Cirrus Logic AN241 - Manual (Page 4)

AN241
4
Figure 2 implements a common mode capacitor between the positive and negative nodes of the differen-
tial inputs. 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 response and the amount of atten-
uation at the modulator sampling rate. This capacitor also acts as a charge reservoir for the internal sam-
pling 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 linear-
ity.
3. Single-Ended to Differential Input Buffer
3.1 Applicable Converters
A single-ended to differential analog input buffer is ideal for use with the following Cirrus Logic audio con-
verters:
- CS5361
- CS4272
- CS42528/26/18/16
- CS42428/26/18/16
3.2 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.
3.3 Recommended Filter Topology
Figure 3 shows a single-ended to differential analog input buffer. This buffer provides proper biasing, iso-
lation from the switched capacitor currents, low output impedance, and anti-alias filtering. The second op-
amp stage is set up in an inverting configuration to produce the negative node of the differential input. In
the input buffer shown below, the second stage has unity gain, and the single-ended input level will effec-
tively be doubled when presented differentially to the converter. For example, a 2Vpp single-ended input
will provide a 4Vpp differential input to the converter.