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參數(shù)資料
| 型號(hào)： | AD7865 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Simultaneous Sampling, Fast, 14-Bit ADC(四通道,同時(shí)采樣,高速,14位A/D轉(zhuǎn)換器) |
| 中文描述： | 同步采樣，快速，14位ADC（四通道，同時(shí)采樣，高速，14位的A / D轉(zhuǎn)換器） |
| 文件頁數(shù)： | 16/21頁 |
| 文件大?。?/td> | 375K |
| 代理商： | AD7865 |
AD7865
–16–
Rev. PrC
PRELMNARY
DATA
TECHNCAL
An alternative scheme for adjusting full-scale error in
systems which use an external reference is to adjust the
voltage at the VREF pin until the full-scale error for any
of the channels is adjusted out. The good full-scale
matching of the channels will ensure small full-scale
errors on the other channels.
V
1
R1
10k
W
R2
500
W
R3
10k
W
AGND
AD7865*
*ADDITIONAL PINS OMITTED FOR CLARITY
INPUT
RANGE = ±10V
10k
W
R5
10k
W
R4
V
INXA
Figure 13. AD7865 Full-Scale Adjust Circuit
T.B.D.
Figure 14. Histogram of 8192 Conversions of a DC Input
The output spectrum from the ADC is evaluated by
applying a sine wave signal of very low distortion to the
analog input. A Fast Fourier Transform (FFT) plot is
generated from which the SNR data can be obtained.
Figure 15 shows a typical 4096 point FFT plot of the
AD7865 with an input signal of 99.9 kHz and a sam-
pling frequency of 500 kHz. The SNR obtained from this
graph is XX.X dB. It should be noted that the harmonics
T.B.D
Figure 15. AD7865 FFT Plot
Effective Number of Bits
The formula given in Equation 1 relates the SNR to the
number of bits. Rewriting the formula, as in Equation 2, it
is possible to get a measure of performance expressed in
effective number of bits (N).
N
=
SNR
1.76
6.02
(2)
AD7865 DYNAMIC SPECIFICATIONS
The AD7865 is specified and 100% tested for dynamic
performance specifications as well as traditional dc
specifications such as Integral and Differential
Nonlinearity. These ac specifications are required for the
signal processing applications such as phased array sonar,
adaptive filters and spectrum analysis. These applications
require information on the ADC’s effect on the spectral
content of the input signal. Hence, the parameters for
which the AD7865 is specified include SNR, harmonic
distortion, intermodulation distortion and peak harmonics.
These terms are discussed in more detail in the following
sections.
Signal-to-Noise Ratio (SNR)
SNR is the measured signal to noise ratio at the output of
the ADC. The signal is the rms magnitude of the funda-
mental. Noise is the rms sum of all the nonfundamental
signals up to half the sampling frequency (fs/2) excluding
dc. SNR is dependent upon the number of quantization
levels used in the digitization process; the more levels, the
smaller the quantization noise. The theoretical signal to
noise ratio for a sine wave input is given by
SNR = (6.02N + 1.76) dB
where N is the number of bits.
Thus for an ideal 14-bit converter, SNR = 86.04 dB.
Figure 14 shows a histogram plot for 8192 conversions of
a dc input using the AD7865 with 5 V supply. The
analog input was set at the center of a code transition. It
can be seen that all the codes appear in the one output bin
indicating very good noise performance from the ADC.
(1)
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