参数资料
型号: DC571A
厂商: Linear Technology
文件页数: 27/48页
文件大小: 0K
描述: BOARD DELTA SIGMA ADC LTC2418
软件下载: QuikEval System
设计资源: DC571A Design File
DC571A Schematic
标准包装: 1
系列: QuikEval™
ADC 的数量: 1
位数: 24
采样率(每秒): 7.5
数据接口: MICROWIRE?,串行,SPI?
工作温度: 0°C ~ 70°C
已用 IC / 零件: LTC2418
已供物品:
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LTC2414/LTC2418
33
241418fa
APPLICATIO S I FOR ATIO
WU
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When external amplifiers are driving the LTC2414/
LTC2418, the ADC input referred system noise calculation
can be simplified by Figure 32. The noise of an amplifier
driving the LTC2414/LTC2418 input pin can be modeled
as a band limited white noise source. Its bandwidth can be
approximated by the bandwidth of a single pole lowpass
filter with a corner frequency fi. The amplifier noise spec-
tral density is ni. From Figure 32, using fi as the x-axis
selector, we can find on the y-axis the noise equivalent
bandwidth freqi of the input driving amplifier. This band-
width includes the band limiting effects of the ADC internal
calibration and filtering. The noise of the driving amplifier
referred to the converter input and including all these
effects can be calculated as N = ni √freqi. The total system
noise (referred to the LTC2414/LTC2418 input) can now
be obtained by summing as square root of sum of squares
the three ADC input referred noise sources: the LTC2414/
LTC2418 internal noise (1
V), the noise of the IN+ driving
amplifier and the noise of the INdriving amplifier.
If the FO pin is driven by an external oscillator of frequency
fEOSC, Figure 32 can still be used for noise calculation if the
x-axis is scaled by fEOSC/153600. For large values of the
ratio fEOSC/153600, the Figure 32 plot accuracy begins to
decrease, but in the same time the LTC2414/LTC2418
noise floor rises and the noise contribution of the driving
amplifiers lose significance.
Normal Mode Rejection and Antialiasing
One of the advantages delta-sigma ADCs offer over con-
ventional ADCs is on-chip digital filtering. Combined with
a large oversampling ratio, the LTC2414/LTC2418 signifi-
cantly simplify antialiasing filter requirements.
The Sinc4 digital filter provides greater than 120dB normal
mode rejection at all frequencies except DC and integer
multiples of the modulator sampling frequency (fS). The
LTC2414/LTC2418’s autocalibration circuits further sim-
plify the antialiasing requirements by additional normal
mode signal filtering both in the analog and digital domain.
Independent of the operating mode, fS = 256 fN = 2048
fOUTMAX where fN in the notch frequency and fOUTMAX is
the maximum output data rate. In the internal oscillator
mode with a 50Hz notch setting, fS = 12800Hz and with a
60Hz notch setting fS = 15360Hz. In the external oscillator
mode, fS = fEOSC/10.
Figure 33. Input Normal Mode Rejection,
Internal Oscillator and 50Hz Notch
Figure 34. Input Normal Mode Rejection, Internal
Oscillator and 60Hz Notch or External Oscillator
Figure 32. Input Referred Noise Equivalent Bandwidth
of an Input Connected White Noise Source
INPUT NOISE SOURCE SINGLE POLE
EQUIVALENT BANDWIDTH (Hz)
1
INPUT
REFERRED
NOISE
EQUIVALENT
BANDWIDTH
(Hz)
10
0.1
1
10
100
1k
10k
100k
1M
2414/18 F32
0.1
100
FO = HIGH
FO = LOW
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0fS 2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS10fS11fS12fS
INPUT
NORMAL
MODE
REJECTION
(dB)
2414/18 F33
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
FO = HIGH
FO = LOW OR
FO = EXTERNAL
OSCILLATOR,
fEOSC = 10 fS
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0fS
INPUT
NORMAL
MODE
REJECTION
(dB)
2414/18 F34
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS 10fS
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