8

ADS7862

®

REFERENCE

internal +2.5V reference to the ADS7862. The ADS7862

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 ADS7862 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 2 (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

ADS7862: 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 ADS7862 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 A/Ds, two differential sample-and-hold am-

pins and a high speed parallel interface. There are four

analog inputs that are grouped into two channels (A and B)

selected by the A0 input (A0 LOW selects Channels A0 and

B0, while A0 HIGH selects Channels A1 and B1). Each

A/D converter has two inputs (A0 and A1 and B0 and B1)

that can be sampled and converted simultaneously, thus

preserving the relative phase information of the signals on

both analog inputs. The part accepts an analog input voltage

+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 ADS7862 by bringing the

CONVST pin LOW for a minimum of 15ns. CONVST

LOW places both sample-and-hold amplifiers in the hold

state simultaneously and the conversion process is started on

both channels. The BUSY output will then go HIGH and

remain HIGH for the duration of the conversion cycle.

Depending on the status of the A0 pin, the data will either

reflect a conversion of Channel 0 (A0 LOW) or Channel 1

(A0 HIGH). The data can be read from the parallel output

bus following the conversion by bringing both RD and CS

LOW.

Conversion time for the ADS7862 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 immediately 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 ADS7862 allow the

A/Ds 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 A/D even when

the A/D 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

ADS7862 to switch from the sample to the hold mode

following the CONVST pulse is 3.5ns. The average delta of

repeated aperture delay values is typically 50ps (also known

as aperture jitter). These specifications reflect the ability of

the ADS7862 to capture AC input signals accurately at the

exact same moment in time.

ADS7862

ADS7862

Single-Ended Input

Common

Voltage

–V

peak-to-peak

Differential Input

Common

Voltage

V

REF

peak-to-peak

V

REF

peak-to-peak

FIGURE 1. Methods of Driving the ADS7862 Single-Ended

or Differential.