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LTC2489

2489f

The SINC4 digital ﬁlter provides excellent normal mode

rejection at all frequencies except DC and integer multiples

of the modulator sampling frequency (fS). The modulator

sampling frequency is fS = 15,360Hz while operating with

its internal oscillator and fS = fEOSC/20 when operating with

an external oscillator of frequency fEOSC.

When using the internal oscillator, the LTC2489 is designed

to reject line frequencies. As shown in Figure 15, rejection

nulls occur at multiples of frequency fN, where fN = 55Hz

for simultaneous 50Hz/60Hz rejection. Multiples of the

modulator sampling rate (fS = fN • 256) only reject noise

to 15dB (see Figure 16); 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 17.

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 LTC2489 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 18 shows mea-

surement results for the rejection of a 7.5V peak-to-peak

noise source (150% of full scale) applied to the LTC2489.

This curve shows that the rejection performance is main-

tained even in extremely noisy environments.

Output Data Rate

When using its internal oscillator, the LTC2489 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

INPUT SIGNAL FREQUENCY (Hz)

2489 F15

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

2fN 3fN 4fN 5fN 6fN 7fN 8fN

fN = fEOSC/5120

INPUT SIGNAL FREQUENCY (Hz)

250fN 252fN 254fN 256fN 258fN 260fN 262fN

2489 F16

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

fN = fEOSC/5120

Figure 16. Input Normal Mode Rejection at fS = 256 • fN

Figure 15. Input Normal Mode Rejection at DC

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