Datasheet

LTC2484
34
2484fd
APPLICATIONS INFORMATION
Normal Mode Rejection and Antialiasing
One of the advantages delta-sigma ADCs offer over
conventional ADCs is on-chip digital fi ltering. Combined
with a large oversampling ratio, the LTC2484 signifi cantly
simplifi es antialiasing fi lter requirements. Additionally,
the input current cancellation feature of the LTC2484 al-
lows external lowpass fi ltering without degrading the DC
performance of the device.
The SINC
4
digital fi lter provides greater than 120dB normal
mode rejection at all frequencies except DC and integer
multiples of the modulator sampling frequency (f
S
). The
LTC2484’s autocalibration circuits further simplify the
antialiasing requirements by additional normal mode
signal fi ltering 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
=
f
EOSC
/20. The performance of the normal mode rejection
is shown in Figures 30 and 31.
In 1x speed mode, the regions of low rejection occurring
at integer multiples of f
S
have a very narrow bandwidth.
Magnifi ed 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 exter-
nal oscillator mode but they can be used in all operating
modes by appropriately selecting the f
N
value.
The user can expect to achieve this level of performance
using the internal oscillator as it is demonstrated by
Figures 34, 35 and 36. Typical measured values of the
normal mode rejection of the LTC2484 operating with an
internal oscillator and a 60Hz notch setting are shown in
Figure 34 superimposed over the theoretical calculated
curve. Similarly, the measured normal mode rejection of
the LTC2484 for the 50Hz rejection mode and 50Hz/60Hz
rejection mode are shown in Figures 35 and 36.
As a result of these remarkable normal mode specifi ca-
tions, minimal (if any) antialias fi ltering is required in front
of the LTC2484. If passive RC components are placed in
front of the LTC2484, the input dynamic current should
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)
2484 F30
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0f
S
INPUT NORMAL MODE REJECTION (dB)
2484 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
Figure 30. Input Normal Mode Rejection,
Internal Oscillator and 50Hz Notch Mode
Figure 31. Input Normal Mode Rejection, Internal
Oscillator and 60Hz Notch Mode of External Oscillator