2
LC MOS High Speed, P-Compatible
a
8-Bit ADC with Track/Hold Function
AD7821
FEATURES
FUNCTIO NAL BLO CK D IAGRAM
Fast Conversion Tim e: 660 ns m ax
100 kHz Track-and-Hold Function
1 MHz Sam ple Rate
Unipolar and Bipolar Input Ranges
Ratiom etric Reference Inputs
No External Clock
Extended Tem perature Range Operation
Skinny 20-Pin DlPs, SOIC and 20-Term inal
Surface Mount Packages
GENERAL D ESCRIP TIO N
P RO D UCT H IGH LIGH TS
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 µs 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 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 sup-
ply, like the AD7820.
1. Fast Conversion T ime
T he half-flash conversion technique, coupled with fabrication
on Analog Devices’ LC2MOS 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/µs 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 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 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 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 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.
T he AD7821 is fabricated in Linear-Compatible CMOS
(LC2MOS), 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.
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.
REV. A
Inform ation furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assum ed by Analog Devices for its
use, nor for any infringem ents of patents or other rights of third parties
which m ay result from its use. No license is granted by im plication or
otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norw ood, MA 02062-9106, U.S.A.
Tel: 617/ 329-4700
Fax: 617/ 326-8703