Datasheet

EVM Overview

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1 EVM Overview

1.1 Features

• Full-featured Evaluation Board for the ADS8556/ADS8557 or ADS8558 16/14/12-Bit, 6-Channel

Simultaneous Sampling Analog to Digital Converters

• Analog inputs can be configured for direct application to the ADC or applied through buffer circuits

• On board external reference option

• Supports Serial or High-Speed Parallel Interface Modes

1.2 Introduction

The ADS855xEVM is an evaluation board for the ADS8556, ADS8557 and ADS8558 family of high

precision analog to digital converters. The ADS8556 is featured throughout this Users Guide, but the

ADS8557 or the ADS8558 device may be installed on the evaluation board. All EVM functions are

identical regardless of the installed device.

The ADS8556 is a high speed, low power, six channel 16-bit A/D converter that operates from

independent ±HVDD (±5V to ±15V), +5V AVdd and DVdd supplies. The digital output is delivered through

a built in buffer circuit that can be powered from DVdd or separate 2.7-5.25V (BVdd) sources. This allows

for flexibility when designing within mixed voltage environments.

The six sample and hold circuits are divided into three pairs (A, B and C). Each pair of channels has a

hold signal (CONVST_A, CONVST_B, and CONVST_C) which, when strobed together, allows

simultaneous sampling on all 6 analog inputs. The part accepts bipolar analog input voltages in the range

of up to ±12V.

Conversion time for the ADS8556 is 1.19µs maximum when operated in hardware mode. Throughput is up

to 670kSPS when operated in the parallel interface mode or 470kSPS when operated in serial interface

mode. To achieve the maximum throughput of 800kSPS (530kSPS serial), the EVM allows the user to

apply an external conversion clock and reference.

1.3 Analog Interface

The analog circuit of the board is divided in four parts. The first part is the analog input buffer or front-end

circuit of the A/D converter. Its function is to provide optional gain and impedance matching of the input

signal. The analog power supplies for both the high voltage rails on the ADC and the input buffer is part

two. The final parts of the analog input include the basic analog functions of the ADS8556 and the optional

external reference circuit. Each of these is described below.

1.3.1 Analog Inputs

The analog input to the ADS855xEVM board is comprised of three independent THS4032 operational

amplifiers. The THS4032’s are powered from a user provided (up to) ±15V analog supply. The amplifiers

are arranged as non-inverting unity gain buffers by default. The input circuit for CHA is shown in Figure 1

and is typical of all six input channels. JPx can be used to bypass the input buffer circuit.

ADS855xEVM SLAU298–November 2009

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