CS5571

DS768PP1

27

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10:56

3.13 Using the CS5571 in Multiplexing Applications

The CS5571 is a delta-sigma A/D converter. Delta-sigma converters use oversampling as means to

achieve high signal-to-noise performance. This means that once a conversion is started, the converter

takes many samples to compute the resulting output word. The analog input for the signal to be converted

must remain active during the entire conversion until RDY falls.

The CS5571 can be used in multiplexing applications, but the system timing for changing the multiplexer

channel and for starting a new conversion will depend upon the multiplexer system architecture.

The simplest system is illustrated in Figure 27. Any time the multiplexer is changed, the analog signal

presented to the converter must fully settle. After the signal has settled, the CONV signal is issued to the

converter to start a conversion. Being a delta-sigma converter, the signal must remain present at the input

of the converter until the conversion is completed. Once the conversion is completed, RDY falls. At this

time the multiplexer can be changed to the next channel and the data can be read from the serial port.

The CONV signal should be delayed until after the data is read and until the new analog signal has settled.

In this configuration, the throughput of the converter will be dictated by the settling time of the analog input

circuit and the conversion time of the converter. The conversion data can be read from the serial port after

the multiplexer is changed to the new channel while the analog input signal is settling.

Figure 27. Simple Multiplexing Scheme

A more complex multiplexing scheme can be used to increase the throughput of the converter is illustrated

in Figure 28. In this circuit, two banks of multiplexers are used.

Amplifier

Settling Time

Conversion Time

Amplifier

Settling Time

CH1

CH2

CONV

RDY

Advance

Mux

Throughput

CS5571

AIN

ACOM

CH1

CH2

CH3

CH4

90

150pF

2k

4700pF

C0G