Datasheet

LTC2412
31
2412f
APPLICATIO S I FOR ATIO
WUU
U
input driving amplifier. This bandwidth includes the band
limiting effects of the ADC 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
LTC2412 input) can now be obtained by summing as
square root of sum of squares the three ADC input referred
noise sources: the LTC2412 internal noise (800nV), 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 32 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 32 plot accuracy begins to
decrease, but in the same time the LTC2412 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 LTC2412 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
LTC2412’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 with a 50Hz notch setting, f
S
= 12800Hz and with a
60Hz notch setting f
S
= 15360Hz. In the external oscillator
mode, f
S
= f
EOSC
/10.
The combined normal mode rejection performance is
shown in Figure␣ 33 for the internal oscillator with 50Hz
notch setting (F
O
= HIGH) and in Figure␣ 34 for the internal
oscillator with 60Hz notch setting (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␣ 35 (rejection near DC) and
Figure 33. Input Normal Mode Rejection,
Internal Oscillator and 50Hz Notch
Figure 34. Input Normal Mode Rejection, Internal
Oscillator and 60Hz Notch or External Oscillator
Figure 32. Input Referred Noise Equivalent Bandwidth
of an Input Connected White Noise Source
INPUT NOISE SOURCE SINGLE POLE
EQUIVALENT BANDWIDTH (Hz)
1
INPUT REFERRED NOISE
EQUIVALENT BANDWIDTH (Hz)
10
0.1 1 10 100 1k 10k 100k 1M
2412 F32
0.1
100
F
O
= HIGH
F
O
= LOW
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)
2412 F33
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)
2412 F34
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