Cirrus Logic AN299 - Manual

Copyright © Cirrus Logic, Inc. 2006
(All Rights Reserved)
http://www.cirrus.com
Optimizing the Performance of CS553x ADCs
1. INTRODUCTION
Getting optimum performance with high-performance converters is not a trivial task. Good system grounding tech-
niques, power supply filtering, careful board layout, and control of system clocks and high-speed digital signals is of
utmost importance. We will cover these and other subjects in this application note in order to allow your design to
get the best performance from the CS5531/2/3/4 converters.
The CS5531/2/3/4 family of Delta-Sigma converters are some of the lowest-noise DC measurement devices in the
market. These devices are suitable for many applications where high-resolution measurements of very small DC
voltages are required, such as weigh scales, pressure transducers, and thermocouples. With a built in, low-noise,
programmable gain amplifier, the full-scale input range can be as low as 39.0625 mV DC in unipolar mode with a
2.5 V or 5 V VREF. As will be demonstrated in this application note, the input scale can be reduced even further,
allowing very small differential inputs to be measured to a high degree of resolution. For example, as explained in
this document, a 10 mV full-scale unipolar input can be measured to 0.596 nV per least-significant bit with the 24-
bit devices. Obviously the output will contain noise, but with software averaging, that noise can be resolved to an
impressive number of noise-free bits.
2. THE IMPORTANCE OF A GOOD GROUND PLANE
In order to reduce board costs, some designs may attempt to use single- or double-sided boards and fill unrouted
areas with a ground plane. Please be aware that this is not an effective practice in a high-performance analog-to-
digital conversion circuit. A low-noise, low-impedance, uninterrupted ground plane is extremely important in order
to get optimum performance from the ADC. However, filling unused areas around the input traces with copper and
connecting that copper to the ground plane does have some additional advantages regarding Faraday shielding.
The ground plane is the point of reference for many signals on the board. The extremely low-level analog input sig-
nals are very susceptible to corruption from noise voltage transients on the ground plane. Since the ground plane is
used to conduct supply and return currents for high-speed signals, it is possible to introduce unwanted noise voltage
drops across the ground plane.
Consider the best possible PCB layout for the simple schematic shown in Figure 1 below.
Figure 1. Analog Input Circuit for a CS5534 ADC
AIN+
AIN-
VA+
VA-
C1
C2
CS5534
V+
AIN+
AIN-
R1
R2
C1 C2 C4
C3
1
2
5
6
7
8
AN299
SEP
‘
06
AN299REV1