
LTC2435/LTC2435-1
34
24351fc
applicaTions inForMaTion
Normal Mode Rejection and Anti-Aliasing
One of the advantages delta-sigma ADCs offer over
conventional ADCs is on-chip digital filtering. Combined
with a large oversampling ratio, the LTC2435/LTC2435-1
significantly simplifies anti-aliasing filter requirements.
TheSINC4digitalfilterprovidesgreaterthan120dBnormal
mode rejection at all frequencies except DC and integer
multiples of the modulator sampling frequency (fS). In-
dependent 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 Fig-
ure 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).
Figure 34a. Input Normal Mode Rejection,
Internal Oscillator and 50Hz Notch (LTC2435)
Figure 34b. Input Normal Mode Rejection, Internal
Oscillator and FO = Low or External Oscillator
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0 fS 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 35. Input Normal Mode Rejection
INPUT SIGNAL FREQUENCY (fN)
INPUT
NORMAL
REJECTION
(dB)
2435 F35
0
–20
–40
–60
–80
–100
–120
0
fN 2fN 3fN 4fN 5fN 6fN 7fN 8fN
Figure 36. Input Normal Mode Rejection
INPUT SIGNAL FREQUENCY (fN)
INPUT
NORMAL
REJECTION
(dB)
2435 F36
0
–20
–40
–60
–80
–100
–120
250
248
252 254 256 258 260 262 264