Datasheet

LTC2497
23
2497fa
INPUT SIGNAL FREQUENCY (Hz)
INPUT NORMAL MODE REJECTION (dB)
2497 F16
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)
2497 F17
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
f
N
= f
EOSC/5120
Figure 17. Input Normal Mode Rejection at f
S
= 256 • f
N
Figure 16. Input Normal Mode Rejection at DC
The SINC
4
digital filter provides excellent normal mode
rejection at all frequencies except DC and integer multiples
of the modulator sampling frequency (f
S
). 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 LTC2497 is designed
to reject line frequencies. As shown in Figure 16, rejection
nulls occur at multiples of frequency f
N
, where f
N
= 55Hz
for simultaneous 50Hz/60Hz rejection. Multiples of the
modulator sampling rate (f
S
= f
N
• 256) only reject noise
to 15dB (see Figure 17); if noise sources are present at
these frequencies antialiasing will reduce their effects.
The user can expect to achieve this level of performance us-
ing the internal oscillator, as shown in Figure 18. Measured
values of normal mode rejection are shown superimposed
over the theoretical values.
Traditional high order delta-sigma modulators suffer from
potential instabilities at large input signal levels. The
proprietary architecture used for the LTC2497 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. Figure 19 shows mea-
surement results for the rejection of a 7.5V peak-to-peak
noise source (150% of full scale) applied to the LTC2497.
This curve shows that the rejection performance is main-
tained even in extremely noisy environments.
Output Data Rate
When using its internal oscillator, the LTC2497 produces
up to 7.5 samples per second (sps) with a notch frequency
of 60Hz. The actual output data rate depends upon the length
of the sleep and data output cycles which are controlled
applicaTions inForMaTion