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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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Error

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AN314

AUG ‘07

AN314REV1

1 2 3 4 5 6 7 8 9 10

Summary of Contents

Page 1 - 1. INTRODUCTION

Page 2

10 AN314REV1AN314REVISION HISTORYRelease Date ChangesREV1 AUG 2007 Initial ReleaseContacting Cirrus Logic SupportFor all product questions and inquiri

Page 3 - 4. SELECTING A MULTIPLEXER

2 AN314REV1AN3143. REQUIREMENTS FOR MULTIPLEXING THE CS556X/7X/8XFor higher-throughput in multiplexed applications, it is desirable for the converter

Page 4

AN314REV1 3AN314Figure 4. Amplifier(s) and Filter(s) Placed Before MultiplexerAs illustrated, not every input may require an amplifier or buffer. Th

Page 5 - ½ CS3004

4 AN314REV1AN314selected input's voltage level and this charge will be dumped into the next input’s source when that switch closes.If an op-amp o

Page 6 - 5. SELECTING AN AMPLIFIER

AN314REV1 5AN314of approximately 6 dB in the full-scale signal-to-noise ratio. This is because the full-scale input range of the differ-ential input

Page 7

6 AN314REV1AN3145. SELECTING AN AMPLIFIERIf an amplifier is needed to change the full-scale range or to buffer the inputs and it is placed between the

Page 8

AN314REV1 7AN314Figure 6. 2-input, Single-ended-to-differential, Multiple Input Range, Multiplexed SystemIn Figure 6, the differential device, CS5560

Page 9 - 7. CONCLUSION

8 AN314REV1AN314Figure 7. 8-input, Single-ended, Multiple Input Range, Multiplexed SystemThe circuit shown in Figure 7 is similar to the differential

Page 10 - REVISION HISTORY

AN314REV1 9AN3147. CONCLUSIONThe CS556x/7x/8x family of devices offers an exceptional alternative to SAR devices in multiplexed applications.Due to th

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