Datasheet

LTC2499
28
2499fd
Figure 21. Input Normal Mode Rejection at f
S
= 256 • f
N
Figure 20. Input Normal Mode Rejection at DC
When using the internal oscillator, the LTC2499 is de-
signed to reject line frequencies. As shown in Figure 20,
rejection 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 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.
applications inForMation
Traditional high order delta-sigma modulators suffer from
potential instabilities at large input signal levels. The
proprietary architecture used for the LTC2499 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 25 and 26 show
measurement results for the rejection of a 7.5V peak-
to-peak noise source (150% of full scale) applied to the
LTC2499. These curves show that the rejection performance
is maintained even in extremely noisy environments.
INPUT SIGNAL FREQUENCY (Hz)
INPUT NORMAL MODE REJECTION (dB)
2499 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
INPUT SIGNAL FREQUENCY (Hz)
250f
N
252f
N
254f
N
256f
N
258f
N
260f
N
262f
N
INPUT NORMAL MODE REJECTION (dB)
2499 F21
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
f
N
= f
EOSC/5120