a
AD7891
REV. D
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LC
2
MOS 8-Channel, 12-Bit
High Speed Data Acquisition System
FUNCTIONAL BLOCK DIAGRAM
2.5V
REFERENCE
TRACK/HOLD
V
IN1A
V
IN1B
V
IN2A
V
IN2B
V
IN3A
V
IN3B
V
IN4A
V
IN4B
V
IN5A
V
IN5B
V
IN6A
V
IN6B
V
IN7A
V
IN7B
V
IN8A
V
IN8B
CONTROL LOGIC
WR CS RD EOC CONVST
MODE AGND
AGND DGND
CLOCK
V
DD
V
DD
AD7891
12-BIT
ADC
ADDRESS
DECODE
REF OUT/
REF IN
REF GND
STANDBY
M
U
X
DATA/
CONTROL
LINES
FEATURES
Fast 12-Bit ADC with 1.6 s Conversion Time
8 Single-Ended Analog Input Channels
Overvoltage Protection on Each Channel
Selection of Input Ranges:
ⴞ5 V, ⴞ10 V for AD7891-1
0 to +2.5 V, 0 to +5 V, ⴞ2.5 V for AD7891-2
Parallel and Serial Interface
On-Chip Track/Hold Amplifier
On-Chip Reference
Single-Supply, Low Power Operation (100 mW Max)
Power-Down Mode (75 W Typ)
APPLICATIONS
Data Acquisition Systems
Motor Control
Mobile Communication Base Stations
Instrumentation
GENERAL DESCRIPTION
The AD7891 is an 8-channel, 12-bit data acquisition system
with a choice of either parallel or serial interface structure. The
part contains an input multiplexer, an on-chip track/hold ampli-
fier, a high speed 12-bit ADC, a 2.5 V reference, and a high
speed interface. The part operates from a single 5 V supply and
accepts a variety of analog input ranges across two models, the
AD7891-1 (± 5V and ± 10 V) and the AD7891-2 (0 V to +2.5 V,
0V to +5 V, and ± 2.5 V).
The AD7891 provides the option of either a parallel or serial
interface structure determined by the MODE pin. The part
has standard control inputs and fast data access times for both
the serial and parallel interfaces, ensuring easy interfacing to
modern microprocessors, microcontrollers, and digital signal
processors.
In addition to the traditional dc accuracy specifications, such as
linearity, full-scale and offset errors, the part is also specified for
dynamic performance parameters, including harmonic distortion
and signal-to-noise ratio.
Power dissipation in normal mode is 82 mW typical; in
the standby mode, this is reduced to 75 mW typ. The part is
available in a 44-terminal MQFP and a 44-lead PLCC.
PRODUCT HIGHLIGHTS
1. The AD7891 is a complete monolithic 12-bit data acquisition
system that combines an 8-channel multiplexer, 12-bit ADC,
2.5 V reference, and track/hold amplifier on a single chip.
2. The AD7891-2 features a conversion time of 1.6 ms and an
acquisition time of 0.4 ms. This allows a sample rate of
500 kSPS when sampling one channel and 62.5 kSPS when
channel hopping. These sample rates can be achieved using
either a software or hardware convert start. The AD7891-1
has an acquisition time of 0.6 ms when using a hardware
convert start and an acquisition time of 0.7 ms when using a
software convert start. These acquisition times allow sample
rates of 454.5 kSPS and 435 kSPS, respectively, for hardware
and software convert start.
3. Each channel on the AD7891 has overvoltage protection. This
means an overvoltage on an unselected channel does not affect
the conversion on a selected channel. The AD7891-1 can
withstand overvoltages of ± 17 V.
