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AN239

CS5361/CS5381 Compatibility

by Kevin L Tretter

1. Analog Input Buffer Design

1.1 Introduction

There are many considerations that must be taken into account when designing and implementing an an-

alog input buffer. These include negligible noise contribution, input biasing, isolation from switched ca-

pacitor currents, maintaining a low output impedance so as not to cause distortion, and providing anti-

alias filtering appropriate for the modulator sample rate.

1.2 Recommended Differential Input Buffer

Figure 1 shows the recommended fully differential input buffer for the CS5361 and CS5381. This input

buffer will work well with both devices. However, if full performance is to be realized, then care must be

taken when selecting the op amps to ensure that the noise level of the input buffer is insignificant relative

to the converter. The CDB5361 and CDB5381 implement the NE5532 op amp, which provides an ade-

quate noise floor to see full performance from these two converters.

Figure 1. Recommended Differential Input Buffer for the CS5361 and CS5381

AIN+

AIN-

470 pF

C0G

470 pF

C0G

CS5361/CS5381

634 Ω

91 Ω

634 Ω

91 Ω

2700 pF

C0G

10 µF

10 kΩ

0.01 µF

1 µF

0.01 µF

100 kΩ

470 pF

C0G

470 pF

C0G

634 Ω

91 Ω

634 Ω

91 Ω

2700 pF

C0G

10 µF

10 kΩ

0.01 µF

100 kΩ

AIN+

AIN-

100 kΩ

10 µF

NE5532

MAY ‘03

AN239REV1

Summary of content (4 pages)

PAGE 1
AN239 CS5361/CS5381 Compatibility by Kevin L Tretter 1. Analog Input Buffer Design 1.1 Introduction There are many considerations that must be taken into account when designing and implementing an analog input buffer. These include negligible noise contribution, input biasing, isolation from switched capacitor currents, maintaining a low output impedance so as not to cause distortion, and providing antialias filtering appropriate for the modulator sample rate. 1.
PAGE 2
AN239 2. Capacitor Requirements for FILT+ (pin 24) 2.1 Introduction The CS5361 and CS5381 utilize a positive reference voltage for the internal sampling circuits. This reference pin, called FILT+ (pin 24), requires external capacitors for decoupling purposes. The size of the decoupling capacitor will affect the low frequency (<200 Hz) distortion performance of the converter. The larger the capacitor, the better the low frequency distortion performance. 2.
PAGE 3
• Notes •
PAGE 4