Datasheet

LTC2481
29
2481fc
APPLICATIONS INFORMATION
high bandwidth (at least 500kHz) necessary to drive the
input switched-capacitor network. A possible solution is
a high gain, low bandwidth amplifi er stage followed by a
high bandwidth unity-gain buffer.
When external amplifi ers are driving the LTC2481, the
ADC input referred system noise calculation can be
simplifi ed by Figure 30. The noise of an amplifi er driving
the LTC2481 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 fi lter with a
corner frequency f
i
. The amplifi er noise spectral density
is n
i
. From Figure 30, using f
i
as the x-axis selector, we
can fi nd on the y-axis the noise equivalent bandwidth freq
i
of the input driving amplifi er. This bandwidth includes
the band limiting effects of the ADC internal calibration
and fi ltering. The noise of the driving amplifi er 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 LTC2481 input) can now be obtained by
summing as square root of sum of squares the three
ADC input referred noise sources: the LTC2481 internal
noise, the noise of the IN
+
driving amplifi er and the noise
of the IN
driving amplifi er.
If the CA0/f
0
pin is driven by an external oscillator of
frequency f
EOSC
, Figure 30 can still be used for noise
calculation if the x-axis is scaled by f
EOSC
/307200.
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)
2481 F31
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
For large values of the ratio f
EOSC
/307200, the Figure 30
plot accuracy begins to decrease, but at the same time
the LTC2481 noise fl oor rises and the noise contribution
of the driving amplifi ers lose signifi cance.
Normal Mode Rejection and Antialiasing
One of the advantages delta-sigma ADCs offer over
conventional ADCs is on-chip digital fi ltering. Combined
with a large oversampling ratio, the LTC2481 signifi cantly
simplifi es antialiasing lter requirements. Additionally,
the input current cancellation feature of the LTC2481
allows external lowpass fi ltering without degrading the
DC performance of the device.
The SINC
4
digital fi lter provides greater than 120dB normal
mode rejection at all frequencies except DC and integer
multiples of the modulator sampling frequency (f
S
). The
LTC2481’s autocalibration circuits further simplify the
antialiasing requirements by additional normal mode
signal fi ltering 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 oscilla-
tor mode with a 50Hz notch setting, f
S
= 12800Hz, with
50Hz/60Hz rejection, f
S
= 13960Hz and with a 60Hz notch
setting f
S
= 15360Hz. In the external oscillator mode, f
S
=
f
EOSC
/20. The performance of the normal mode rejection
is shown in Figures 31 and 32.
Figure 31. Input Normal Mode Rejection,
Internal Oscillator and 50Hz Notch Mode
Figure 32. Input Normal Mode Rejection at DC
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0f
S
INPUT NORMAL MODE REJECTION (dB)
2481 F32
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