AD7650 datasheet(19/21 Pages) AD

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REV. 0

AD7650

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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 AD7650 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 20 shows a connection diagram which allows that. Compo-

nents values required and resulting full-scale ranges are shown

in Table II.

2.5V REF

ANALOG

INPUT

100nF

CREF

IN+

IN–

REF

REFGND

1mF

AD7650

10nF

Figure 20. Using the AD7650 in 16-Bit Bipolar

and/or Wider Input Ranges

Table II. Component Values and Input Ranges

Input Range

±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

2 k

Ω

None

Ω

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 20.

Layout

The AD7650 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 AD7650 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 AD7650 or, at least, as close as possible to the

AD7650. If the AD7650 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 AD7650.

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 AD7650 to avoid noise

coupling. Fast switching signals like

CNVST or clocks should be

shielded with digital ground to avoid radiating noise to other sec-

tions 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 AD7650 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 AD7650 and reduce the magnitude of the supply spikes.

Decoupling 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 correspond-

ing 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 AD7650 can either be 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, 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 inter-

face 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 AD7650 has five different ground pins: IN–, REFGND,

AGND, DGND, and OGND. IN– 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.

Evaluating the AD7650 Performance

A recommended layout for the AD7650 is outlined in the

evaluation board for the AD7650. The evaluation board package

includes a fully assembled and tested evaluation board,

documentation, and software for controlling the board from a

PC via the Eval-Control Board.

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