REV. 0

AD7664

–18–

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 23 shows a connection diagram which allows that. Com-

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

Table II.

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 in Figure 23. Also,

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 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,

which should be established as close as possible to the AD7664.

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 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 will reduce the effect of feedthrough

through the board.

The power supplies lines to the AD7664 should use as large 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 reduce the magnitude of the supply spikes. Decou-

pling ceramic capacitors, typically 100 nF, should be placed on

each power supplies pins AVDD, DVDD, and OVDD close to,

and ideally right up against, these pins and their corresponding

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 available, connect the DVDD digital supply to

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

Figure 5, and 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.

U1

2.5V REF

ANALOG

INPUT

R2

R3

R4

100nF

R1

U2

IN

INGND

REF

REFGND

100nF

AD7664

Figure 23. Using the AD7664 in 16-Bit Bipolar and/or

Wider Input Ranges

Table II. Component Values and Input Ranges

Input Range

R1

R2

R3

R4

±10 V

250

Ω

2 k

Ω

10 k

Ω

8 k

Ω

±5 V

500

Ω

2 k

Ω

10 k

Ω

6.67 k

Ω

0 V to –5 V

1 k

Ω

1 k

Ω

None

0

Ω

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

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

log 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.

Evaluating the AD7664 Performance

A recommended layout for the AD7664 is outlined in the

evaluation board for the AD7664. The evaluation board pack-

age includes a fully assembled and tested evaluation board,

documentation, and software for controlling the board from a

PC via the Eval-Control Board.