AD7824/AD7828
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.
1
AIN4
APPLYING THE AD7824/AD7828
REFERENCE AND INPUT
V
DD 24
NC
23
A0
22
5V
ANALOG INPUTS
0V TO 5V
2
AIN3
3
AIN2
4
AIN1
5
NC
6
DB0
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
can easily be varied. By reducing the reference span, V
REF
(+) to
V
REF
(–), to less than 5 V, the sensitivity of the converter can be
increased (e.g., if V
REF
= 2 V then 1 LSB = 7.8 mV). The input/
reference arrangement also facilitates ratiometric operation.
This reference flexibility also allows the input channel voltage
span to be offset from zero. The voltage at V
REF
(–) sets the
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.
V
IN
(+)
V
IN
(–)
5V
0.1 F
47 F
AIN1
GND
AD7824*
AD7824
A1
21
DB7
20
DB6
19
DB5
18
DB4
17
CS
16
RDY
15
P ADDRESS
BUS
P 4LSB
DATA BUS
7
DB1
8
DB2
9
DB3
P 4MSB
DATA BUS
P CONTROL INPUT
STATUS OUTPUT
5V
P CONTROL INPUT
STATUS OUTPUT
10
RD
11
INT
12
GND
V
REF
(+)
14
V
REF
(–)
13
NC = NO CONNECT
Figure 3. AD7824 Operational Diagram
1
2
AD7828*
V
DD
V
REF
(+)
V
REF
(–)
AIN6
AIN5
AIN4
AIN3
AIN2
AIN1
NC
DB0
DB1
DB2
DB3
RD
INT
GND
AIN7
28
AIN8
27
V
DD 26
A0
25
A1
24
ANALOG INPUTS
0V TO 5V
5V
P ADDRESS
BUS
ANALOG INPUTS
0V TO 5V
3
4
5
6
7
8
*
ADDITIONAL PINS OMITTED FOR CLARITY.
ONLY CHANNEL 1 SHOWN.
AD7828
A2
23
DB7
22
DB6
21
DB5
20
DB4
19
CS
18
RDY
17
P CONTROL INPUT
STATUS OUTPUT
5V
P 4MSB
DATA BUS
Figure 5. Power Supply as Reference
V
IN
(+)
V
IN
(–)
5V
0.1 F
47 F
AD580
10 F
0.1 F
V
REF
(–)
AIN1
GND
AD7824*
P 4LSB
DATA BUS
9
10
11
AD7828*
V
DD
V
REF
(+)
P CONTROL INPUT
STATUS OUTPUT
12
13
14
V
REF
(+)
16
V
REF
(–)
15
NC = NO CONNECT
Figure 4. AD7828 Operational Diagram
CIRCUIT INFORMATION
BASIC DESCRIPTION
*
ADDITIONAL PINS OMITTED FOR CLARITY.
ONLY CHANNEL 1 SHOWN.
Figure 6. External Reference Using the AD580, Full-Scale
Input is 2.5 V
V
IN
(+)
AIN1
GND
AD7824*
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.
AD7828*
5V
0.1 F
47 F
V1
V2
V
DD
DB7
V
REF
(+)
V
REF
(–)
DB0
DATA
*
ADDITIONAL PINS OMITTED FOR CLARITY.
ONLY CHANNEL 1 SHOWN.
DATA =
V
IN
(+)
V1 – V2
256 (FOR ALL CHANNELS)
Figure 7. Input Not Referenced to GND
–6–
REV. F
AD [ ANALOG DEVICES ]
