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參數資料
| 型號： | AD7725 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | 16-Bit 900 kSPS ADC with a Programmable Postprocessor |
| 中文描述： | 16位900 ksps模數轉換器的可編程后處理器 |
| 文件頁數： | 18/28頁 |
| 文件大小： | 442K |
| 代理商： | AD7725 |
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REV. A
–18–
AD7725
XTAL
MCLK
1M
Figure 18. Crystal Oscillator Connection
When an external clock source is being used, the internal
oscil-
lator circuit can be disabled by tying XTAL_OFF high. A low
phase noise clock should be used to generate the ADC sam-
pling clock because sampling clock jitter effectively modulates
the input signal and raises the noise floor. The sampling
clock
generator should be isolated from noisy digital circuits, grounded,
and heavily decoupled to the analog ground plane.
The sampling clock generator should be referenced to the analog
ground in a split ground system; however, this is not always pos-
sible because of system constraints. In many applications, the
sampling clock must be derived from a higher frequency multi-
purpose system clock that is generated on the digital ground
plane. If the clock signal is passed between its origin on a digital
ground plane to the AD7725 on the analog ground plane, the
ground noise between the two planes adds directly to the clock
and will produce excess jitter. The jitter can cause degradation
in the signal-to-noise ratio and also produce unwanted harmon-
ics. This can be remedied somewhat by transmitting the sampling
signal as a differential one, using either a small RF transformer
or a high speed differential driver and a receiver such as PECL.
In either case, the original master system clock should be gener-
ated from a low phase noise crystal oscillator.
SYSTEM SYNCHRONIZATION
The SYNC input provides a synchronization function for use in
parallel or serial mode. SYNC allows the user to begin gathering
samples of the analog input from a known point in time.
This
allows a system using multiple AD7725s, operated from a
common master clock, to be synchronized so that each ADC
simultaneously updates its output register. In a system using
multiple AD7725s, a common signal to their SYNC inputs
will synchronize their operation. When SYNC is high, the digi-
tal filter sequencer is reset to zero. A SYNC pulse, one CLKIN
cycle long, can be applied. This way, SYNC is sensed
low on
the next rising edge of CLKIN. When SYNC is sensed low,
normal conversion continues. Following a SYNC, the
modula-
tor and filter need time to settle before data can be read from
the AD7725. Also, when INIT is taken high, it activates SYNC,
which ensures that multiple devices cascaded in serial mode will
sample their analog inputs simultaneously.
FILTERING
The Preset Filter
The preset filter is the digital filter directly following the modu-
lator. This is a fixed filter whose main function is to remove the
large out-of-band quantization noise shaped by the modulator.
This filter is made up of three cascaded half-band FIR filters,
and each filter decimates by two. The word rate into the preset
filter is CLKIN, and due to the decimation in the three subse-
quent filter stages, the output word rate of the preset filter, and
thus the input word rate to the postprocessor, is CLKIN/8. See
Figure 19.
POST-
PROCESSOR
MODULATOR
FIR 1
DEC 2
FIR 2
DEC 2
FIR 3
DEC 2
PRESET
FILTER
INPUT WORD
RATE = CLKIN
OUTPUT WORD
RATE = CLKIN/8
Figure 19. The Preset Filter
The Postprocessor
The AD7725 contains Systolix’s PulseDSP
mable postprocessor. The postprocessor directly follows the
preset filter. The postprocessor core is a systolic array of simple
high performance processors. These processors are grouped into
36 multiply accumulate (MAC) blocks, with each block consist-
ing of three multipliers and one adder. Each block can process
three filter taps, thus the postprocessor allows up to 36 3 = 108
filter taps. In a systolic array, numerical data is pumped around
processors. Each of these processors is allocated to a dedicated
function and only performs that single function. The data is
passed between processors and, in this manner, complex opera-
tions are performed on the input signal. In the AD7725, data
transfers between processors are fully synchronous. As a result,
the user does not have to consider timing issues.
The postprocessor core is optimized for signal conditioning
applications. In this type of application, generally the most
common function is filtering. The core can support any filter
structure, whether FIR, IIR, recursive, or nonrecursive. The
core also supports polynomial functions, commonly used in
linearization algorithms.
Data can be transparently decimated or interpolated when
passed between processors. This simplifies the design of multirate
filtering and gives great flexibility when specifying the final
output word rate. The AD7725 postprocessor supports decima-
tion/interpolation by factors up to 256.
TM
user-program-
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