AD7872JRZ-REEL 查看數據表（PDF） - Analog Devices
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AD7872JRZ-REEL Datasheet PDF : 24 Pages
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AD7871/AD7872
Data Sheet
DYNAMIC SPECIFICATIONS
The AD7871/AD7872 are specified and tested for dynamic
performance specifications as well as traditional dc
specifications such as INL and DNL. These ac specifications are
required for signal processing applications such as speech
recognition, spectrum analysis and high speed modems. These
applications require information on the effects on the spectral
content of the input signal. Therefore, the parameters for which
the AD7871/AD7872 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(dB) = (6.02N + 1.76)
(1)
where N is the number of bits in the ADC. Thus, for an ideal
14-bit converter, SNR = 86 dB.
The output spectrum from the ADC is evaluated by applying a
sine wave signal of very low distortion to the VIN input, which is
sampled at an 83 kHz sampling rate. A Fast Fourier Transform
(FFT) plot is generated from which the SNR data can be
obtained. Figure 18 shows a typical 2048 point FFT plot of the
AD7871/AD7872, with an input signal of 10 kHz and a
sampling frequency of 83 kHz. The SNR obtained from this
graph is 80 dB. Note that the harmonics are included when
calculating the SNR.
0
INPUT FREQUENCY = 10kHz
SAMPLE FREQUENCY = 60kHz
SNR = 80dB
–30
TA = 25°C
–60
–90
–120
get a measure of performance expressed in an effective number
of bits (N).
N = SNR − 1.76
(2)
6.02
The effective number of bits for a device can be calculated
directly from its measured SNR. Figure 19 shows a typical plot
of effective number of bits vs. frequency for the
AD7871/AD7872 with a sampling frequency of 60 kHz.
14.0
SAMPLE FREQUENCY = 60kHz
TA = 25°C
13.5
13.0
12.5
12.0
0
10
20
30
FREQUENCY (kHz)
Figure 19. Effective Number of Bits vs. Frequency
Harmonic Distortion
Harmonic distortion is the ratio of the rms sum of harmonics to
the fundamental. For the AD7871/AD7872, total harmonic
distortion (THD) is defined as
THD(dB) = 20log V22 + V32 + V4 2 + V52 + V6 2
V1
where:
V1 is the rms amplitude of the fundamental.
V2, V3, V4, V5 and V6 are the rms amplitudes of the second
through the sixth harmonic.
The THD is also derived from the FFT plot of the ADC output
spectrum. Figure 20 shows how the THD varies with input
frequency.
110
SAMPLE FREQUENCY = 60kHz
TA = 25°C
100
–150
0
10
20
30
90
FREQUENCY (kHz)
Figure 18. Fast Fourier Transform 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
80
0
10
20
30
INPUT FREQUENCY (kHz)
Figure 20. Total Harmonic Distortion vs. Frequency
Rev. E | Page 14 of 24
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