AD7653ACPZ datasheet(26/27 Pages) AD

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Data Sheet

AD7653

Rev. C | Page 25 of 26

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 AD7653 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 38 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 38.

ANALOG

INPUT

100nF

CREF

INGND

REF

REFGND

AD7653

02966-0-022

15Ω

2.7nF

Figure 38. Using the AD7653 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 AD7653 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 AD7653 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 separated easily. Digital and

analog ground planes should be joined in only one place,

preferably underneath the AD7653, or as close as possible to the

AD7653. If the AD7653 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 AD7653.

Running digital lines under the device should be avoided since

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

should be allowed to run under the AD7653 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 to will reduce the effect of

crosstalk through the board.

The power supply lines to the AD7653 should use as large a

trace as possible to provide low impedance paths and to reduce

the effect of glitches on the power supply lines. Good

decoupling is also important to lower the supply’s impedance

presented to the AD7653, 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 AD7653 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 22), 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 AD7653 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 configuration. OGND is connected to the

digital system ground.

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