参数资料
型号: AD7863BR-3REEL
厂商: Analog Devices Inc
文件页数: 6/24页
文件大小: 0K
描述: IC ADC 14BIT DUAL 2CH 28-SOIC
标准包装: 1,000
位数: 14
采样率(每秒): 175k
数据接口: 并联
转换器数目: 2
功率耗散(最大): 94.5mW
电压电源: 模拟和数字
工作温度: -40°C ~ 85°C
安装类型: 表面贴装
封装/外壳: 28-SOIC(0.295",7.50mm 宽)
供应商设备封装: 28-SOIC W
包装: 带卷 (TR)
输入数目和类型: 4 个单端,单极;4 个单端,双极
AD7863
Rev. B | Page 14 of 24
AD7863 DYNAMIC SPECIFICATIONS
The AD7863 is specified and tested for dynamic performance 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
AD7863 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 fundamental.
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
(1)
where N is the number of bits.
Thus for an ideal 14-bit converter, SNR = 86.04 dB.
Figure 12 shows a histogram plot for 8192 conversions of a dc
input using the AD7863 with 5 V supply. The analog input was
set at the center of a code transition. It can be seen that the
codes appear mainly in the one output bin, indicating very good
noise performance from the ADC.
746
747
748
749
750
751
752
753
754
755
06
41
1-
0
12
CO
UN
T
S
CODE
8000
7000
6000
5000
4000
3000
2000
1000
0
Figure 12. 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 VAX/BX input,
which is sampled at a 175 kHz sampling rate. A fast fourier
transform (FFT) plot is generated from which the SNR data can
be obtained. Figure 13 shows a typical 8192 point FFT plot of
the AD7863 with an input signal of 10 kHz and a sampling
frequency of 175 kHz. The SNR obtained from this graph is
80.72 dB. It should be noted that the harmonics are taken into
account when calculating the SNR.
0
1020
304050
607080
90
06
41
1-
0
13
(d
B
)
FREQUENCY (kHz)
0
–130
–120
–110
–100
–90
–80
–70
–60
–50
–40
–30
–20
–10
–140
–150
fSAMPLE = 175kHz
fIN = 10kHz
SNR = +80.72dB
THD = –92.96dB
Figure 13. AD7863 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
obtain a measure of performance expressed in effective number
of bits (N).
02
.
6
76
.
1
=
SNR
N
(2)
The effective number of bits for a device can be calculated
directly from its measured SNR.
Figure 14 shows a typical plot of effective numbers of bits vs.
frequency for an AD7863-2 with a sampling frequency of
175 kHz. The effective number of bits typically falls between
13.11 and 11.05 corresponding to SNR figures of 80.68 dB
and 68.28 dB.
0
200
400
600
800
1000
06
41
1-
0
14
EN
O
B
FREQUENCY (kHz)
14.0
10.5
11.0
11.5
12.0
12.5
13.0
13.5
10.0
Figure 14. Effective Numbers of Bits vs. Frequency
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