Datasheet

LTC2414/LTC2418
33
241418fa
APPLICATIO S I FOR ATIO
WUUU
When external amplifiers are driving the LTC2414/
LTC2418, the ADC input referred system noise calculation
can be simplified by Figure 32. The noise of an amplifier
driving the LTC2414/LTC2418 input pin can be modeled
as a band limited white noise source. Its bandwidth can be
approximated by the bandwidth of a single pole lowpass
filter with a corner frequency f
i
. The amplifier noise spec-
tral density is n
i
. From Figure 32, using f
i
as the x-axis
selector, we can find on the y-axis the noise equivalent
bandwidth freq
i
of the input driving amplifier. This band-
width 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 LTC2414/LTC2418 input) can now
be obtained by summing as square root of sum of squares
the three ADC input referred noise sources: the LTC2414/
LTC2418 internal noise (1µV), 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 LTC2414/LTC2418
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 LTC2414/LTC2418 signifi-
cantly simplify 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
LTC2414/LTC2418’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
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.
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
2414/18 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)
2414/18 F33
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
100
110
120
F
O
= HIGH