Datasheet

LTC2482
28
2482fc
APPLICATIONS INFORMATION
(referred to the LTC2482 input) can now be obtained by
summing as square root of sum of squares the three ADC
input referred noise sources: the LTC2482 internal noise,
the noise of the IN+ driving amplifi er and the noise of the
IN– driving amplifi er.
If the f
O
pin is driven by an external oscillator of frequency
f
EOSC
, Figure 26 can still be used for noise calculation if
the x-axis is scaled by f
EOSC
/307200. For large values of
the ratio f
EOSC
/307200, the Figure 26 plot accuracy begins
to decrease, but at the same time the LTC2482 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 LTC2482 signifi cantly
simplifi es antialiasing fi lter requirements. Additionally,
the input current cancellation feature of the LTC2482 al-
lows 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
LTC2482’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 50Hz/60Hz rejection, f
S
= 13960Hz. In the
external oscillator mode, f
S
= f
EOSC
/20.
The regions of low rejection occurring at integer multiples
of f
S
have a very narrow bandwidth. Magnifi ed details of
the normal mode rejection curves are shown in Figure 27
(rejection near DC) and Figure 28 (rejection at f
S
= 256f
N
)
where f
N
represents the notch frequency. These curves
have been derived for the external oscillator mode but
they can be used in all operating modes by appropriately
selecting the f
N
value.
The user can expect to achieve this level of performance
using the internal oscillator as it is demonstrated by
Figure 29. Typical measured values of the normal mode
rejection of the LTC2482 operating with an internal oscil-
lator (50Hz/60Hz rejection) is shown in Figure 29.
As a result of these remarkable normal mode specifi ca-
tions, minimal (if any) antialias fi ltering is required in front
of the LTC2482. If passive RC components are placed in
front of the LTC2482, the input dynamic current should
be considered (see Input Current section). In this case,
the differential input current cancellation feature of the
LTC2482 allows external RC networks without signifi cant
degradation in DC performance.
Traditional high order delta-sigma modulators, while
providing very good linearity and resolution, suffer
from potential instabilities at large input signal levels.
The proprietary architecture used for the LTC2482 third
INPUT SIGNAL FREQUENCY (Hz)
INPUT NORMAL MODE REJECTION (dB)
2482 F27
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
f
N
0 2f
N
3f
N
4f
N
5f
N
6f
N
7f
N
8f
N
f
N
= f
EOSC
/5120
INPUT SIGNAL FREQUENCY (Hz)
250f
N
252f
N
254f
N
256f
N
258f
N
260f
N
262f
N
INPUT NORMAL MODE REJECTION (dB)
2482 F28
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
Figure 27. Input Normal Mode Rejection at DC Figure 28. Input Normal Mode Rejection at f
S
= 256f
N