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參數資料
| 型號： | AD7875BQ |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | ADC |
| 英文描述： | LC2MOS Complete, 12-Bit, 100 kHz, Sampling ADCs |
| 中文描述： | 1-CH 12-BIT SUCCESSIVE APPROXIMATION ADC, SERIAL/PARALLEL ACCESS, CDIP24 |
| 封裝： | 0.300 INCH, HERMETIC SEALED, CERAMIC, DIP-24 |
| 文件頁數： | 11/20頁 |
| 文件大小： | 331K |
| 代理商： | AD7875BQ |
REV. B
2
AD7870/AD7875/AD7876
–11–
Figure 12. Mode 2 Timing Diagram, Byte or Serial Read
T he Mode 2 timing diagram for byte and serial data is shown in
Figure 12. For two-byte data read, the lower byte (DB0–DB7)
has to be accessed first since HBEN must be low to start con-
version. T he ADC behaves like slow memory for this first read,
but the second read to access the upper byte of data is a normal
read. Operation of the serial functions is identical between
Mode 1 and Mode 2. T he timing diagram of Figure 12 shows
both a noncontinuously and a continuously running SCLK
(dashed line).
DY NAMIC SPE CIFICAT IONS
T he AD7870 and AD7875 are specified and 100% tested for
dynamic performance specifications as well as traditional dc
specifications such as integral and differential nonlinearity. Al-
though the AD7876 is not production tested for ac parameters,
its dynamic performance is similar to the AD7870 and AD7875.
T he ac specifications are required for signal processing applica-
tions such as speech recognition, spectrum analysis and high
speed modems. T hese applications require information on the
ADC’s effect on the spectral content of the input signal. Hence,
the parameters for which the AD7870 and AD7875 are speci-
fied include SNR, harmonic distortion, intermodulation distor-
tion and peak harmonics. T hese 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. T he 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 depen-
dent upon the number of quantization levels used in the digiti-
zation process; the more levels, the smaller the quantization
noise. T he theoretical signal-to-noise ratio for a sine wave input
is given by
SNR
= (6.02
N
+ 1.76)
dB
(1)
where
N
is the number of bits. T hus for an ideal 12-bit con-
verter,
SNR
= 74 dB.
sine-wave signal of very low distortion to the V
IN
input which is
sampled at a 100 kHz sampling rate. A Fast Fourier T ransform
(FFT ) plot is generated from which the SNR data can be ob-
tained. Figure 13 shows a typical 2048 point FFT plot of the
AD7870K N/AD7875K N with an input signal of 25 kHz and a
sampling frequency of 100 kHz. T he SNR obtained from this
graph is 72.6 dB. It should be noted that the harmonics are
taken into account when calculating the SNR.
Figure 13. FFT Plot
E ffective Number of Bits
T he formula given in (1) relates SNR to the number of bits.
Rewriting the formula, as in (2), it is possible to get a measure
of performance expressed in effective number of bits (N).
N
=
SNR –
1.76
6.02
(2)
T he effective number of bits for a device can be calculated di-
rectly from its measured SNR.
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| AD7875KNZ1 | 制造商:AD 制造商全稱:Analog Devices 功能描述:LC2MOS Complete, 12-Bit, 100 kHz, Sampling ADCs |
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