Rainbow-electronics ADC10464 Manual do Utilizador descarregar pdf (Página 12)

Applications Information (Continued)

TL/H/11108– 14

FIGURE 4. Typical Connection. Note the multiple bypass capacitors on the reference and power supply pins. If V

REF

is not grounded, it should also be bypassed to analog ground using multiple capacitors (see 5.0 ‘‘Power Supply

Considerations’’). AGND and DGND should be at the same potential. V

IN0

is shown with an input protection network.

Pin 17 is normally left open, but optional ‘‘speedup’’ resistor R

can be used to reduce the conversion time.

4.0 INHERENT SAMPLE-AND-HOLD

Because the ADC10461, ADC10462, and ADC10464 sam-

ple the input signal once during each conversion, they are

capable of measuring relatively fast input signals without the

help of an external sample-hold. In a non-sampling succes-

sive-approximation A/D converter, regardless of speed, the

input signal must be stable to better than

1/2 LSB during

each conversion cycle or significant errors will result. Con-

sequently, even for many relatively slow input signals, the

signals must be externally sampled and held constant dur-

ing each conversion if a SAR with no internal sample-and-

hold is used.

Because they incorporate a direct sample/hold control in-

put, the ADC10461, ADC10462, and ADC10464 are suitable

for use in DSP-based systems. The S

/H input allows syn-

chronization of the A/D converter to the DSP system’s sam-

pling rate and to other ADC10461s, ADC10462s, and

ADC10464s.

The ADC10461, ADC10462, and ADC10464 can perform

accurate conversions of input signals with frequency com-

ponents from DC to over 250 kHz.

5.0 POWER SUPPLY CONSIDERATIONS

The ADC10461, ADC10462, and ADC10464 are designed

to operate from a

5V (nominal) power supply. There are

two supply pins, AV

and DV

. These pins allow sepa-

rate external bypass capacitors for the analog and digital

portions of the circuit. To guarantee accurate conversions,

the two supply pins should be connected to the same volt-

age source, and each should be bypassed with a 0.1 mF

ceramic capacitor in parallel with a 10 mF tantalum capaci-

tor. Depending on the circuit board layout and other system

considerations, more bypassing may be necessary.

The ADC10461 has a single ground pin, and the ADC10462

and ADC10464 each have separate analog and digital

ground pins for separate bypassing of the analog and digital

supplies. The devices with separate analog and digital

ground pins should have their ground pins connected to the

same potential, and all grounds should be ‘‘clean’’ and free

of noise.

In systems with multiple power supplies, careful attention to

power supply sequencing may be necessary to avoid over-

driving inputs. The A/D converter’s power supply pins

should be at the proper voltage before digital or analog sig-

nals are applied to any of the other pins.

6.0 LAYOUT AND GROUNDING

In order to ensure fast, accurate conversions from the

ADC10461, ADC10462, and ADC10464, it is necessary to

use appropriate circuit board layout techniques. The analog

ground return path should be low-impedance and free of

noise from other parts of the system. Noise from digital cir-

cuitry can be especially troublesome, so digital grounds

should always be separate from analog grounds. For best

performance, separate ground planes should be provided

for the digital and analog parts of the system.

All bypass capacitors should be located as close to the con-

verter as possible and should connect to the converter and

to ground with short traces. The analog input should be iso-

lated from noisy signal traces to avoid having spurious sig-

nals couple to the input. Any external component (e.g., a

filter capacitor) connected across the converter’s input

should be connected to a very clean ground return point.

Grounding the component at the wrong point will result in

reduced conversion accuracy.

7.0 DYNAMIC PERFORMANCE

Many applications require the A/D converter to digitize AC

signals, but conventional DC integral and differential nonlin-

earity specifications don’t accurately predict the A/D con-

verter’s performance with AC input signals. The important

specifications for AC applications reflect the converter’s

ability to digitize AC signals without significant spectral er-

rors and without adding noise to the digitized signal. Dynam-

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