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参数资料
| 型号： | LT2435IGN#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | ADC |
| 英文描述： | 1-CH 20-BIT DELTA-SIGMA ADC, SERIAL ACCESS, PDSO16 |
| 封装： | 0.150 INCH, LEAD FREE, PLASTIC, SSOP-16 |
| 文件页数： | 26/40页 |
| 文件大小： | 458K |
| 代理商： | LT2435IGN#PBF |
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LTC2435/LTC2435-1
32
24351fb
Figure 34a. Input Normal Mode Rejection,
Internal Oscillator and 50Hz Notch (LTC2435)
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 LTC2435/LTC2435-1 sig-
nificantly 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). Inde-
pendent of the operating mode, fS = 256 fN = 1024
fOUTMAX where fN is the notch frequency and fOUTMAX is the
maximum output data rate. In the internal oscillator mode,
for the LTC2435, FS = 12800Hz with a 50Hz notch setting
and fS = 15360Hz with a 60Hz notch setting. For the
LTC2435-1, fS = 13980Hz (FO = LOW). In the external
oscillator mode, fS = fEOSC/10.
The normal mode rejection performance is shown in
Figure 34. 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 35 (rejection near DC) and Figure 36
(rejection at fS = 256fN) where fN represents the notch
frequency. For the LTC2435, the bandwidth is 13.6Hz
(FO = GND) and 11.4Hz (FO = VCC). The Bandwidth is
12.4Hz for the LTC2435-1 (FO = GND).
Through FO connection, the LTC2435 provides better than
110dB input differential mode rejection at 50Hz or 60Hz
±2%. While for the LTC2435-1, it has a notch frequency of
about 55Hz with better than 70db rejection over 48Hz to
62.4Hz, which covers both 50Hz
±2% and 60Hz ±2%. In
order to achieve better rejection over the range of 48Hz to
62.4Hz, a running average can be performed. By averaging
two consecutive LTC2435-1 readings, a sinc1 notch is
combined with the sinc4 digital filter, yielding the fre-
quency response shown in Figure 37. The averaging
operation still keeps the output rate with the following
algorithm:
Result 1 = average (sample 0, sample 1)
Result 2 = average (sample 1, sample 2)
Result n = average (sample n-1, sample n)
The user can expect to achieve in practice this level of
performance using the internal oscillator as it is demon-
strated by Figures 38 to 40. Typical measured values of the
normal mode rejection of the LTC2435-1 operating with an
internal oscillator and a 54.6Hz notch setting are shown in
Figure 38 and 39 superimposed over the theoretical calcu-
lated curve. The same normal mode rejection perfor-
mance is obtained for the LTC2435 with the frequency
scaled to have the notch frequency at 60Hz (FO = GND) or
50Hz (FO = VCC).
APPLICATIO S I FOR ATIO
WU
U
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0fS 2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS10fS11fS12fS
INPUT
NORMAL
MODE
REJECTION
(dB)
2435 F34a
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
FO = HIGH
INPUT FREQUENCY
0
–60
–40
0
2435 F34b
–80
–100
fS/2
fS
–120
–140
–20
REJECTION
(dB)
Figure 34b. Input Normal Mode Rejection, Internal
Oscillator and FO = Low or External Oscillator
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