AD7664ASTZ datasheet(20/25 Pages) AD

20 / 25 page

REV.

AD7664

–19–

APPLICATION HINTS

Bipolar and Wider Input Ranges

In some applications, it is desired to use a bipolar or wider ana-

log input range like, for instance,

±10 V, ± 5 V, or 0 V to 5 V.

Although the AD7664 has only one unipolar range, by simple

modifications of the input driver circuitry, bipolar and wider

input ranges can be used without any performance degradation.

Figure 22 shows a connection diagram that allows this.

Component values required and resulting full-scale ranges are

shown in Table III.

For applications where accurate gain and offset are desired, they

can be calibrated by acquiring a ground and a voltage reference

using an analog multiplexer, U2, as shown for bipolar input

ranges in Figure 22.

2.5V REF

ANALOG

INPUT

100nF

CREF

INGND

REF

REFGND

1 F

AD7664

10nF

Figure 22. Using the AD7664 in 16-Bit Bipolar and/or

Wider Input Ranges

Table III. Component Values and Input Ranges

Input Range

R1 (k )

R2 (k )

R3 (k )

R4 (k )

±10 V

±5 V

6.67

0 V to –5 V

None

Layout

The AD7664 has very good immunity to noise on the power

supplies, as can be seen in Figure 9. However, care should still

be taken with regard to grounding layout.

The printed circuit board that houses the AD7664 should be

designed so that the analog and digital sections are separated

and confined to certain areas of the board. This facilitates the

use of ground planes that can be easily separated. Digital and

analog ground planes should be joined in only one place,

preferably underneath the AD7664, or, at least, as close as

possible to the AD7664. If the AD7664 is in a system where

multiple devices require analog-to-digital ground connections,

the connection should still be made at one point only, a star

ground point that should be established as close as possible to

the AD7664.

It is recommended to avoid running digital lines under the

device, because these couple noise onto the die. The analog

ground plane should be allowed to run under the AD7664 to avoid

noise coupling. Fast switching signals like

CNVST or clocks

should be shielded with digital ground to avoid radiating

noise to other sections of the board and should never run

near analog signal paths. Crossover of digital and analog signals

should be avoided. Traces on different but close layers of the

board should run at right angles to each other. This reduces the

effect of feedthrough through the board.

The power supplies’ lines to the AD7664 should use as large a

trace as possible to provide low impedance paths and reduce the

effect of glitches on the power supplies’ lines. Good decoupling

is also important to lower the supplies’ impedance presented to

the AD7664 and to reduce the magnitude of the supply spikes.

Decoupling ceramic capacitors, typically 100 nF, should be

placed on each power supply’s pins AVDD, DVDD, and OVDD

close to, and ideally right up against, these pins and their corre-

sponding ground pins. Additionally, low ESR 10

µF capacitors

should be located in the vicinity of the ADC to further reduce low

frequency ripple.

The DVDD supply of the AD7664 can be either a separate supply

or come from the analog supply AVDD or the digital interface

supply OVDD. When the system digital supply is noisy, or fast

switching digital signals are present, it is recommended that, if no

separate supply is available, to connect the DVDD digital supply

to the analog supply, AVDD, through an RC filter, as shown

in Figure 5, and to connect the system supply to the interface

digital supply, OVDD, and the remaining digital circuitry. When

DVDD is powered from the system supply, it is useful to insert

a bead to further reduce high frequency spikes.

The AD7664 has five different ground pins: INGND, REFGND,

AGND, DGND, and OGND. INGND is used to sense the

analog input signal. REFGND senses the reference, voltage and

should be a low impedance return to the reference because it

carries pulsed currents. AGND is the ground to which most

internal ADC analog signals are referenced. This ground must

be connected with the least resistance to the analog ground

plane. DGND must be tied to the analog or digital ground

plane, depending on the configuration. OGND is connected to

the digital system ground.

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