®

ADS774

6

THEORY OF OPERATION

In the ADS774, the advantages of advanced CMOS technol-

ogy—high logic density, stable capacitors, precision analog

switches—and Burr-Brown’s state of the art laser trimming

techniques are combined to produce a fast, low power

analog-to-digital converter with internal sample/hold.

The charge-redistribution successive-approximation circuitry

converts analog input voltages into digital words.

A simple example of a charge-redistribution A/D converter

with only 3 bits is shown in Figure 1.

approximation is made by connecting S

comparator. After three successive approximation steps have

been made the voltage level at the comparator will be within

1/2LSB of GND, and a digital word which represents the

and S

OPERATION

BASIC OPERATION

Figure 2 shows the minimum connections required to oper-

ate the ADS774 in a basic

±10V range in the Control Mode

(discussed in detail in a later section.) The falling edge of a

Convert Command (a pulse taking pin 5 LOW for a mini-

mum of 25ns) both switches the ADS774 input to the hold

state and initiates the conversion. Pin 28 (STATUS) will

output a HIGH during the conversion, and falls only after the

conversion is completed and the data has been latched on the

data output pins (pins 16 to 27.) Thus, the falling edge of

STATUS on pin 28 can be used to read the data from the

conversion. Also, during conversion, the STATUS signal

puts the data output pins in a High-Z state and inhibits the

input lines. This means that pulses on pin 5 are ignored, so

that new conversions cannot be initiated during the conver-

sion, either as a result of spurious signals or to short-cycle

the ADS774.

The ADS774 will begin acquiring a new sample as soon as

the conversion is completed, even before the STATUS

output falls, and will track the input signal until the next

conversion is started. The ADS774 is designed to complete

a conversion and accurately acquire a new signal in 8.5

µs

max over the full operating temperature range, so that

conversions can take place at a full 117kHz.

CONTROLLING THE ADS774

The Burr-Brown ADS774 can be easily interfaced to most

microprocessor systems and other digital systems. The

microprocessor may take full control of each conversion, or

the converter may operate in a stand-alone mode, controlled

only by the R/C input. Full control consists of selecting an

8- or 12-bit conversion cycle, initiating the conversion, and

reading the output data when ready—choosing either 12 bits

all at once, or the 8 MSB bits followed by the 4 LSB bits in

R/C, and CE) are all TTL/CMOS-compatible. The functions

of the control inputs are described in Table II. The control

function truth table is shown in Table III.

STAND-ALONE OPERATION

For stand-alone operation, control of the converter is accom-

plished by a single control line connected to R/C. In this

and 12/8 are connected to +5V. The output data are

FIGURE 1. 3-Bit Charge Redistribution A/D.

INPUT SCALING

Precision laser-trimmed scaling resistors at the input divide

standard input ranges (0V to +10V, 0V to +20V,

±5V or

±10V) into levels compatible with the CMOS characteristics

of the internal capacitor array.

SAMPLING

While sampling, the capacitor array switch for the MSB

MSB capacitor is proportional to the voltage level of the

comparator input offset to zero.

CONVERSION

opened to trap a charge on the MSB capacitor proportional

to the analog input level at the time of the sampling com-

mand, and switch S

The charge trapped in the capacitor array can now be moved

between the three capacitors in the array by connecting

reference) or “G” (to connect to GND), thus changing the

voltage generated at the comparator input.

During the first approximation, the MSB capacitor is con-

nected through switch S

comparator output is HIGH or LOW, the logic will then

C

R

G

2C

R

G

4C

R

G

+

–

S

3

S

2

S

1

S

C

Comparator

L

o

g

i

c

Reference

Input

S

Signal

Analog

Input

Out