Datasheet
LTC2492
30
2492fd
APPLICATIONS INFORMATION
The SINC
4
digital fi lter provides excellent normal mode
rejection at all frequencies except DC and integer multiples
of the modulator sampling frequency (f
S
) (see Figures
18 and 19). The modulator sampling frequency is f
S
=
15,360Hz while operating with its internal oscillator and
f
S
= F
EOSC
/20 when operating with an external oscillator
of frequency F
EOSC
.
When using the internal oscillator, the LTC2492 is designed
to reject line frequencies. As shown in Figure 20, rejection
nulls occur at multiples of frequency f
N
, where f
N
is
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0f
S
2f
S
3f
S
4f
S
5f
S
6f
S
7f
S
8f
S
9f
S
10f
S
11f
S
12f
S
INPUT NORMAL MODE REJECTION (dB)
2492 F18
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
Figure 18. Input Normal Mode Rejection, Internal Oscillator
and 50Hz Rejection Mode
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0f
S
INPUT NORMAL MODE REJECTION (dB)
2492 F19
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
2f
S
3f
S
4f
S
5f
S
6f
S
7f
S
8f
S
9f
S
10f
S
Figure 19. Input Normal Mode Rejection, Internal Oscillator
and 60Hz Rejection Mode
INPUT SIGNAL FREQUENCY (Hz)
INPUT NORMAL MODE REJECTION (dB)
2492 F20
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
f
N
0 2f
N
3f
N
4f
N
5f
N
6f
N
7f
N
8f
N
f
N
= f
EOSC/5120
Figure 20. Input Normal Mode Rejection at DC
INPUT SIGNAL FREQUENCY (Hz)
250f
N
252f
N
254f
N
256f
N
258f
N
260f
N
262f
N
INPUT NORMAL MODE REJECTION (dB)
2492 F21
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
Figure 21. Input Normal Mode Rejection at f
S
= 256 • f
N
determined by the input control bits FA and FB (f
N
= 50Hz
or 60Hz or 55Hz for simultaneous rejection). Multiples of
the modulator sampling rate (f
S
= f
N
• 256) only reject noise
to 15dB (see Figure 21); if noise sources are present at
these frequencies anti-aliasing will reduce their effects.
The user can expect to achieve this level of performance
using the internal oscillator, as shown in Figures 22, 23,
and 24. Measured values of normal mode rejection are
shown superimposed over the theoretical values in all
three rejection modes.