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| 型號： | AD7874SQ |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | ADC |
| 英文描述： | LC2MOS 4-Channel, 12-Bit Simultaneous Sampling Data Acquisition System |
| 中文描述： | 4-CH 12-BIT SUCCESSIVE APPROXIMATION ADC, PARALLEL ACCESS, CDIP28 |
| 封裝： | 0.600 INCH, HERMETIC SEALED, CERDIP-28 |
| 文件頁數(shù)： | 8/16頁 |
| 文件大?。?/td> | 414K |
| 代理商： | AD7874SQ |
AD7874
REV. C
–8–
T he first read operation to the AD7874 after conversion always
accesses data from Data Register 1 (i.e., the conversion result
from the V
IN1
input).
INT
is reset high on the falling edge of
RD
during this first read operation. T he second read always ac-
cesses data from Data Register 2 and so on. T he address pointer
is reset to point to Data Register 1 on the rising edge of
CONVST
. A read operation to the AD7874 should not be at-
tempted during conversion. T he timing diagram for the
AD7874 conversion sequence is shown in Figure 7.
CH1
CH2
CH3
CH4
DATA
INT
CS
RD
CONVST
HIGH-IMPEDANCE
TRACK/HOLDS GO
INTO HOLD
HIGH-
Z
HIGH-Z
t
1
t
CONV
t
ACQUISITION
t
5
t
2
t
8
t
4
t
3
t
7
t
6
HIGH-
Z
HIGH-
Z
TIMES
t
2
,
t
3
,
t
4
,
t
6
,
t
7
, AND
t
8
ARE THE SAME FOR ALL FOUR READ OPERATIONS.
Figure 7. AD7874 Timing Diagram
AD7874 DY NAMIC SPE CIFICAT IONS
T he AD7874 is specified and 100% tested for dynamic perfor-
mance specifications as well as traditional dc specifications such
as Integral and Differential Nonlinearity. T hese ac specifications
are required for the signal processing applications such as
phased array sonar, adaptive filters and spectrum analysis.
T hese applications require information on the ADC’s effect on
the spectral content of the input signal. Hence, the parameters
for which the AD7874 is specified include SNR, harmonic dis-
tortion, intermodulation distortion 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
where N is the number of bits.
T hus for an ideal 12-bit converter, SNR = 74 dB.
T he output spectrum from the ADC is evaluated by applying a
sine wave signal of very low distortion to the V
IN
input which is
sampled at a 29 kHz sampling rate. A Fast Fourier T ransform
(FFT ) plot is generated from which the SNR data can be ob-
tained. Figure 8 shows a typical 2048 point FFT plot of the
AD7874BN with an input signal of 10 kHz and a sampling
frequency of 29 kHz. T he SNR obtained from this graph is
73.2 dB. It should be noted that the harmonics are taken into
account when calculating the SNR.
(1)
Figure 8. AD7874 FFT Plot
E ffective Number of Bits
T he 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)
T he effective number of bits for a device can be calculated di-
rectly from its measured SNR.
Figure 9 shows a typical plot of effective number of bits versus
frequency for an AD7874BN with a sampling frequency of
29 kHz. T he effective number of bits typically falls between
11.75 and 11.87 corresponding to SNR figures of 72.5 dB and
73.2 dB.
z
Figure 9. Effective Numbers of Bits vs. Frequency
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參數(shù)描述 |
|---|---|
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