參數(shù)資料
型號(hào): AD7863
廠商: Analog Devices, Inc.
英文描述: Simultaneous Sampling Dual 175 kSPS 14-Bit ADC(同時(shí)采樣175kSPS14位雙A/D轉(zhuǎn)換器)
中文描述: 雙同步采樣175 kSPS的14位ADC(同時(shí)采樣175kSPS14位雙的A / D轉(zhuǎn)換器)
文件頁(yè)數(shù): 13/18頁(yè)
文件大小: 291K
代理商: AD7863
AD7863
–13–
REV. 0
Total Harmonic Distortion (THD)
Total Harmonic Distortion (THD) is the ratio of the rms sum
of harmonics to the rms value of the fundamental. For the
AD7863, THD is defined as
THD
(
dB
)
=
20 log
V
2
2
+
V
3
2
+
V
4
V
1
2
+
V
5
2
where
V
1
is the rms amplitude of the fundamental and
V
2
,
V
3
,
V
4
and
V
5
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.
Intermodulation Distortion
With inputs consisting of sine waves at two frequencies, fa and
fb, any active device with nonlinearities will create distortion
products at sum and difference frequencies of mfa
±
nfb where
m, n = 0, 1, 2, 3 . . ., etc. Intermodulation terms are those for
which neither m nor n are equal to zero. For example, the second
order terms include (fa + fb) and (fa – fb) while the third order
terms include (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb).
In this case, the second and third order terms are of different
significance. The second order terms are usually distanced in
frequency from the original sine waves while the third order
terms are usually at a frequency close to the input frequencies.
As a result, the second and third order terms are specified sepa-
rately. The calculation of the intermodulation distortion is as
per the THD specification where it is the ratio of the rms sum
of the individual distortion products to the rms amplitude of the
fundamental expressed in dBs. In this case, the input consists of
two equal amplitude, low distortion sine waves. Figure 10 shows
a typical IMD plot for the AD7863.
FREQUENCY – kHz
0
–130
–140
–150
10
d
–120
–110
–100
–90
–80
–70
–60
–50
–40
–30
–20
–10
20
30
40
50
60
70
80
INPUT FREQUENCIES
F1 = 50.13kHz
F2 = 49.13kHz
F
SAMPLE
= 175kHz
IMD:
2ND ORDER TERM
–98.21dB
3RD ORDER TERM
–93.91dB
0
90
Figure 10. IMD Plot
Peak Harmonic or Spurious Noise
Harmonic or Spurious Noise is defined as the ratio of the rms
value of the next largest component in the ADC output spec-
trum (up to f
S
/2 and excluding dc) to the rms value of the fun-
damental. Normally, the value of this specification will be
determined by the largest harmonic in the spectrum, but for
parts where the harmonics are buried in the noise floor the peak
will be a noise peak.
DC Linearity Plot
Figures 11 and 12 show typical DNL and INL plots for the
AD7863.
1
0
–1
D
–0.5
0.5
0
2048
4096
6144
8192
10240 12288
14336 16383
ADC CODE
Figure 11. DC DNL Plot
1
0
–1
I
–0.5
0.5
0
2048
4096
6144
8192
10240
12288
14336 16383
ADC CODE
Figure 12. DC INL Plot
Power Considerations
In the automatic power-down mode then the part may be oper-
ated at a sample rate that is considerably less than 175 kHz. In
this case, the power consumption will be reduced and will de-
pend on the sample rate. Figure 13 shows a graph of the power
consumption versus sampling rates from 1 Hz to 100 kHz in the
automatic power-down mode. The conditions are 5 V supply
25
°
C.
FREQUENCY – kHz
50
00
P
45
40
35
30
25
20
15
10
5
10
20
30
40
50
60
70
80
90
100
Figure 13.Power vs. Sample Rate in Auto Power-Down
Mode
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