CS5126 datasheet(13/32 Pages) CIRRUS

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ANALOG CIRCUIT CONNECTIONS

Most popular successive-approximation A/D

converters generate dynamic loads at their ana-

log connections. The CS5126 internally buffers

all analog inputs (AIN, VREF, and AGND) to

ease the demands placed on external circuitry.

However, accurate system operation still requires

careful attention to details at the design stage re-

garding source impedances as well as grounding

and decoupling schemes.

Reference Considerations

An application note titled "Voltage references

for the CS501X/CSZ511X Series of A/D Convert-

ers" is available which describes the dynamic

load conditions presented by the VREF input on

Crystal’s self-calibrating SAR A/D converters

(including the CS5126). As the CS5126 se-

quences through bit decisions it switches por-

tions of the capacitor array to the VREF pin in

accordance with the successive-approximation

algorithm. For proper operation, the source im-

pedance at the VREF pin must remain low at

frequencies up to 1MHz.

A large capacitor connected between VREF and

AGND can provide sufficiently low output im-

pedance at the frequencies of interest, so the ref-

erence voltage can simply be derived as shown

in Figure 13a. Although very low cost, this ref-

erence has almost no power supply rejection

from the VA+ line.

Alternatively, a more stable and precise refer-

ence can be generated using a TL431 shunt ref-

erence from T.I. or Motorola, as shown in Fig-

ure 13b.

The magnitude of the current load on the exter-

nal reference circuitry will scale to the master

clock frequency. At the full-rated 24 MHz clock

the reference must supply a maximum load cur-

rent of 20

µA peak-to-peak (2µA typical). An

output impedance of 2

Ω will therefore yield a

maximum error of 40mV. With a 4.5V reference

and LSB size of 138mV this would insure ap-

proximately 1/4 LSB accuracy. A 10

µF capaci-

tor exhibits an impedance of less than 2

Ω at fre-

quencies greater than 16kHz. A high-quality tan-

talum capacitor in parallel with a smaller ce-

ramic capacitor is recommended.

VA+

IN4148

100

µF

AGND

VREF

10 k

Ω

0.1

µF

100

Ω

a. Simple Reference

+12 or +15 V

µF

AGND

VREF

1.6 k

Ω

0.1

µF

Ω

0.1

µF

2 k

Ω

2 k

Ω

TL431

b. Low-cost Shunt Reference

Figure 13. Suggested Voltage Reference Circuits

CS5126

DS32F1

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