Datasheet

LTC2415/LTC2415-1
32
sn2415 24151fs
APPLICATIO S I FOR ATIO
WUU
U
FREQUENCY AT V
IN
(Hz)
1
–120
REJECTION (dB)
–100
–80
–60
–40
–20
0
50 100 150 200
2415 F30
250
V
CC
= 5V
V
REF
= 5V
V
IN
= 2.5V
F
O
= 0
INPUT FREQUENCY
0
–60
–40
0
2415 F31
–80
–100
f
S
/2 f
S
–120
–140
–20
REJECTION (dB)
INPUT FREQUENCY DEVIATION FROM NOTCH FREQUENCY (%)
–12–8–404812
REJECTION (dB)
2415 F32
–60
–70
–80
–90
–100
–110
–120
–130
–140
48 50 52 54 56 58 60 62
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
NORMAL MODE REECTION RATIO (dB)
2415 F33
–80
–90
–100
–100
–120
–130
–140
INPUT FREQUENCY (Hz)
0
20 40 60 80 100 120 140 160 180 200 220
NORMAL MODE REJECTION (dB)
2415 F34
0
–20
–40
–60
–80
–100
–120
V
CC
= 5V
REF
+
= 5V
REF
–
= GND
V
INCM
= 2.5V
V
IN(P-P)
= 5V
T
A
= 25°C
MEASURED DATA
CALCULATED DATA
Figure 34. Input Normal Mode Rejection vs Input
Frequency with Input Perturbation of 100% of Full Scale
Figure 30. Rejection vs Frequency at V
IN Figure 31. Rejection vs Frequency at V
IN
Figure 32. Rejection vs Frequency at V
IN
Figure 33. Normal Mode Rejection
when Using an Internal Oscillator
Normal Mode Rejection, Output Rate and Running
Averages
The LTC2415/LTC2415-1 both contain an identical Sinc
4
digital filter (see Figures 30 and 31) which offers excellent
line frequency noise rejection. For the LTC2415, a notch
frequency of either 50Hz or 60Hz (see Figure 32) is user
selectable by tying pin F
O
high or Low, respectively. On the
other hand, the LTC2415-1 offers simultaneous rejection
of 50Hz and 60Hz by tying F
O
low. This sets the notch
frequency to approximately 55Hz (see Figure 32).
At a notch frequency of 55Hz, the LTC2415-1 rejects 50Hz
±2% and 60Hz ±2% better than 72dB. In order to achieve
better than 87dB rejection of both 50Hz and 60Hz ±2%, a
running average can be performed. By averaging two
consecutive ADC readings, a Sinc
1
notch is combined with
the Sinc
4
digital filter yielding the frequency response
shown in Figures 33 and 34. In order to preserve the 2×
output rate, adjacent results are averaged with the follow-
ing algorithm:
Result 1 = average (sample 0, sample 1)
Result 2 = average (sample 1, sample 2)
Result 3 = average (sample 2, sample 3)
…
Result N = average (sample n-1, sample n)