti-aliasing
capacitor, and performs several functions. The value of the capacitor affects the overall low pass filter re-
sponse and the amount of attenuation at the modulator sampling rate. This capacitor also acts as a charge
reservoir for the internal sampling capacitors. Since this capacitor is located in the signal path, it is very
important not to use capacitors with a large voltage coefficient (such as general purpose ceramics) since
they can degrade signal linearity.
5. Single-Ended Input Buffer with a Common Reference Pin
5.1 Applicable Converters
A single-ended analog input buffer with a common reference pin is ideal for use with the following Cirrus
Logic audio converters:
- CS5340
- CS5341
- CS5342
- CS42406
5.2 Introduction
A single-ended signal consists of only one signal line and hence requires only one input pin. Single-ended
inputs typically require less input buffer components, but also have several disadvantages. First, at a giv-
en voltage level, a single-ended input cannot produce as much signal swing as a differential signal, hence
reducing the dynamic range. Also, single-ended inputs are more prone to errors caused by DC offsets and
coupled-noise.
5.3 Recommended Filter Topology
Figure 7 shows a single-ended analog input buffer. This buffer provides proper biasing, isolation from the
switched capacitor currents, low output impedance, and anti-alias filtering.