Cirrus Logic AN314 User Manual

An314, Introduction

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Multiplexing the CS556x/7x/8x Delta-Sigma ADCs

1. INTRODUCTION

The CS556x/7x/8x devices represent a new generation of delta-sigma analog-to-digital converters that compete
with SAR (successive approximation register) converters, with some distinct advantages. The unique features of
these over-sampling converters allow them to be multiplexed at high speed across many input channels, because
every conversion produces a fully settled result. In this application note the similarities and differences between
SAR converters and the CS556x/7x/8x family of ADCs will be discussed as well as some design tips on choosing
the right multiplexer and input amplifiers to drive these ADCs.

2. COMPARING THE CS556X/7X/8X WITH A SAR

Typically SAR ADCs have been used in the past for any high-throughput multiplexed application. This is because
most delta-sigma converters require multiple conversions to fully settle. In other words, a large step change on the
input takes a considerable amount of time to be reflected accurately (settled) in the output of the digital filter. Most
delta-sigma converters use single-bit modulators and sample at kilohertz rates, and many samples are required as
input to the digital filter to achieve the required accuracy. The CS556x/7x/8x family of converters is unique in that
the modulator output is multi-bit and the sample rate is 8 MSps (megasamples per second). This, in combination
with a specially designed FIR filter that only takes a few clock cycles to compute a result, produces a converter that
can fully settle in a single conversion.

There are several distinct advantages of delta-sigma converters over SAR converters such as their superior DNL
(differential non-linearity) performance and excellent noise immunity. One of the main reasons for the increased
noise immunity is that the digital filter attenuates the out-of-band noise. However, a SAR ADC typically has a band-
width much higher than Nyquist, and out-of-band noise can alias down into the pass band. Another reason is that
a SAR converter samples the input signal once per conversion (Figure 1a) while the delta-sigma converter averages
many samples per conversion (Figure 1b). As can be seen in Figure 1a, a noisy input signal (one with noise tran-
sients as illustrated) can result in significant errors within a SAR ADC unless multiple conversions are taken and
averaged in software. In Figure 1b, the delta-sigma converter does this internally in the digital filter resulting in a
conversion much less susceptible to transient noise.

Figure 1a. SAR ADC Sampling (One Sample per Conversion)

Figure 1b. Delta-Sigma ADC Sampling (Many Sample per Conversion)

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