LTC2480
2480fd
applicaTions inForMaTion
fEOSC/20. The performance of the normal mode rejection
is shown in Figures 30 and 31.
In 1x speed mode, the regions of low rejection occurring
at integer multiples of fS have a very narrow bandwidth.
Magnified details of the normal mode rejection curves
are shown in Figure 32 (rejection near DC) and Figure 33
(rejection at fS = 256fN) where fN represents the notch
frequency. These curves have been derived for the exter-
nal oscillator mode but they can be used in all operating
modes by appropriately selecting the fN value.
The user can expect to achieve this level of performance
using the internal oscillator as it is demonstrated by
Figures 34, 35 and 36. Typical measured values of the
normal mode rejection of the LTC2480 operating with an
internal oscillator and a 60Hz notch setting are shown in
Figure 34 superimposed over the theoretical calculated
curve. Similarly, the measured normal mode rejection of
the LTC2480 for the 50Hz rejection mode and 50Hz/60Hz
rejection mode are shown in Figures 35 and 36.
As a result of these remarkable normal mode specifica-
tions,minimal(ifany)anti-aliasfilteringisrequiredinfront
of the LTC2480. If passive RC components are placed in
front of the LTC2480, the input dynamic current should
be considered (see Input Current section). In this case,
the differential input current cancellation feature of the
LTC2480 allows external RC networks without significant
degradation in DC performance.
Traditional high order delta-sigma modulators, while pro-
vidingverygoodlinearityandresolution,sufferfrompoten-
tial instabilities at large input signal levels. The proprietary
architecture used for the LTC2480 third order modulator
resolves this problem and guarantees a predictable stable
behavior at input signal levels of up to 150% of full-scale.
Inmanyindustrialapplications,itisnotuncommontohave
INPUT FREQUENCY (Hz)
0
15
30
45
60
75
90 105 120 135 150 165 180 195 210 225 240
NORMAL
MODE
REJECTION
(dB)
2480 F34
0
–20
–40
–60
–80
–100
–120
VCC = 5V
VREF = 5V
VIN(CM) = 2.5V
VIN(P-P) = 5V
TA = 25°C
MEASURED DATA
CALCULATED DATA
INPUT FREQUENCY (Hz)
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
NORMAL
MODE
REJECTION
(dB)
2480 F35
0
–20
–40
–60
–80
–100
–120
VCC = 5V
VREF = 5V
VIN(CM) = 2.5V
VIN(P-P) = 5V
TA = 25°C
MEASURED DATA
CALCULATED DATA
INPUT FREQUENCY (Hz)
0
20
40
60
80
100
120
140
160
180
200
220
NORMAL
MODE
REJECTION
(dB)
2480 F36
0
–20
–40
–60
–80
–100
–120
VCC = 5V
VREF = 5V
VIN(CM) = 2.5V
VIN(P-P) = 5V
TA = 25°C
MEASURED DATA
CALCULATED DATA
Figure 34. Input Normal Mode Rejection vs Input Frequency
with Input Perturbation of 100% Full-Scale (60Hz Notch)
Figure 35. Input Normal Mode Rejection vs Input Frequency
with Input Perturbation of 100% Full-Scale (50Hz Notch)
Figure 36. Input Normal Mode Rejection vs Input Frequency
with Input Perturbation of 100% Full-Scale (50Hz/60Hz Mode)
INPUT FREQUENCY (Hz)
0
15
30
45
60
75
90 105 120 135 150 165 180 195 210 225 240
NORMAL
MODE
REJECTION
(dB)
2480 F37
0
–20
–40
–60
–80
–100
–120
VCC = 5V
VREF = 5V
VIN(CM) = 2.5V
TA = 25°C
VIN(P-P) = 5V
VIN(P-P) = 7.5V
(150% OF FULL SCALE)
Figure 37. Measured Input Normal Mode Rejection
vs Input Frequency with Input Perturbation of 150%
Full-Scale (60Hz Notch)