Datasheet

25
LTC2400
APPLICATIONS INFORMATION
WUU
U
INPUT FREQUENCY
0
–60
–40
0
2400 F26
–80
–100
f
S
/2 f
S
–120
–140
–20
REJECTION (dB)
Figure 26. Sinc
4
Filter Rejection
RESISTANCE AT V
REF
(Ω)
0
–20
INL ERROR (ppm)
0
40
60
80
400
800
1000
160
2400 F25
20
200 600
100
120
140
C
VREF
= 0.01µF
C
VREF
= 0.1µF
C
VREF
= 1µF
C
VREF
= 10µF
V
CC
= 5V
V
REF
= 5V
T
A
= 25°C
Figure 25. INL Error vs R
VREF
(Large C)
In addition to the dynamic reference current, the V
REF
ESD
protection diodes have a temperature dependent leakage
current. This leakage current, nominally 1nA (±10nA max),
results in a fixed full-scale shift of 10µV for a 10k source
resistance.
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 LTC2400 significantly
simplifies antialiasing filter requirements.
The digital filter provides very high rejection except at
integer multiples of the modulator sampling frequency
(f
S
), see Figure 26. The modulator sampling frequency is
256 • F
O
, where F
O
is the notch frequency (typically 50Hz
or 60Hz). The bandwidth of signals not rejected by the
digital filter is narrow (≈0.2%) compared to the bandwidth
of the frequencies rejected.
As a result of the oversampling ratio (256) and the digital
filter, minimal (if any) antialias filtering is required in front
of the LTC2400. If passive RC components are placed in
front of the LTC2400 the input dynamic current should be
considered (see Input Current section). In cases where
large effective RC time constants are used, an external
buffer amplifier may be required to minimize the effects of
input dynamic current.
The modulator contained within the LTC2400 can handle
large-signal level perturbations without saturating. Signal
levels up to 40% of V
REF
do not saturate the analog modu-
lator. These signals are limited by the input ESD protection
to 300mV below ground and 300mV above V
CC
.