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PDF AD7821 Data sheet ( Hoja de datos )
| Número de pieza | AD7821 | |
| Descripción | LC2MOS High Speed/ mP-Compatible 8-Bit ADC with Track/Hold Function | |
| Fabricantes | Analog Devices | |
| Logotipo |  |
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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
LC2MOS High Speed, P-Compatible
8-Bit ADC with Track/Hold Function
AD7821
FUNCTIONAL BLOCK DIAGRAM
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 (vs. 1.36 µs for
the AD7820) and 100 kHz signal bandwidth (vs. 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 sup-
ply, 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
(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.
PRODUCT HIGHLIGHTS
1. Fast Conversion Time
The half-flash conversion technique, coupled with fabrication
on Analog Devices’ LC2MOS 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 fig-
ure 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 sup-
ply (± 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.
REV. A
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
which 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: 617/329-4700
Fax: 617/326-8703
1 page  
Typical Performance Curves–AD7821
Conversion Time (RD Mode) vs.
Temperature
Power Supply Current vs. Tempera-
ture (Not Including Reference Ladder)
Accuracy vs. tWR
Accuracy vs. tRD
Accuracy vs. tP
Accuracy vs. VREF
[VREF = VREF(+) – VREF(–)]
Effective Number of Bits vs. Input
Signal (±2.5 V) Frequency
tINTL, Internal Time Delay vs.
Temperature
Output Current vs. Temperature
REV. A
–5–
5 Page 
AD7821
APPLYING THE AD7821
The AD7821 is specified for a unipolar input range of 0 to +5 V
and a bipolar input range of –2.5 V to +2.5 V. The VREF(–) and
VREF(+) voltages required for these input ranges are outlined be-
low. See the Typical Performance Characteristics section for op-
eration with unspecified input voltage ranges.
BIPOLAR OPERATION
Figure 18 gives the configuration and reference voltages
required for –2.5 V to +2.5 V operation. The nominal transfer
characteristic for this input range is shown in Figure 20.
The output code is Offset Binary with 1 LSB = ([+2.5 –
(–2.5)]/256) V = 19.5 mV.
UNIPOLAR OPERATION
Figure 18 gives the configuration and reference voltages re-
quired for 0 V to +5 V operation. The nominal transfer charac-
teristic for this input range is shown in Figure 19. The output
code is Natural Binary with 1 LSB = (5/256) V = 19.5 mV.
Figure 18. AD7821 Unipolar/Bipolar Operation
Figure 20. Nominal Transfer Characteristic for Bipolar
(–2.5 V to +2.5 V) Operation
16-CHANNEL TELECOM A/D CONVERTER
The fast sampling rate (1 MHz) and bipolar operation of the
AD7821 makes it useful in Telecom applications for sampling a
number of input channels using a multiplexer. Figure 21 shows
a circuit for such an application.
The maximum signal frequency required for acceptable quality
in Telecom applications is 3 kHz. The circuit given in Figure 21
permits each of the 16-input channels to be sampled at a rate of
16 kHz maximum. The sampling rate takes account of such
multiplexer parameters as tON, settling time etc. The circuit also
eases the problem of the antialiasing filter design by sampling at
a rate much greater than that required by the Nyquist criterion.
Figure 19. Nominal Transfer Characteristic for Unipolar
(0 V to +5 V) Operation
Figure 21. 16-Channel Telecom A/D Converter System
REV. A
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11 Page | ||
| Páginas | Total 12 Páginas | |
| PDF Descargar | [ Datasheet AD7821.PDF ] | |
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