LTC2497 Linear Technology, LTC2497 Datasheet - Page 24

• Manufacturer Part Number: LTC2497
• Description: 16-Bit 8-/16-Channel ADC
• Manufacturer: Linear Technology

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LTC2497
APPLICATIONS INFORMATION
The SINC
rejection at all frequencies except DC and integer multiples
of the modulator sampling frequency (f
sampling frequency is f
its internal oscillator and f
an external oscillator of frequency f
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
for simultaneous 50Hz/60Hz rejection. Multiples of the
modulator sampling rate (f
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
using the internal oscillator, as shown in Figure 18.
Measured values of normal mode rejection are shown
superimposed over the theoretical values.
24
Figure 16. Input Normal Mode Rejection at DC
4
–100
–110
–120
digital fi lter provides excellent normal mode
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
0
f
N
INPUT SIGNAL FREQUENCY (Hz)
2f
S
N
= 15,360Hz while operating with
S
3f
= f
N
S
EOSC
= f
4f
N
N
5f
/20 when operating with
• 256) only reject noise
N
f
EOSC
N
= f
6f
N
N
EOSC/5120
S
, where f
). The modulator
.
7f
N
2497 F16
8f
N
N
= 55Hz
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
Figure 17. Input Normal Mode Rejection at f
–100
–110
–120
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
250f
N
252f
INPUT SIGNAL FREQUENCY (Hz)
N
254f
N
256f
N
258f
f
N
N
= f
EOSC/5120
260f
N
2497 F17
262f
S
= 256 • f
N
N
2497f