LTC2415/LTC2415-1
32
sn2415 24151fs
APPLICATIO S I FOR ATIO
WU
U
FREQUENCY AT VIN (Hz)
1
–120
REJECTION
(dB)
–100
–80
–60
–40
–20
0
50
100
150
200
2415 F30
250
VCC = 5V
VREF = 5V
VIN = 2.5V
FO = 0
INPUT FREQUENCY
0
–60
–40
0
2415 F31
–80
–100
fS/2
fS
–120
–140
–20
REJECTION
(dB)
INPUT FREQUENCY DEVIATION FROM NOTCH FREQUENCY (%)
–12
–8
–4
048
12
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
–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
VCC = 5V
REF+ = 5V
REF – = GND
VINCM = 2.5V
VIN(P-P) = 5V
TA = 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 VIN
Figure 31. Rejection vs Frequency at VIN
Figure 32. Rejection vs Frequency at VIN
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 Sinc4
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 FO high or Low, respectively. On the
other hand, the LTC2415-1 offers simultaneous rejection
of 50Hz and 60Hz by tying FO 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 Sinc1 notch is combined with
the Sinc4 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)