Datasheet

LTC2413
33
sn2413 2413fs
APPLICATIO S I FOR ATIO
WUU
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Figure 39. Input Normal Mode Rejection
Figure 38. Input Normal Mode Rejection
Figure 40. Input Normal Mode Rejection
F
O
= LOW OR
F
O
= EXTERNAL OSCILLATOR,
f
EOSC
= 10 • f
S
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0f
S
INPUT NORMAL MODE REJECTION (dB)
2413 F38
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)
2413 F39
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
INPUT SIGNAL FREQUENCY (Hz)
250f
N
252f
N
254f
N
256f
N
258f
N
260f
N
262f
N
INPUT NORMAL MODE REJECTION (dB)
2413 F40
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
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 LTC2413 significantly
simplifies antialiasing filter requirements.
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
LTC2413’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, f
S
= 256 • f
N
= 2048 •
f
OUTMAX
where f
N
in the notch frequency and f
OUTMAX
is
the maximum output data rate. In the internal oscillator
mode, f
S
= 13980Hz. In the external oscillator mode, f
S
=
f
EOSC
/10.
The combined normal mode rejection performance is
shown in Figure␣ 38. The regions of low rejection occurring
at integer multiples of f
S
have a very narrow bandwidth.
Magnified details of the normal mode rejection curves are
shown in Figure␣ 39 (rejection near DC) and Figure␣ 40
(rejection at f
S
= 256f
N
) where f
N
represents the notch
frequency. These curves have been derived for the exter-
nal oscillator mode but they can be used in all operating
modes by appropriately selecting the f
N
value.