AD7660 datasheet(19/21 Pages) AD

19 / 21 page

REV.

AD7660

–18–

APPLICATION HINTS

Bipolar and Wider Input Ranges

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

analog input range like, for instance,

±10 V, ±5 V, or 0 V to

5 V. Although the AD7660 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. Compo-

nent values required and resulting full-scale ranges are shown in

Table III.

2.5V REF

ANALOG

INPUT

100nF

CREF

INGND

REF

REFGND

100nF

AD7660

Figure 22. Using the AD7660 in 16-Bit Bipolar and/or

Wider Input Ranges

Table III. Component Values and Input Ranges

Input Range

R1 (k

W) R2 (kW)

R3 (k

W) R4 (kW)

±10 V

±5 V

6.67

0 V to –5 V

None

For bipolar range 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 in

Figure 22. Also, CF can be used as a one-pole antialiasing filter.

Layout

The AD7660 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 AD7660 should be

designed so 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 ana-

log ground planes should be joined in only one place,

preferably underneath the AD7660, or, at least, as close as

possible to the AD7660. If the AD7660 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 AD7660.

It is recommended to avoid running digital lines under the device

as these will couple noise onto the die. The analog ground plane

should be allowed to run under the AD7660 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 will reduce the effect of feedthrough

through the board.

The power supply lines to the AD7660 should use as large a

trace as possible to provide low impedance paths and reduce the

effect of glitches on the power supply lines. Good decoupling is

also important to lower the supply’s impedance presented to the

AD7660 and to reduce the magnitude of the supply spikes.

Decoupling ceramic capacitors, typically 100 nF, should be placed

on each power supply pins AVDD, DVDD, and OVDD close to,

and ideally right up against, these pins and their corresponding

ground pins. Additionally, low ESR 10

mF capacitors should

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

frequency ripple.

The DVDD supply of the AD7660 can be either a separate

supply or come from the analog supply, AVDD, or from the

digital interface supply, OVDD. When the system digital supply

is noisy, or fast switching digital signals are present, it is recom-

mended 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 6 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 AD7660 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 depend-

ing on the configuration. OGND is connected to the digital

system ground.

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