a
LC2MOS High Speed, P Compatible
8-Bit ADC with Track/Hold Function
AD7821
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
FUNCTIONAL BLOCK DIAGRAM
GENERAL DESCRIPTION
PRODUCT HIGHLIGHTS
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 VSS pin (Pin 19)
allows the part to operate from ± 5 V supplies and to digitize
bipolar input signals. Alternatively, for unipolar inputs, the VSS pin
can be grounded and the AD7821 will operate from a single +5 V
supply, like the AD7820.
1. Fast Conversion Time
The half-flash conversion technique, coupled with fabrication
on Analog Devices’ LC2MOS process, enables a very fast conversion time. The conversion time for the WR-RD mode is
660 ns, with 700 ns for the RD mode.
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 generate 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
(LC2MOS), an advanced, mixed technology process combining
precision bipolar circuits with low power CMOS logic. The part
features a low power dissipation of 50 mW.
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-tonoise ratio, distortion, and slew rate.
REV. B
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties that
may result from its use. No license is granted by implication or otherwise
under any patent or patent rights of Analog Devices.
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
