Datasheet

LTC2484
36
2484fd
APPLICATIONS INFORMATION
be considered (see Input Current section). In this case,
the differential input current cancellation feature of the
LTC2484 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 LTC2484 third
order modulator resolves this problem and guarantees
a predictable stable behavior at input signal levels of up
to 150% of full-scale. In many industrial applications,
it is not uncommon to have to measure microvolt level
signals superimposed over volt level perturbations and
the LTC2484 is eminently suited for such tasks. When the
perturbation is differential, the specifi cation of interest is
the normal mode rejection for large input signal levels. With
a reference voltage V
REF
= 5V, the LTC2484 has a full-scale
differential input range of 5V peak-to-peak. Figures 37 and
38 show measurement results for the LTC2484 normal
mode rejection ratio with a 7.5V peak-to-peak (150% of
full-scale) input signal superimposed over the more tradi-
tional normal mode rejection ratio results obtained with a
5V peak-to-peak (full-scale) input signal. In Figure 37, the
LTC2484 uses the internal oscillator with the notch set
at 60Hz (f
O
= LOW) and in Figure 38 it uses the internal
oscillator with the notch set at 50Hz. It is clear that the
LTC2484 rejection performance is maintained with no
compromises in this extreme situation. When operating
with large input signal levels, the user must observe that
such signals do not violate the device absolute maximum
ratings.
INPUT FREQUENCY (Hz)
0 15 30 45 60 75 90 105 120 135 150 165 180 195 210
225 240
NORMAL MODE REJECTION (dB)
2484 F37
0
–20
–40
–60
–80
–100
–120
V
CC
= 5V
V
REF
= 5V
V
INCM
= 2.5V
T
A
= 25°C
V
IN(P-P)
= 5V
V
IN(P-P)
= 7.5V
(150% OF FULL SCALE)
INPUT FREQUENCY (Hz)
0
NORMAL MODE REJECTION (dB)
2484 F38
0
–20
–40
–60
–80
–100
–120
V
CC
= 5V
V
REF
= 5V
V
IN(CM)
= 2.5V
T
A
= 25°C
V
IN(P-P)
= 5V
V
IN(P-P)
= 7.5V
(150% OF FULL SCALE)
12.5 25 37.5 50 62.5 75 87.5 100 112.5 125 137.5 150 162.5 175 187.5 200
Figure 37. Measured Input Normal Mode Rejection vs Input Freq-
uency with Input Perturbation of 150% Full-Scale (60Hz Notch)
Figure 38. Measured Input Normal Mode Rejection vs Input Freq-
uency with Input Perturbation of 150% Full-Scale (50Hz Notch)