AD7824 Analog Devices, AD7824 Datasheet - Page 6

• Manufacturer Part Number: AD7824
• Description: LC2MOS High Speed 4- & 8-Channel 8-Bit ADCs
• Manufacturer: Analog Devices

AD7824/AD7828
OPERATIONAL DIAGRAM
The AD7824 is a 4-channel 8-bit A/D converter and the
AD7828 is an 8-channel 8-bit A/D converter. Operational dia-
grams 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.
CIRCUIT INFORMATION
BASIC DESCRIPTION
The AD7824/AD7828 uses a half-flash conversion technique
whereby two 4-bit flash A/D converters are used to achieve an
8-bit result. Each 4-bit flash ADC contains 15 comparators
which 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 conver-
sion to provide the 4 most significant data bits. An internal
DAC, driven by the 4 MSBs, then recreates an analog approxi-
mation 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 4 least
significant bits of the output data.
APPLYING THE AD7824/AD7828
REFERENCE AND INPUT
The two reference inputs on the AD7824/AD7828 are fully dif-
ferential and define the zero to full-scale input range of the A/D
converter. As a result, the span of the analog input voltage for
all channels can easily be varied. By reducing the reference
span, V
converter can be increased (e.g., if V
7.8 mV). The input/reference arrangement also facilitates ratio-
metric operation.
This reference flexibility also allows the input channel voltage
span to be offset from zero. The voltage at V
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.
REF
(+) – V
REF
(–), to less than 5 V the sensitivity of the
REF
= 2 V then 1 LSB =
REF
(–) sets the