Datasheet

LTC2430/LTC2431
30
24301f
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
/153600. For large values of the
ratio f
EOSC
/153600, the Figure 29 plot accuracy begins to
decrease, but in the same time the LTC2430/LTC2431
noise floor rises and the noise contribution of the driving
amplifiers lose significance.
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 LTC2430/LTC2431 signifi-
cantly 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
LTC2430/LTC2431’s autocalibration circuits further sim-
plify the antialiasing 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
OUTMAX
where f
N
is the notch frequency and f
OUTMAX
is
the maximum output data rate. In the internal oscillator
mode, f
S
= 12,800Hz with a 50Hz notch setting and f
S
=
15,360Hz with a 60Hz notch setting. In the external
oscillator mode, f
S
= f
EOSC
/10.
The combined normal mode rejection performance is
shown in Figure␣ 30 for the internal oscillator with 50Hz
notch setting (F
O
= HIGH) and in Figure␣ 31 for the internal
oscillator with F
O
= LOW and for the external oscillator
mode. 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␣ 32 (rejection near DC) and Figure␣ 33 (rejection at
f
S
= 256f
N
) where f
N
represents the notch frequency.
These curves have been derived for the external oscillator
mode but they can be used in all operating modes by
appropriately selecting the f
N
value.
The user can expect to achieve in practice this level of
performance using the internal oscillator as it is demon-
strated by Figures 34 to 36. Typical measured values of the
normal mode rejection of the LTC2430/LTC2431 operat-
ing with an internal oscillator and a 60Hz notch setting are
shown in Figure 34 superimposed over the theoretical
calculated curve. Similarly, typical measured values of the
normal mode rejection of the LTC2430/LTC2431 operat-
ing with an internal oscillator and a 50Hz notch setting are
shown in Figure 35 superimposed over the theoretical
calculated curve.
As a result of these remarkable normal mode specifica-
tions, minimal (if any) antialias filtering is required in front
of the LTC2430/LTC2431. If passive RC components are
placed in front of the LTC2430/LTC2431, the input dy-
namic current should be considered (see Input Current
section). In cases where large effective RC time constants
are used, an external buffer amplifier may be required to
minimize the effects of dynamic input current.
APPLICATIO S I FOR ATIO
WUUU
Figure 30. Input Normal Mode Rejection,
Internal Oscillator and 50Hz Notch
Figure 31. Input Normal Mode Rejection, Internal
Oscillator and F
O
= LOW or External Oscillator
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0f
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)
2431 F30
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
F
O
= HIGH
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)
2431 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