Datasheet
LTC2480
35
2480fd
applicaTions inForMaTion
If the f
O
pin is driven by an external oscillator of frequency
f
EOSC
, Figure 29 can still be used for noise calculation if
the x-axis is scaled by f
EOSC
/307200. For large values of
the ratio f
EOSC
/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.
The SINC
4
digital filter provides greater than 120dB normal
mode rejection at all frequencies except DC and integer
multiples of the modulator sampling frequency (f
S
). 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, f
S
= 256 • f
N
= 2048
• f
OUT(MAX)
where f
N
is the notch frequency and f
OUT(MAX)
is the maximum output data rate. In the internal oscilla-
tor mode with a 50Hz notch setting, f
S
= 12800Hz, with
50Hz/60Hz rejection, f
S
= 13960Hz and with a 60Hz notch
setting f
S
= 15360Hz. In the external oscillator mode, f
S
=
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0 f
S
2f
S
3f
S
4f
S
5f
S
6f
S
7f
S
8f
S
9f
S
10f
S
11f
S
12f
S
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 f
S
INPUT NORMAL MODE REJECTION (dB)
2480 F31
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
2f
S
3f
S
4f
S
5f
S
6f
S
7f
S
8f
S
9f
S
10f
S
INPUT SIGNAL FREQUENCY (Hz)
INPUT NORMAL MODE REJECTION (dB)
2480 F32
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
f
N
0 2f
N
3f
N
4f
N
5f
N
6f
N
7f
N
8f
N
f
N
= f
EOSC/5120
INPUT SIGNAL FREQUENCY (Hz)
250f
N
252f
N
254f
N
256f
N
258f
N
260f
N
262f
N
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 f
S
= 256f
N