AD7872JRZ-REEL 查看數據表（PDF） - Analog Devices
零件编号
产品描述 (功能)
生产厂家
AD7872JRZ-REEL Datasheet PDF : 24 Pages
| |||
Data Sheet
APPLICATION INFORMATION
Good printed circuit board (PCB) layout is as important as the
circuit design itself in achieving high speed ADC performance.
The AD7871/AD7872 is required to make bit decisions on an
LSB size of 366 µV. Thus, the designer has to be conscious of
noise both in the ADC itself and in the preceding analog
circuitry. Switching mode power supplies are not recommended
as the switching spikes will feed through to the comparator
causing noisy code transitions. Other causes of concern are
ground loops and digital feedthrough from microprocessors.
These are factors that influence any ADC; a proper PCB layout
that minimizes these effects is essential for best performance.
LAYOUT HINTS
Ensure that the layout for the PCB has the digital and analog
signal lines separated as much as possible. Take care not to run
a digital track alongside an analog signal track. Guard (screen)
the analog input with AGND.
Establish a single point analog ground (star ground) separate
from the logic system ground at the AD7871/AD7872 AGND
pin or as close as possible to the AD7871/AD7872. Connect all
other grounds and the AD7871/AD7872 DGND to this single
analog ground point. Do not connect any other digital grounds
to this analog ground point.
Low impedance analog and digital power supply common
returns are essential to low noise operation of the ADC, so
make the foil width for these tracks as wide as possible. The use
of ground planes minimizes impedance paths and also guards
the analog circuitry from digital noise. The circuit layout of
Figure 31 and Figure 32 have both analog and digital ground
planes that are kept separated and joined together only at the
AD7871/AD7872 AGND pin.
NOISE
Keep the input signal leads to VIN and signal return leads from
AGND as short as possible to minimize input noise coupling. In
applications where this is not possible, use a shielded cable
between the source and the ADC. Reduce the ground circuit
impedance as much as possible because any potential difference
in grounds between the signal source and the ADC appears as
an error voltage in series with the input signal.
AD7871/AD7872
DATA ACQUISITION BOARD
Figure 29 shows the AD7871/AD7872 in a data acquisition
circuit. The corresponding PCB layout has three interface ports:
one serial and two parallel. Note that the AD7871/AD7872
serial lines are buffered by a 74HC244. This allows long lines
with large capacitive loads to be driven. One of the parallel
ports is directly compatible with the DSP processor evaluation
board expansion connector.
The only additional component required for a full data
acquisition system is an antialiasing filter. There is a component
grid provided near the analog input on the PCB, which may be
used for such a filter or any other input conditioning circuitry.
To facilitate this option, there is a shorting plug (labelled LK1
on the PCB) on the analog input track. If this shorting plug is
used, the analog input connects to the buffer amplifier driving
the AD7871/AD7872; if this shorting plug is omitted, a wire
link can be used to connect the analog input to the PCB
component grid.
Rev. E | Page 17 of 24
Share Link: 