Datasheet

LTC2480
37
2480fd
applicaTions inForMaTion
INPUT FREQUENCY (Hz)
0
NORMAL MODE REJECTION (dB)
2480 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 38. Measured Input Normal Mode Rejection
vs Input Frequency with Input Perturbation of 150%
Full-Scale (50Hz Notch)
INPUT SIGNAL FREQUENCY (f
N
)
INPUT NORMAL REJECTION (dB)
2480 F39
0
–20
–40
–60
–80
–100
–120
0
f
N
2f
N
3f
N
4f
N
5f
N
6f
N
7f
N
8f
N
INPUT SIGNAL FREQUENCY (f
N
)
INPUT NORMAL REJECTION (dB)
2480 F48
0
–20
–40
–60
–80
–100
–120
250248 252 254 256 258 260 262 264
Figure 39. Input Normal Mode
Rejection 2x Speed Mode
Figure 40. Input Normal Mode
Rejection 2x Speed Mode
INPUT FREQUENCY (Hz)
0
NORMAL MODE REJECTION (dB)
50
100
125 225
2480 F41
25 75
150
175
200
0
–20
–40
–60
–80
–100
–120
V
CC
= 5V
V
REF
= 5V
V
INCM
= 2.5V
V
IN(P-P)
= 5V
F
O
= GND
T
A
= 25oC
MEASURED DATA
CALCULATED DATA
Figure 41. Input Normal Mode
Rejection vs Input Frequency,
2x Speed Mode and 50Hz/60Hz Mode
Figure 42. Input Normal Mode
Rejection 2x Speed Mode
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
48
–70
–80
–90
–100
–110
–120
–130
–140
54 58
2480 F42
50 52
56 60 62
NORMAL MODE REJECTION (dB)
NO AVERAGE
WITH
RUNNING
AVERAGE
to measure microvolt level signals superimposed over volt
level perturbations and the LTC2480 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
REF
= 5V,
the LTC2480 has a full-scale differential input range of
5V peak-to-peak. Figures 37 and 38 show measurement
results for the LTC2480 normal mode rejection ratio with a
7.5V peak-to-peak (150% of full-scale) input signal super-
imposed over the more traditional normal mode rejection
ratio results obtained with a 5V peak-to-peak (full-scale)
input signal. In Figure 37, the LTC2480 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 LTC2480 rejection performance
is maintained with no compromises in this extreme situ-
ation. 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 LTC2480, 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
S
= 256f
N
are different,
see 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
mode. Typical measured values of the normal mode rejec-
tion of the LTC2480 operating with the internal oscillator
and 2x speed mode is shown in Figure 41.
When the LTC2480 is configured in 2x speed mode, by
performing a running average, a SINC1 notch is combined
with the SINC4 digital filter, yielding the normal mode