LTC2418_15 Datasheet, PDF (34/48 Pages) Linear Technology – 8-/16-Channel 24-Bit No Latency ADCs

LTC2414/LTC2418
APPLICATIO S I FOR ATIO
The combined normal mode rejection performance is
shown in Figure 33 for the internal oscillator with 50Hz
notch setting (F_O = HIGH) and in Figure 34 for the internal
oscillator with 60Hz notch setting (F_O = LOW) and for the
external oscillator mode. The regions of low rejection
occurring at integer multiples of f_S have a very narrow
bandwidth. Magnified details of the normal mode rejection
curves are shown in Figure 35 (rejection near DC) and
Figure 36 (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 in practice this level of
performance using the internal oscillator as it is demon-
strated by Figures 37 and 38. Typical measured values of
the normal mode rejection of the LTC2414/LTC2418
operating with an internal oscillator and a 60Hz notch
setting are shown in Figure 37 superimposed over the
theoretical calculated curve. Similarly, typical measured
values of the normal mode rejection of the LTC2414/
LTC2418 operating with an internal oscillator and a 50Hz
notch setting are shown in Figure 38 superimposed over
the theoretical calculated curve.
As a result of these remarkable normal mode specifica-
tions, minimal (if any) antialias filtering is required in front
of the LTC2414/LTC2418. If passive RC components are
placed in front of the LTC2414/LTC2418, the input dy-
namic 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 dynamic input current.
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
0
f_N 2f_N 3f_N 4f_N 5f_N 6f_N 7f_N 8f_N
INPUT SIGNAL FREQUENCY (Hz)
2414/18 F35
Figure 35. Input Normal Mode Rejection
0
MEASURED DATA
V_CC = 5V
–20
CALCULATED DATA
REF⁺ = 5V
REF ^– = GND
V_INCM = 2.5V
–40
V_IN(P-P) = 5V
SDI = GND
– 60
F_O = GND
T_A = 25°C
–80
–100
–120
0 15 30 45 60 75 90 105 120 135 150 165 180 195 210 225 240
INPUT FREQUENCY (Hz)
2414/18 F37
Figure 37. Input Normal Mode Rejection vs Input Frequency
with Input Perturbation of 100% Full Scale (60Hz Notch)
34
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
250f_N 252f_N 254f_N 256f_N 258f_N 260f_N 262f_N
INPUT SIGNAL FREQUENCY (Hz)
2414/18 F36
Figure 36. Input Normal Mode Rejection
0
MEASURED DATA
V_CC = 5V
–20
CALCULATED DATA
REF⁺ = 5V
REF ^– = GND
V_INCM = 2.5V
–40
V_IN(P-P) = 5V
SDI = GND
– 60
F_O = 5V
T_A = 25°C
–80
–100
–120
0 12.5 25 37.5 50 62.5 75 87.5 100 112.5 125 137.5 150 162.5 175 187.5 200
INPUT FREQUENCY (Hz)
2414/18 F38
Figure 38. Input Normal Mode Rejection vs Input Frequency
with Input Perturbation of 100% Full Scale (50Hz Notch)
241418fa