Datasheet

26
LTC2420
APPLICATIO S I FOR ATIO
WUU
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As a result of the oversampling ratio (256) and the digital
filter, minimal (if any) antialias filtering is required in front
of the LTC2420. If passive RC components are placed in
front of the LTC2420 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 LTC2420 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
.
Operation at Higher Data Output Rates
The LTC2420 typically operates with an internal oscillator
of 153.6kHz. This corresponds to a notch frequency of
60Hz and an output rate of 7.5 samples/second. The
internal oscillator is enabled if the F
O
pin is logic LOW
(logic HIGH for a 50Hz notch). It is possible to drive the F
O
pin with an external oscillator for higher data output rates.
As shown in Figure 28, an external clock of 2.048MHz
applied to the F
O
pin results in a notch frequency of 800Hz
with a data output rate of 100 samples/second.
Figure 29 shows the total unadjusted error (Offset Error +
Full-Scale Error + INL + DNL) as a function of the output
data rate with a 5V reference. The relationship between the
output data rate (ODR) and the frequency applied to the F
O
pin (F
O
) is:
ODR = F
O
/20480
For output data rates up to 50 samples/second, the total
unadjusted error (TUE) is better than 16 bits, and better
than 12 bits at 100 samples/second. As shown in Figure
30, for output data rates of 100 samples/second, the TUE
is better than 15 bits for V
REF
below 2.5V. Figure 31 shows
an unaveraged total unadjusted error for the LTC2420 op-
erating at 100 samples/second with V
REF
= 2.5V. Figure 32
shows the same device operating with a 5V reference and
an output data rate of 7.5 samples/second.
At 100 samples/second, the LTC2420 can be used to
capture transient data. This is useful for monitoring set-
tling or auto gain ranging in a system. The LTC2420 can
monitor signals at an output rate of 100 samples/second.
INPUT FREQUENCY
0
–60
–40
0
2420 F27
–80
–100
f
S
/2 f
S
–120
–140
–20
REJECTION (dB)
Figure 27. Sinc
4
Filter Rejection
F
O
SCK
SDO
CS
V
CC
V
REF
V
IN
GND
8
7
6
5
1
2
3
4
LTC2420
800Hz NOTCH (100 SAMPLES/SECOND)
60Hz NOTCH (7.5 SAMPLES/SECOND)
EXTERNAL 2.048MHz CLOCK SOURCE
INTERNAL 153.6kHz OSCILLATOR
2420 F28
Figure 28. Selectable 100 Samples/Second Turbo Mode
OUTPUT RATE (SAMPLES/SEC)
0
TOTAL UNADJUSTED ERROR (ppm)
96
128
160
12 BITS
13 BITS
14 BITS
16 BITS
2420 F29
64
32
0
50 100
192
224
256
V
REF
= 5V
150
Figure 29. Total Error vs Output Rate (V
REF
= 5V)