8

ADS7864

®

REFERENCE

internal +2.5V reference to the ADS7864. The ADS7864

can operate, however, with an external reference in the range

of 1.2V to 2.6V for a corresponding full-scale range of 2.4V

to 5.2V.

The internal reference of the ADS7864 is double-buffered.

If the internal reference is used to drive an external load, a

buffer is provided between the reference and the load ap-

plied to pin 33 (the internal reference can typically source

2mA of current—load capacitance should not exceed 100pF).

If an external reference is used, the second buffer provides

isolation between the external reference and the CDAC.

This buffer is also used to recharge all of the capacitors of

both CDACs during conversion.

ANALOG INPUT

The analog input is bipolar and fully differential. There are

two general methods of driving the analog input of the

ADS7864: single-ended or differential (see Figures 1 and 2).

When the input is single-ended, the –IN input is held at the

common-mode voltage. The +IN input swings around the

same common voltage and the peak-to-peak amplitude is the

common-mode voltage may vary (see Figure 3).

When the input is differential, the amplitude of the input is the

difference between the +IN and –IN input, or: (+IN) – (–IN).

The peak-to-peak amplitude of each input is

±1/2V

this common voltage. However, since the inputs are 180

° out

of phase, the peak-to-peak amplitude of the differential voltage

range of the voltage that may be common to both inputs (see

Figure 4).

INTRODUCTION

The ADS7864 is a high speed, low power, dual 12-bit A/D

converter that operates from a single +5V supply. The input

channels are fully differential with a typical common-mode

rejection of 80dB. The part contains dual 2

µs successive

approximation ADCs, six differential sample-and-hold am-

pins and a high speed parallel interface. There are six analog

inputs that are grouped into three channels (A, B and C).

Each A/D converter has three inputs (A0/A1, B0/B1 and C0/

C1) that can be sampled and converted simultaneously, thus

preserving the relative phase information of the signals on

both analog inputs. Each pair of channels has a hold signal

(HOLDA, HOLDB, HOLDC) to allow simultaneous sam-

pling on all 6 channels. The part accepts an analog input

internal +2.5V reference. The part will also accept bipolar

input ranges when a level shift circuit is used at the front end

(see Figure 7).

A conversion is initiated on the ADS7864 by bringing the

HOLDX pin LOW for a minimum of 15ns. HOLDX LOW

places both sample-and-hold amplifiers of the X channels in

the hold state simultaneously and the conversion process is

started on both channels. The BUSY output will then go

LOW and remain LOW for the duration of the conversion

cycle. The data can be read from the parallel output bus

following the conversion by bringing both RD and CS

LOW.

Conversion time for the ADS7864 is 1.75

µs when an 8MHz

external clock is used. The corresponding acquisition time is

0.25

µs. To achieve maximum output rate (500kHz), the read

function can be performed during at the start of the next

conversion.

NOTE: This mode of operation is described in more detail

in the Timing and Control section of this data sheet.

SAMPLE-AND-HOLD SECTION

The sample-and-hold amplifiers on the ADS7864 allow the

ADCs to accurately convert an input sine wave of full-scale

amplitude to 12-bit accuracy. The input bandwidth of the

sample-and-hold is greater than the Nyquist rate (Nyquist

equals one-half of the sampling rate) of the ADC even when

the ADC is operated at its maximum throughput rate of

500kHz. The typical small-signal bandwidth of the sample-

and-hold amplifiers is 40MHz.

Typical aperture delay time or the time it takes for the

ADS7864 to switch from the sample to the hold mode

following the negative edge of HOLDX signal is 5ns. The

average delta of repeated aperture delay values is typically

50ps (also known as aperture jitter). These specifications

reflect the ability of the ADS7864 to capture AC input

signals accurately at the exact same moment in time.

FIGURE 1. Methods of Driving the ADS7864 Single-Ended

or Differential.

ADS7864

ADS7864

Single-Ended Input

Common

Voltage

–V

peak-to-peak

Differential Input

Common

Voltage

V

REF

peak-to-peak

V

REF

peak-to-peak