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參數資料
型號: AD7885
廠商: Analog Devices, Inc.
英文描述: LC2MOS 16-Bit, High Speed Sampling ADCs
中文描述: LC2MOS 16位,高速采樣ADC
文件頁數: 11/16頁
文件大?。?/td> 319K
代理商: AD7885
AD7884/AD7885
REV. C
–11–
If the noise in the converter is too high for an application, it can
be reduced by oversampling and digital filtering. T his involves
sampling the input at higher than the required word rate and
then averaging to arrive at the final result. T he very fast conver-
sion time of the AD7884/AD7885 makes it very suitable for
oversampling. For example, if the required input bandwidth is
40 kHz, the AD7884/AD7885 could be oversampled by a factor
of 2. T his yields a 3 dB improvement in the effective SNR per-
formance. T he noise performance in the
±
5 volt input range is
now effectively 85
μ
V rms and the resultant spread of codes for
2500 conversions will be four. T his is shown in Figure 15.
1500
0
1000
500
C
(X – 1)
(X)
(X + 1) (X + 2)
CODE
Figure 15. Histogram of 2500 Conversions of a DC Input
Using a
×
2 Oversampling Ratio
Dynamic Performance
With a combined conversion and acquisition time of 6
μ
s, the
AD7884/AD7885 is ideal for wide bandwidth signal processing
applications. Signal to (Noise + Distortion), T otal Harmonic
Distortion, Peak Harmonic or Spurious Noise and Intermodula-
tion Distortion are all specified. Figure 16 shows a typical
FFT plot of a 1.8 kHz,
±
5 V input after being digitized by the
AD7884/AD7885.
0
–150
–60
–120
–90
–30
2048 POINT FFT
d
f = 1.8kHz,
±
5V SINE WAVE
f = 163kHz
SNR = 87dB
THD = –95dB
Figure 16. AD7884/AD7885 FFT Plot
E ffective Number of Bits
T he formula for SNR (see T erminology section) is related to
the resolution or number of bits in the converter. Rewriting the
formula, below, gives a measure of performance expressed in
effective number of bits (N).
N
= (
SNR
– 1.76)/6.02
16
10
80
13
11
20
12
0
15
14
60
40
FREQUENCY – kHz
E
Figure 17. Effective Number of Bits vs. Frequency
T he effective number of bits for a device can be calculated from
its measured SNR. Figure 17 shows a typical plot of effective
number of bits versus frequency for the AD7884. T he sampling
frequency is 166 kHz.
MICROPROCE SSOR INT E RFACING
T he AD7884/AD7885 is designed on a high speed process
which results in very fast interfacing timing (Data Access T ime
of 57 ns max). T he AD7884 has a full 16-bit parallel bus, and
the AD7885 has an 8-bit wide bus. T he AD7884, with its paral-
lel interface, is suited to 16-bit parallel machines whereas the
AD7885, with its byte interface, is suited to 8-bit machines.
Some examples of typical interface configurations follow.
AD7884 to MC68000 Interface
Figure 18 shows a general interface diagram for the MC68000,
16-bit microprocessor to the AD7884. In Figure 18, conversion
is initiated by bringing
CSA
low (i.e., writing to the appropriate
address). T his allows the processor to maintain control over the
complete conversion process. In some cases it may be more
desirable to control conversion independent from the processor.
T his can be done by using an external sampling timer.
MC68000
AD7884
ADDRESS
DECODE LOGIC
CONVST
CS
RD
DB15 – DB0
R/W
DATA BUS
ADDRESS BUS
A23 – A1
D15 – D0
DTACK
AS
CSA
CSB
Figure 18. AD7884 to MC68000 Interface
Once conversion has been started, the processor must wait until
it is completed before reading the result. T here are two ways of
ensuring this. T he first way is to simply use a software delay to
wait for 6.5
μ
s before bringing
CS
and
RD
low to read the data.
相關PDF資料
PDF描述
AD7885AAP LC2MOS 16-Bit, High Speed Sampling ADCs
AD7885ABP LC2MOS 16-Bit, High Speed Sampling ADCs
AD7885AN LC2MOS 16-Bit, High Speed Sampling ADCs
AD7885BN LC2MOS 16-Bit, High Speed Sampling ADCs
AD7886JP LC2MOS 12-Bit, 750 kHz/1 MHz, Sampling ADC
相關代理商/技術參數
參數描述
AD7885AAP 制造商:Analog Devices 功能描述:16-Bit, High Speed Sampling ADCs
ad7885aap-reel 制造商:Analog Devices 功能描述:
AD7885ABP 制造商:Rochester Electronics LLC 功能描述:16-BIT,BYTE O/P SAMPL.A/D - Bulk 制造商:Analog Devices 功能描述:
AD7885ABP-REEL 制造商:Analog Devices 功能描述:
AD7885AQ 制造商:Analog Devices 功能描述: 制造商:Analog Devices 功能描述:IC 16-BIT ADC
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