ltc2484 Linear Technology Corporation, ltc2484 Datasheet - Page 35

• Manufacturer Part Number: ltc2484
• Description: 24-bit Delta Sigma Adc With Easy Drive Input Current Cancellation
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
superimposed over the theoretical calculated curve. Simi-
larly, the measured normal mode rejection of the LTC2484
for the 50Hz rejection mode and 50Hz/60Hz rejection mode
are shown in Figures 35 and 36.
Figure 36. Input Normal Mode Rejection vs Input Frequency
with Input Perturbation of 100% Full Scale (50Hz/60Hz Mode)
–100
–120
–100
–120
–100
–120
–20
–40
– 60
–80
– 60
Figure 34. Input Normal Mode Rejection vs Input Frequency
with Input Perturbation of 100% Full Scale (60Hz Notch)
–20
–40
–80
Figure 35. Input Normal Mode Rejection vs Input Frequency
with Input Perturbation of 100% Full Scale (50Hz Notch)
–20
–40
– 60
–80
0
0
0
0
0
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
15
20
30
40
45
60
60
U
75
80
90
INPUT FREQUENCY (Hz)
INPUT FREQUENCY (Hz)
INPUT FREQUENCY (Hz)
U
105 120 135 150 165 180 195 210 225 240
100
120
MEASURED DATA
CALCULATED DATA
MEASURED DATA
CALCULATED DATA
MEASURED DATA
CALCULATED DATA
W
140
160
180
U
V
V
V
V
T
V
V
V
V
T
V
V
V
V
T
CC
REF
IN(CM)
IN(P-P)
A
CC
REF
IN(CM)
IN(P-P)
A
CC
REF
IN(CM)
IN(P-P)
A
= 25°C
= 25°C
= 25°C
= 5V
= 5V
= 5V
200
= 5V
= 5V
= 5V
= 2.5V
= 5V
= 2.5V
= 5V
= 2.5V
= 5V
2484 F36
2484 F34
2484 F35
220
As a result of these remarkable normal mode specifica-
tions, minimal (if any) antialias filtering is required in front
of the LTC2484. If passive RC components are placed in
front of the LTC2484, the input dynamic current should be
considered (see Input Current section). In this case, the
differential input current cancellation feature of the LTC2484
allows external RC networks without significant degrada-
tion in DC performance.
Traditional high order delta-sigma modulators, while pro-
viding very good linearity and resolution, suffer from po-
tential instabilities at large input signal levels. The
proprietary architecture used for the LTC2484 third order
modulator resolves this problem and guarantees a pre-
dictable stable behavior at input signal levels of up to 150%
of full scale. In many industrial applications, it is not un-
common to have to measure microvolt level signals super-
imposed over volt level perturbations and the LTC2484 is
eminently suited for such tasks. When the perturbation is
differential, the specification of interest is the normal mode
rejection for large input signal levels. With a reference
voltage V
tial 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 traditional nor-
mal 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
= 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.
Using the 2x speed mode of the LTC2484, the device
bypasses the digital offset calibration operation to double
the output data rate. The superior normal mode rejection
is maintained as shown in Figures 30 and 31. However, the
magnified details near DC and f
Figures 39 and 40. In 2x speed mode, the bandwidth is
11.4Hz for the 50Hz rejection mode, 13.6Hz for the 60Hz
rejection mode and 12.4Hz for the 50Hz/60Hz rejection
REF
= 5V, the LTC2484 has a full-scale differen-
S
= 256f
N
are different, see
LTC2484
35
2484fa
O