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參數資料
| 型號： | AD7868BQ |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 模擬信號調理 |
| 英文描述： | LC2MOS Complete, 12-Bit Analog I/O System |
| 中文描述： | SPECIALTY ANALOG CIRCUIT, CDIP24 |
| 封裝： | CERDIP-24 |
| 文件頁數： | 12/16頁 |
| 文件大小： | 337K |
| 代理商： | AD7868BQ |
AD7868
–12–
REV. B
AD7868—T MS32020/T MS320C25 Interface
Figure 18 shows an interface which is suitable for the
T MS32020/T MS320C25 processors. T his interface is config-
ured for synchronous, continuous clock operation. Note, the
AD7868 will not interface correctly to these processors if the
AD7868 is configured for a noncontinuous clock. Conversion
starts and DAC updating are controlled by an external timer.
TMS32020
TMS320C25
FSX
DX
TCLK
DT
TFS
*ADDITIONAL PINS OMITTED FOR CLARITY
LDAC
FSR
CLKR
DR
RCLK
DR
CONVST
RFS
CONTROL
TIMER
AD7868*
4.7k
2k
4.7k
5V
+
5V
–
CLKX
Figure 18. AD7868—TMS32020/TMS320C25 Interface
APPLICAT ION HINT S
Good printed circuit board (PCB) layout is as important as the
circuit design itself in achieving high speed A/D performance.
T he AD7868’s comparator is required to make bit decisions on
an LSB size of 1.465 mV. T o achieve this, the designer has to
be conscious of noise both in the ADC itself and in the preced-
ing analog circuitry. Switching mode power supplies are not rec-
ommended as the switching spikes will feed through to the
comparator causing noisy code transitions. Other causes of con-
cern are ground loops and digital feedthrough from micropro-
cessors. T hese are factors which influence any ADC, and a
proper PCB layout which minimizes these effects is essential for
best performance.
LAY OUT HINT S
Ensure that the layout for the printed circuit board has the digi-
tal and analog signal lines separated as much as possible. T ake
care not to run any 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 as close as possible to the AD7868
AGND pins. Connect all other grounds and the AD7868
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 re-
turns are essential to low noise operation of the ADC, so make
the foil width for these tracks as wide as possible. T he use of
ground planes minimizes impedance paths and also guards the
analog circuitry from digital noise. T he circuit layout of Figures
22 and 23 have both analog and digital ground planes which are
kept separated and only joined together at the AD7868 AGND
pins.
NOISE
K eep the input signal leads to V
IN
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 be-
tween the source and the ADC. Reduce the ground circuit im-
pedance as much as possible since any potential difference in
grounds between the signal source and the ADC appears as an
error voltage in series with the input signal.
INPUT /OUT PUT BOARD
Figure 19 shows an analog I/O board based on the AD7868.
T he corresponding printed circuit board (PCB) layout and
silkscreen are shown in Figures 21 to 23.
T he analog input to the AD7868 is buffered with an AD711 op
amp. T here is a component grid provided near the analog input
on the PCB which may be used for an antialiasing filter for the
ADC or a reconstruction filter for the DAC or any other condi-
tioning circuitry. T o facilitate this option, there are two wire
links (labeled LK 1 and LK 2) required on the analog input and
output tracks.
T he board contains a SHA circuit which can be used on the
output of the AD7868 DAC to extend the very good perfor-
mance of the part over a wider frequency range. T he increased
performance from the SHA can be seen in Figures 14 and 15 of
this data sheet. A wire link (labeled LK 3) connects the board
output to either the SHA output or directly to the AD7868
DAC output.
T here are three
LDAC
link options on the board;
LDAC
can be
driven from an external source independent of
CONVST
,
LDAC
can be tied to
CONVST
or
LDAC
can be tied to GND.
Choosing the latter option of tying
LDAC
to GND disables the
SHA operation, and places the SHA permanently in the track
mode.
Microprocessor connections to the board are made by a 9-way
D-type connector. T he pinout is shown in Figure 20. T he
ADC’s digital outputs are buffered with 74HC4050s. T hese
buffers provide a higher current output capability for high
capacitance loads or cables. Normally, these buffers are not
required as the AD7868 will be sitting on the same board as the
processor.
POWE R SUPPLY CONNE CT IONS
T he PCB requires two analog power supplies and one 5 V digi-
tal supply. Connections to the analog supply are made directly
to the PCB as shown on the silkscreen in Figure 21. T he con-
nections are labeled V+ and V– and the range for both of these
supplies is 12 V to 15 V. Connections to the 5 V digital supply
are made through the D-type connector SK T 6. T he
±
5 V ana-
log supply required by the AD7868 are generated from two volt-
age regulators on the V+ and V– supplies.
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| AD7868BR | 制造商:Analog Devices 功能描述:Data Acquisition System Single ADC Single DAC 12-Bit 28-Pin SOIC W 制造商:Rochester Electronics LLC 功能描述:ANALOG I/O PORT IC - Bulk |
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