Datasheet
LTC2484
37
2484fd
APPLICATIONS INFORMATION
INPUT SIGNAL FREQUENCY (f
N
)
INPUT NORMAL REJECTION (dB)
2484 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)
2484 F4
0
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
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
= 25°C
MEASURED DATA
CALCULATED DATA
Figure 41. Input Normal Mode Rejection vs Input Frequency,
2x Speed Mode and 50Hz/60Hz Mode
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
48
–70
–80
–90
–100
–110
–120
–130
–140
54 58
50 52
56 60 62
NORMAL MODE REJECTION (dB)
NO AVERAGE
WITH
RUNNING
AVERAGE
Figure 42. Input Normal Mode Rejection 2x Speed Mode
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 magnifi ed 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 LTC2484 operating with the internal oscillator
and 2x speed mode is shown in Figure 41.
When the LTC2484 is confi gured in 2x speed mode, by
performing a running average, a SINC
1
notch is combined
with the SINC
4
digital fi lter, yielding the normal mode
rejection identical as that for the 1x speed mode. The
averaging operation still keeps the output rate with the
following algorithm:
Result 1 = average (sample 0, sample 1)
Result 2 = average (sample 1, sample 2)
……
Result n = average (sample n – 1, sample n)
The main advantage of the running average is that it
achieves simultaneous 50Hz/60Hz rejection at twice the
effective output rate, as shown in Figure 42. The raw
output data provides a better than 70dB rejection over
48Hz to 62.4Hz, which covers both 50Hz ±2% and 60Hz
±2%. With running average on, the rejection is better than
87dB for both 50Hz ±2% and 60Hz ±2%.