REV. B
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under any patent or patent rights of Analog Devices.
a
LC
2
MOS High Speed, P Compatible
8-Bit ADC with Track/Hold Function
AD7821
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700 www.analog.com
Fax: 781/326-8703 © Analog Devices, Inc., 2002
FUNCTIONAL 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-Lead DlPs, SOIC, and 20-Terminal
Surface-Mount Packages
GENERAL DESCRIPTION
The 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 (versus 1.36 µs
for the AD7820) and 100 kHz signal bandwidth (versus 6.4
kHz). The sampling instant is better defined and occurs on the
falling edge of WR or RD. The 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
supply, like the AD7820.
The 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.
The 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.
The AD7821 is fabricated in Linear Compatible CMOS
(LC
2
MOS), an advanced, mixed technology process combining
precision bipolar circuits with low power CMOS logic. The part
features a low power dissipation of 50 mW.
PRODUCT HIGHLIGHTS
1. Fast Conversion Time
The half-flash conversion technique, coupled with fabrication
on Analog Devices’ LC
2
MOS process, enables a very fast con-
version time. The conversion time for the WR-RD mode is
660 ns, with 700 ns for the RD mode.
2. Built-In Track-and-Hold
This allows input signals with slew rates up to 1.6 V/µs to be
converted to 8 bits without an external track-and-hold. This
corresponds to a 5 V peak-to-peak, 100 kHz sine wave signal.
3. Total Unadjusted Error
The AD7821 features an excellent total unadjusted error figure
of less than ±1 LSB over the full operating temperature range.
4. Unipolar/Bipolar Input Ranges
The AD7821 is specified for single-supply (+5 V) operation
with a unipolar full-scale range of 0 to +5 V, and for dual-supply
(±5 V) operation with a bipolar input range of ±2.5 V. Typical
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.
