參數(shù)資料
型號: LTC2411CMS
英文描述: Analog-to-Digital Converter, 24-Bit
中文描述: 模擬到數(shù)字轉換器,24比特
文件頁數(shù): 24/40頁
文件大小: 450K
代理商: LTC2411CMS
LTC2411
30
on the y-axis the noise equivalent bandwidth freqi of the
input driving amplifier. This bandwidth 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
LTC2411 input) can now be obtained by summing as
square root of sum of squares the three ADC input referred
noise sources: the LTC2411 internal noise (1.45
V), the
noise of the IN+ driving amplifier and the noise of the IN
driving amplifier.
If the FO pin is driven by an external oscillator of frequency
fEOSC, Figure 29 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 29 plot accuracy begins to
decrease, but in the same time the LTC2411 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 LTC2411 significantly
simplifies 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
LTC2411’s autocalibration circuits further simplify the
antialiasing requirements by additional normal mode sig-
nal filtering both in the analog and digital domain. Inde-
pendent 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.
The combined normal mode rejection performance is
shown in Figure 30 for the internal oscillator with 50Hz
notch setting (FO = HIGH) and in Figure 31 for the internal
oscillator with 60Hz notch setting (FO = LOW) and for the
external oscillator mode. The regions of low rejection
occurring at integer multiples of fS have a very narrow
bandwidth. Magnified details of the normal mode rejection
curves are shown in Figure 32 (rejection near DC) and
Figure 33 (rejection at fS = 256fN) where fN represents the
notch frequency. These curves have been derived for the
external oscillator mode but they can be used in all
operating modes by appropriately selecting the fN value.
The user can expect to achieve in practice this level of
performance using the internal oscillator as it is demon-
strated by Figures 34 and 35. Typical measured values of
the normal mode rejection of the LTC2411 operating with
an internal oscillator and a 60Hz notch setting are shown
in Figure 34 superimposed over the theoretical calculated
APPLICATIO S I FOR ATIO
WU
UU
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0fS 2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS10fS11fS12fS
INPUT
NORMAL
MODE
REJECTION
(dB)
2411 F30
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)
2411 F31
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS 10fS
Figure 30. Input Normal Mode Rejection,
Internal Oscillator and 50Hz Notch
Figure 31. Input Normal Mode Rejection, Internal
Oscillator and 60Hz Notch or External Oscillator
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