AD7721

REV. A

–15–

CS

AD7721

DRDY

WR

RD

CS

1Y1

1Y2

1Y3

2Y1

2Y2

2Y3

2Y4

HC244

1G 2G 1A1

1A2

1A3

1A4

2A1

2A2

2A3

2A4

1Y4

1Y1

1Y2

1Y3

2A1

2A2

2A3

2A4

HC244

1G 2G 1A1

1A2

1A3

1A4

2Y1

2Y2

2Y3

2Y4

1Y4

DB11

DB10

DB9

DB8

DB7

DB6

DB5

DB4

DB3

DB2

DB1

DB0

EN ADDR

DECODE

A15–A0

D11–D0

TMS320C20/

C25

RD/

W

IRQ

IS

STRB

Figure 19. AD7721 to TMS320C20/C25 Interface

CS

AD7721

DRDY

WR

RD

CS

1Y1

1Y2

1Y3

2Y1

2Y2

2Y3

2Y4

HC244

1G 2G 1A1

1A2

1A3

1A4

2A1

2A2

2A3

2A4

1Y4

1Y1

1Y2

1Y3

2A1

2A2

2A3

2A4

HC244

1G 2G 1A1

1A2

1A3

1A4

2Y1

2Y2

2Y3

2Y4

1Y4

DB11

DB10

DB9

DB8

DB7

DB6

DB5

DB4

DB3

DB2

DB1

DB0

RD

IRQ

WE

DMA13–DMA0

DMD11–DMD

IS

TMS320C5x

EN ADDR

DECODE

Figure 20. AD7721 to TMS320C5x Interface

Grounding and Layout

Since the analog inputs are differential, most of the voltages in

the analog modulator are common-mode voltages. The excellent

common-mode rejection of the part will remove common-mode

noise on these inputs. The analog and digital supplies to the

AD7721 are independent and separately pinned out to minimize

coupling between analog and digital sections of the device. The

digital filter will provide rejection of broadband noise on the

power supplies, except at integer multiples of the modulator

sampling frequency. The digital filter also removes noise from

the analog inputs provided the noise source does not saturate

the analog modulator. As a result, the AD7721 is more immune

to noise interference than a conventional high resolution con-

verter. However, because the resolution of the AD7721 is high

and the noise levels from the AD7721 so low, care must be

taken with regard to grounding and layout.

The printed circuit board that houses the AD7721 should be

designed such 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 easily separated. A minimum

etch technique is generally best for ground planes as it gives the

best shielding. Digital and analog ground planes should only be

joined in one place. If the AD7721 is the only device requiring

an AGND to DGND connection, the ground planes should be

connected at the AGND and DGND pins of the AD7721.

DSUBST, which is the substrate connection for the digital

circuitry of the AD7721, should be connected directly to

AGND. If the AD7721 is in a system where multiple devices

require AGND to DGND 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 AD7721.

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 AD7721 to avoid noise coupling. The power

supply lines to the AD7721 should use as large a trace as pos-

sible to provide low impedance paths and reduce the effects of

glitches on the power supply line. Fast switching signals like

clocks should be shielded with digital ground to avoid radiating

noise to other sections of the board, and clock signals should

never be run near the analog inputs. Avoid crossover of digital

and analog signals. Traces on opposite sides of the board

should run at right angles to each other. This will reduce the

effects of feedthrough through the board. A microstrip tech-

nique is by far the best but is not always possible with a double-

sided board. In this technique, the component side of the board

is dedicated to ground planes while signals are placed on the

other side.

Good decoupling is important when using high resolution

ADCs. All analog and digital supplies should be decoupled to

AGND and DGND respectively with 10 nF ceramic capacitors

in parallel with 1

µF tantalum capacitors. To achieve the best

from these decoupling capacitors, they should be placed as close

as possible to the device, ideally right up against the device. In

systems where a common supply voltage is used to drive both

of the AD7721 and AGND and the recommended digital supply