AD7824/AD7828

–6–

REV. F

OPERATIONAL DIAGRAM

The AD7824 is a 4-channel 8-bit ADC and the AD7828 is an

8-channel 8-bit ADC. Operational diagrams for both of these

devices are shown in Figures 3 and 4. The addition of just a 5 V

reference allows the devices to perform the analog-to-digital function.

24

23

22

21

20

19

18

17

16

15

14

13

1

2

3

4

5

6

7

8

9

10

11

12

AD7824

NC = NO CONNECT

AIN4

AIN3

NC

AIN2

A0

AIN1

A1

NC

DB7

DB0

DB6

DB1

DB5

DB2

DB4

DB3

CS

RD

RDY

INT

GND

ANALOG INPUTS

0V TO 5V

P 4LSB

DATA BUS

P CONTROL INPUT

STATUS OUTPUT

P ADDRESS

BUS

P 4MSB

DATA BUS

P CONTROL INPUT

STATUS OUTPUT

5V

5V

Figure 3. AD7824 Operational Diagram

28

27

26

25

24

23

22

21

20

19

18

17

16

15

1

2

3

4

5

6

7

8

9

10

11

12

13

14

AD7828

NC = NO CONNECT

AIN6

AIN7

AIN5

AIN8

AIN4

AIN3

A0

AIN2

A1

AIN1

A2

NC

DB7

DB0

DB6

DB1

DB5

DB2

DB4

DB3

CS

RD

RDY

INT

GND

ANALOG INPUTS

0V TO 5V

P 4LSB

DATA BUS

P CONTROL INPUT

STATUS OUTPUT

ANALOG INPUTS

0V TO 5V

P ADDRESS

BUS

P 4MSB

DATA BUS

P CONTROL INPUT

STATUS OUTPUT

5V

5V

Figure 4. AD7828 Operational Diagram

CIRCUIT INFORMATION

BASIC DESCRIPTION

The AD7824/AD7828 uses a half-flash conversion technique

whereby two 4-bit flash ADCs are used to achieve an 8-bit result.

Each 4-bit flash ADC contains 15 comparators that compare

the unknown input to a reference ladder to get a 4-bit result.

For a full 8-bit reading to be realized, the upper 4-bit flash, the

most significant (MS) flash, performs a conversion to provide

the four most significant data bits. An internal DAC, driven by

the four MSBs, then recreates an analog approximation of the

input voltage. This analog result is subtracted from the input,

and the difference is converted by the lower flash ADC, the least

significant (LS) flash, to provide the four least significant bits of

the output data. The most significant flash ADC also has one

additional comparator to detect overrange on the analog input.

APPLYING THE AD7824/AD7828

REFERENCE AND INPUT

The two reference inputs on the AD7824/AD7828 are fully differ-

ential and define the zero to full-scale input range of the ADC.

As a result, the span of the analog input voltage for all channels

reference arrangement also facilitates ratiometric operation.

This reference flexibility also allows the input channel voltage

input level for all channels, which produces a digital output of

all zeroes. Therefore, although the analog inputs are not them-

selves differential, they have nearly differential input capability

in most measurement applications because of the reference

design. Figures 5 to 7 show some of the configurations that are

possible.

AD7824*

AD7828*

AIN1

GND

47 F

0.1 F

5V

ADDITIONAL PINS OMITTED FOR CLARITY.

ONLY CHANNEL 1 SHOWN.

*

Figure 5. Power Supply as Reference

AD7824*

AD7828*

AIN1

GND

47 F

0.1 F

5V

ADDITIONAL PINS OMITTED FOR CLARITY.

ONLY CHANNEL 1 SHOWN.

*

AD580

0.1 F

10 F

Figure 6. External Reference Using the AD580, Full-Scale

Input is 2.5 V

AD7824*

AD7828*

AIN1

GND

47 F

0.1 F

5V

ADDITIONAL PINS OMITTED FOR CLARITY.

ONLY CHANNEL 1 SHOWN.

*

DB7

DB0

DATA

V1

V2

DATA =

256 (FOR ALL CHANNELS)

V1 – V2

Figure 7. Input Not Referenced to GND