Datasheet

LTC2410
32
APPLICATIO S I FOR ATIO
WUU
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When external amplifiers are driving the LTC2410, the
ADC input referred system noise calculation can be simpli-
fied by Figure 36. The noise of an amplifier driving the
LTC2410 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 spectral density is n
i
.
From Figure␣ 36, 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 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
LTC2410 input) can now be obtained by summing as
square root of sum of squares the three ADC input referred
noise sources: the LTC2410 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 36 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 36 plot accuracy begins to
decrease, but in the same time the LTC2410 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 LTC2410 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
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
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 37. Input Normal Mode Rejection,
Internal Oscillator and 50Hz Notch
Figure 38. Input Normal Mode Rejection, Internal
Oscillator and 60Hz Notch or External Oscillator
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)
2410 F37
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)
2410 F38
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 36. 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
2410 F36
0.1
100
F
O
= HIGH
F
O
= LOW