ADC10461 datasheet(13/16 Pages) NSC

13 / 16 page

Applications Information (Continued)

damage to the IC. The sum of all the overdrive currents into

all pins must be less than the Absolute Maximum Rating for

Package Input Current. When the input signal is expected to

extend beyond this limit, an input protection scheme should

be used. A simple input protection network using diodes and

resistors is shown in Figure 4. 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 Consider-

ations”). AGND and DGND should be at the same potential.

IN0 is shown with an input protection network. Pin 17 is

normally left open, but optional “speedup” resistor R

SA can

be used to reduce the conversion time.

4.0 INHERENT SAMPLE-AND-HOLD

Because the ADC10461, ADC10462, and ADC10464

sample 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

successive-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. Consequently, even for many relatively slow input

signals, the signals must be externally sampled and held

constant during each conversion if a SAR with no internal

sample-and-hold is used.

Because they incorporate a direct sample/hold control input,

the ADC10461, ADC10462, and ADC10464 are suitable for

use in DSP-based systems. The S/H input allows synchro-

nization of the A/D converter to the DSP system’s sampling

rate

and

other

ADC10461s,

ADC10462s,

and

ADC10464s.

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

CC and DVCC. These pins allow separate

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 voltage

source, and each should be bypassed with a 0.1 µF ceramic

capacitor in parallel with a 10 µF tantalum capacitor. De-

pending on the circuit board layout and other system con-

siderations, 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 signals 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

circuitry can be especially troublesome.

All bypass capacitors should be located as close to the

converter as possible and should connect to the converter

and to ground with short traces. The analog input should be

isolated from noisy signal traces to avoid having spurious

signals 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 convert-

er’s performance with AC input signals. The important speci-

fications for AC applications reflect the converter’s ability to

digitize AC signals without significant spectral errors and

without adding noise to the digitized signal. Dynamic char-

01110814

FIGURE 4. Typical Connection

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