LTC2480
2480fd
applicaTions inForMaTion
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/307200. For large values of
the ratio fEOSC/307200, the Figure 29 plot accuracy begins
to decrease, but at the same time the LTC2480 noise floor
rises and the noise contribution of the driving amplifiers
lose significance.
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 LTC2480 significantly
simplifies anti-aliasing filter requirements. Additionally,
the input current cancellation feature of the LTC2480 al-
lows external lowpass filtering without degrading the DC
performance of the device.
TheSINC4digitalfilterprovidesgreaterthan120dBnormal
mode rejection at all frequencies except DC and integer
multiples of the modulator sampling frequency (fS). The
LTC2480’s autocalibration circuits further simplify the
anti-aliasing requirements by additional normal mode
signal filtering both in the analog and digital domain.
Independent of the operating mode, fS = 256 fN = 2048
fOUT(MAX) where fN is the notch frequency and fOUT(MAX)
is the maximum output data rate. In the internal oscilla-
tor mode with a 50Hz notch setting, fS = 12800Hz, with
50Hz/60Hz rejection, fS = 13960Hz and with a 60Hz notch
setting fS = 15360Hz. In the external oscillator mode, fS =
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0 fS 2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS10fS11fS12fS
INPUT
NORMAL
MODE
REJECTION
(dB)
2480 F30
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0
fS
INPUT
NORMAL
MODE
REJECTION
(dB)
2480 F31
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS 10fS
INPUT SIGNAL FREQUENCY (Hz)
INPUT
NORMAL
MODE
REJECTION
(dB)
2480 F32
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
fN
0
2fN 3fN 4fN 5fN 6fN 7fN 8fN
fN = fEOSC/5120
INPUT SIGNAL FREQUENCY (Hz)
250fN 252fN 254fN 256fN 258fN 260fN 262fN
INPUT
NORMAL
MODE
REJECTION
(dB)
2480 F33
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
Figure 30. Input Normal Mode Rejection,
Internal Oscillator and 50Hz Notch Mode
Figure 31. Input Normal Mode Rejection, Internal
Oscillator and 60Hz Notch Mode or External Oscillator
Figure 32. Input Normal Mode Rejection at DC
Figure 33. Input Normal Mode Rejection at fS = 256fN