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.
a
CMOS 16-Bit,
468.75 kHz, Sigma-Delta ADC
AD7721
Use of a single bit DAC in the modulator guarantees excellent
linearity and dc accuracy. Endpoint accuracy is ensured by on-
chip calibration of offset and gain. This calibration procedure
minimizes the part’s zero-scale and full-scale errors.
The output data is accessed from the output register through a
serial or parallel port. This offers easy, high speed interfacing to
modern microcontrollers and digital signal processors. The
serial interface operates in internal clocking (master) mode, the
AD7721 providing the serial clock.
CMOS construction ensures low power dissipation while a
power-down mode reduces the power consumption to only
100 µW.
GENERAL DESCRIPTION
The AD7721 is a complete low power, 12-/16-bit, sigma-delta
ADC. The part operates from a +5 V supply and accepts a
differential input of 0 V to 2.5 V or ±1.25 V. The analog input
is continuously sampled by an analog modulator at twice the
clock frequency eliminating the need for external sample-and-
hold circuitry. The modulator output is processed by two finite
impulse response (FIR) digital filters in series. The on-chip
filtering reduces the external antialias requirements to first order
in most cases. Settling time for a step input is 97.07 µs while
the group delay for the filter is 48.53 µs when the master clock
equals 15 MHz.
The AD7721 can be operated with input bandwidths up to
229.2 kHz. The corresponding output word rate is 468.75kHz.
The part can be operated with lower clock frequencies also.
The sample rate, filter corner frequency and output word rate
will be reduced also, as these are proportional to the external
clock frequency. The maximum clock frequencies in parallel
mode and serial mode are 10 MHz and 15 MHz respectively.
FEATURES
16-Bit Sigma-Delta ADC
468.75 kHz Output Word Rate (OWR)
No Missing Codes
Low-Pass Digital Filter
High Speed Serial Interface
Linear Phase
229.2 kHz Input Bandwidth
Power Supplies: AV
DD
, DV
DD
: +5 V 6 5%
Standby Mode (70 mW)
Parallel Mode (12-Bit/312.5 kHz OWR)
FUNCTIONAL BLOCK DIAGRAM
VIN1
AV
DD
DV
DD
AGND
DGND
DB8
SDATA/DB11
DB9
DRDY
RFS/DB10
12-BIT A/D CONVERTER
S-D
MODULATOR
FIR
FILTER
CONTROL LOGIC
DB4
AD7721
STBY/DB0
CAL/DB1
CLK
VIN2
DB3
DVAL/SYNC
UNI/DB2
CS
RD
WR
REFIN
DSUBST
DGND
SYNC/
DB5
DB6 SCLK/
DB7
AGND
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700 World Wide Web Site: http://www.analog.com
Fax: 781/326-8703 © Analog Devices, Inc., 1997
