EVAL-AD7731EBZ Analog Devices Inc, EVAL-AD7731EBZ Datasheet

EVAL-AD7731EBZ

Manufacturer Part Number

EVAL-AD7731EBZ

Description

BOARD EVALUATION FOR AD7731

Manufacturer

Analog Devices Inc

Datasheets

1.AD7731BRUZ.pdf (44 pages)

2.EVAL-AD7731EBZ.pdf (11 pages)

3.EVAL-AD7731EBZ.pdf (3 pages)

4.EVAL-AD7731EBZ.pdf (44 pages)

Specifications of EVAL-AD7731EBZ

Number Of Adc's

Number Of Bits

Sampling Rate (per Second)

6.4k

Data Interface

Serial

Inputs Per Adc

3 Differential

Input Range

±1.28 V

Power (typ) @ Conditions

67.5mW @ 6.4kSPS

Voltage Supply Source

Analog and Digital

Operating Temperature

-40°C ~ 85°C

Utilized Ic / Part

AD7731

Lead Free Status / RoHS Status

Lead free / RoHS Compliant

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ONE TECHNOLOGY WAY

INTRODUCTION

The AD7731 is a complete analog front-end for process

control applications. The device has a proprietary program-

mable gain front end which allows it to accept a range of

input signal ranges including low-level signals directly from

a transducer. The sigma delta architecture of the part

consists of an analog modulator and a low-pass program-

mable digital filter, allowing adjustment of filter cutoff,

output rate and settling-time.

The part features three buffered differential programmable

gain analog inputs (which can be configured as five pseudo-

differential inputs) as well as a differential reference input.

The part operates from a single +5 V supply and accepts

seven unipolar analog input ranges: 0 to +20 mV,

+40 mV, +80 mV, +160mV, +320 mV, +640 mV and

+1.28 V and seven bipolar ranges ±20 mV, ±40 mV,

±80 mV, ±160 mV, ±320 mV, ±640 mV and ±1.28 V.

The peak-to-peak resolution achievable directly from the

part is 16 bits at an 800 Hz output rate. The part can

switch between channels with 1ms settling time and

maintain a performance level of 13 bits of peak-to-peak

resolution.

The serial interface on the part can be configured for three-

wire operation and is compatible with microcontrollers and

digital signal processors. The AD7731 contains self-

calibration and system calibration options and features an

offset drift of less than 5nV/°C and a gain drift of less than

2ppm/°C.

As with all high resolution converters care has to be taken

with printed circuit board layout to achieve optimum

performance. The issues that give the greatest concern

include power supply decoupling, grounding techniques

and the general location of the digital and analog signals on

the printed circuit board. This technical note outlines

general guidlines on printed circuit board design and

layout in order to obtain the optimum performance from

the AD7731.

GROUNDING AND LAYOUT

Since the analog inputs and reference input are differential,

most of the voltages in the analog modulator are common-

mode voltages. The excellent Common-Mode Rejection of

the AD7731 will remove common-mode noise on these

inputs. The analog and digital supplies to the AD7731 are

independent and separately pinned out to minimize

coupling between the analog and digital sections of the

Obtaining Optimum Performance From the AD7731

•

P.O.BOX 9106

by: Albert O'Grady

•

NORWOOD, MASSACHUSETTS 02062-9106

Sigma Delta ADC

device. The digital filter will provide rejection of broad-

band noise on the power supplies, except at integer

multiples of the modulator sampling frequency. The digital

filter also removes noise from the analog and reference

inputs provided those noise sources do not saturate the

analog modulator. As a result, the AD7731 is more

immune to noise interference that a conventional high

resolution converter. However, because the resolution of

the AD7731 is so high and the noise levels from the

AD7731 so low, care must be taken with regard to ground-

ing and layout.

The printed circuit board which houses the AD7731 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 which can be separated

easily. 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 AD7731 is the only device requiring an AGND to

DGND connection, then the ground planes should be

connected at the AGND and DGND pins of the AD7731.

If the AD7731 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 which

should be established as close as possible to the AD7731.

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 AD7731 to avoid noise

coupling. The power supply lines to the AD7731 should

use as large a trace as possible 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

technique 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 solder side.

Good decoupling is important when using high resolution

ADCs. All analog supplies should be decoupled with 10µF

tantalum in parallel with 0.1µF capacitors to AGND. To

Technical Note

•

781/329-4700

Related parts for EVAL-AD7731EBZ

EVAL-AD7731EBZ Summary of contents

Page 1

ONE TECHNOLOGY WAY P.O.BOX 9106 Obtaining Optimum Performance From the AD7731 INTRODUCTION The AD7731 is a complete analog front-end for process control applications. The device has a proprietary program- mable gain front end which allows it to accept a ...

Page 2

... A full evaluation board package including a fully as- sembled and tested evaluation board, documentation, software for controlling the board over the printer port and software for analyzing the AD7731’s performance on the PC is available from Analog Devices. The evaluation board order number is EVAL-AD7731EB ...

Page 3

... Figure 2. Eval-AD7731EB Solder Side Artwork. Figure 3. Eval-AD7731EB Silkscreen Artwork ...

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