Datasheet

LTC2493
26
2493fd
Figure 20. Input Normal Mode Rejection at f
S
= 256 • f
N
Figure 19. Input Normal Mode Rejection at DC
When using the internal oscillator, the LTC2493 is designed
to reject line frequencies. As shown in Figure 19, rejec-
tion nulls occur at multiples of frequency f
N
, where f
N
is
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 20); 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 21, 22,
and 23. Measured values of normal mode rejection are
shown superimposed over the theoretical values in all
three rejection modes.
applicaTions inForMaTion
INPUT SIGNAL FREQUENCY (Hz)
INPUT NORMAL MODE REJECTION (dB)
2493 F19
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
INPUT SIGNAL FREQUENCY (Hz)
250f
N
252f
N
254f
N
256f
N
258f
N
260f
N
262f
N
INPUT NORMAL MODE REJECTION (dB)
2493 F20
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
f
N
= f
EOSC/5120
Traditional high order delta-sigma modulators suffer from
potential instabilities at large input signal levels. The
proprietary architecture used for the LTC2493 third order
modulator resolves this problem and guarantees stability
with input signals 150% of full-scale. In many industrial
applications, it is not uncommon to have microvolt level
signals superimposed over unwanted error sources with
several volts if peak-to-peak noise. Figures 24 and 25
show measurement results for the rejection of a 7.5V
peak-to-peak noise source (150% of full-scale) applied to
the LTC2493. These curves show that the rejection perfor-
mance is maintained even in extremely noisy environments.