AD7667AST datasheet(28/29 Pages) AD

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AD7667

Rev. 0 | Page 27 of 28

APPLICATION HINTS

BIPOLAR AND WIDER INPUT RANGES

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

analog input range such as ±10 V, ±5 V, or 0 V to 5 V. Although

the AD7667 has only one unipolar range, simple modifications

of input driver circuitry allow bipolar and wider input ranges to

be used without any performance degradation. Figure 45 shows

a connection diagram that allows this. Component values

required and resulting full-scale ranges are shown in Table 8.

When desired, accurate gain and offset can be calibrated by

acquiring a ground and voltage reference using an analog

multiplexer (U2), as shown in Figure 45.

ANALOG

INPUT

100nF

CREF

INGND

REF

REFGND

AD7667

Ω

2.7nF

Figure 45. Using the AD7667 in 16-Bit Bipolar and/or Wider Input Ranges

Table 8. Component Values and Input Ranges

Input Range

R1 (Ω)

R2 (kΩ)

R3 (kΩ)

R4 (kΩ)

±10 V

500

2.5

±5 V

500

2.5

1.67

0 V to –5 V

500

None

LAYOUT

The AD7667 has very good immunity to noise on the power

supplies. However, care should still be taken with regard to

grounding layout.

The printed circuit board that houses the AD7667 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 separated easily. Digital and analog

ground planes should be joined in only one place, preferably

underneath the AD7667, or as close as possible to the AD7667.

If the AD7667 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 AD7667.

Running digital lines under the device should be avoided since

these couple noise onto the die. The analog ground plane

should be allowed to run under the AD7667 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 crosstalk

through the board.

The power supply lines to the AD7667 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

AD7667 and to reduce the magnitude of the supply spikes.

Decoupling ceramic capacitors, typically 100 nF, should be

placed on each power supply pin—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 near the ADC to further reduce

low frequency ripple.

The DVDD supply of the AD7667 can be a separate supply or

can come from the analog supply AVDD or the digital interface

supply OVDD. When the system digital supply is noisy or when

fast switching digital signals are present, if no separate supply is

available, the user should connect DVDD to AVDD through an

RC filter (see Figure 26) and the system supply to 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 AD7667 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, because

it carries pulsed currents, should be a low impedance return to

the reference. AGND is the ground to which most internal ADC

analog signals are referenced; it 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 config-

uration. OGND is connected to the digital system ground.

EVALUATING THE AD7667’S PERFORMANCE

A recommended layout for the AD7667 is outlined in the

EVAL-AD7667 evaluation board for the AD7667. 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 BRD2.

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