LTC2410 Linear Technology, LTC2410 Datasheet - Page 28

• Manufacturer Part Number: LTC2410
• Description: 24-Bit No Latency ADC with Differential Input and Differential Reference
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC2410
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 LTC2410 significantly
simplifies antialiasing filter requirements.
The Sinc
mode rejection at all frequencies except DC and integer
multiples of the modulator sampling frequency (f
LTC2410’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
f
the maximum output data rate. In the internal oscillator
mode with a 50Hz notch setting, f
60Hz notch setting f
mode, f
The combined normal mode rejection performance is
shown in Figure 36 for the internal oscillator with 50Hz
notch setting (F
oscillator with 60Hz notch setting (F
external oscillator mode. The regions of low rejection
occurring at integer multiples of f
bandwidth. Magnified details of the normal mode rejection
curves are shown in Figure 38 (rejection near DC) and
Figure 39 (rejection at f
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
The user can expect to achieve in practice this level of
performance using the internal oscillator as it is demon-
strated by Figures 40 and 41. Typical measured values of
the normal mode rejection of the LTC2410 operating with
an internal oscillator and a 60Hz notch setting are shown
in Figure 40 superimposed over the theoretical calculated
curve. Similarly, typical measured values of the normal
mode rejection of the LTC2410 operating with an internal
oscillator and a 50Hz notch setting are shown in Figure 41
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 LTC2410. If passive RC components are placed in
28
OUTMAX
S
4
where f
= f
digital filter provides greater than 120dB normal
EOSC
O
N
/10.
= HIGH) and in Figure 37 for the internal
in the notch frequency and f
U
S
= 15360Hz. In the external oscillator
S
= 256f
U
N
) where f
S
S
S
= 12800Hz and with a
W
O
= 256 • f
have a very narrow
= LOW) and for the
N
represents the
N
OUTMAX
U
= 2048 •
N
S
value.
). The
is
Figure 35. Input Signal Bandwidth Using the Internal Oscillator
–100
–110
–120
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
0
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
Figure 37. Input Normal Mode Rejection, Internal
Oscillator and 60Hz Notch or External Oscillator
f
S
Figure 36. Input Normal Mode Rejection,
Internal Oscillator and 50Hz Notch
–100
–110
–120
2f
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
–3.5
–4.0
–4.5
–5.0
–5.5
–6.0
–10
–20
–30
–40
–50
–60
–70
–80
–90
S
0.0
0
3f
0 f
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
S
4f
S
S
F
O
2f
5f
= HIGH
S
S
3f
F
6f
O
S
S
= HIGH
4f
S
7f
5f
S
S
8f
6f
S
S
7f
9f
2410 F37
S
S
F
8f
10f
O
S
= LOW
S
9f
F
F
f
EOSC
S
O
O
10f
= LOW OR
= EXTERNAL OSCILLATOR,
S
= 10 • f
11f
2410 F36
2410 F35
S
12f
S
S