Datasheet

LTC2411/LTC2411-1
30
internal calibration and filtering. The noise of the driving
amplifier referred to the converter input and including all
these effects can be calculated as N␣ = n
i
• √freq
i
. The total
system noise (referred to the LTC2411/LTC2411-1 input)
can now be obtained by summing as square root of sum
of squares the three ADC input referred noise sources: the
LTC2411/LTC2411-1 internal noise (1.45µV), the noise of
the IN
+
driving amplifier and the noise of the IN
–
driving
amplifier.
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 LTC2411/LTC2411-1
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 LTC2411/LTC2411-1 sig-
nificantly 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
LTC2411/LTC2411-1’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, for the LTC2411, F
S
= 12800Hz with a 50Hz notch
setting and f
S
= 15360Hz with a 60Hz notch setting. For the
LTC2411-1, f
S
= 13980Hz (F
O
= LOW). 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
APPLICATIO S I FOR ATIO
WUUU
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)
2411 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)
2411 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 (LTC2411)
Figure 31. Input Normal Mode Rejection, Internal
Oscillator and F
O
= LOW or External Oscillator
INPUT NOISE SOURCE SINGLE POLE
EQUIVALENT BANDWIDTH (Hz)
1
INPUT REFERRED NOISE
EQUIVALENT BANDWIDTH (Hz)
10
100
1000
10 100 1k 10k 100k 1M
2411 G29
0.1
0.1 1
F
O
= LOW
F
O
= HIGH
Figure 29. Input Referred Noise Equivalent Bandwidth
of an Input Connected White Noise Source