
REV. A
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a
LC
2
MOS Hgh Speed,
m
P-Compatible
8-Bit ADC wth Track/Hold Function
AD7821
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 617/329-4700
Fax: 617/326-8703
FUNCT IONAL BLOCK DIAGRAM
FEATURES
Fast Conversion Time: 660 ns max
100 kHz Track-and-Hold Function
1 MHz Sample Rate
Unipolar and Bipolar Input Ranges
Ratiometric Reference Inputs
No External Clock
Extended Temperature Range Operation
Skinny 20-Pin DlPs, SOIC and 20-Terminal
Surface Mount Packages
GE NE RAL DE SCRIPT ION
T he AD7821 is a high speed, 8-bit, sampling, analog-to-digital
converter that offers improved performance over the popular
AD7820. It offers a conversion time of 660 ns (vs. 1.36
μ
s for
the AD7820) and 100 kHz signal bandwidth (vs. 6.4 kHz). T he
sampling instant is better defined and occurs on the falling edge
of
WR
or
RD
. T he provision of a V
SS
pin (Pin 19) allows the
part to operate from
±
5 V supplies and to digitize bipolar input
signals. Alternatively, for unipolar inputs, the V
SS
pin can be
grounded and the AD7821 will operate from a single +5 V sup-
ply, like the AD7820.
T he AD7821 has a built-in track-and-hold function capable of
digitizing full-scale signals up to 100 kHz max. It also uses a
half-flash conversion technique that eliminates the need to gen-
erate a CLK signal for the ADC.
T he AD7821 is designed with standard microprocessor control
signals (
CS
,
RD
,
WR
, RDY,
INT
) and latched, three-state data
outputs capable of interfacing to high speed data buses. An
overflow output (
OFL
) is also provided for cascading devices to
achieve higher resolution.
T he AD7821 is fabricated in Linear-Compatible CMOS
(LC
2
MOS), an advanced, mixed technology process combining
precision bipolar circuits with low power CMOS logic. T he part
features a low power dissipation of 50 mW.
PRODUCT HIGHLIGHT S
1. Fast Conversion T ime
T he half-flash conversion technique, coupled with fabrication
on Analog Devices’ LC
2
MOS process, enables a very fast con-
version time. T he conversion time for the WR-RD mode is
660 ns, with 700 ns for the RD mode.
2. Built-In T rack-and-Hold
T his allows input signals with slew rates up to 1.6 V/
μ
s to be
converted to 8-bits without an external track-and-hold. T his
corresponds to a 5 V peak-to-peak, 100 kHz sine wave signal.
3. T otal Unadjusted Error
T he AD7821 features an excellent total unadjusted error fig-
ure of less than
±
1 LSB over the full operating temperature
range.
4. Unipolar/Bipolar Input Ranges
T he AD7821 is specified for single supply (+5 V) operation
with a unipolar full-scale range of 0 to +5 V, and for dual sup-
ply (
±
5 V) operation with a bipolar input range of
±
2.5 V.
T ypical performance characteristics are given for other input
ranges.
5. Dynamic Specifications for DSP Users
In addition to the traditional ADC specifications, the
AD7821 is specified for ac parameters, including signal-to-
noise ratio, distortion and slew rate.