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AC701 Schematics

Refer page 40 of the schematics. Here for Vccint- the regulator is switching at 250 kHz. page 34 has the sensing signal conditioning for that the inst. amp used is INA333 which has BW of 150 kHz, the BW decreases to 35 kHz when gain is set to 10. They also are using anti-aliasing filter of ~3MHz cut-off.

Now my question is, considering the cross-over frequency of the regulator be 250/5 = 50 kHz and sampling used here [my assumption] 1MS/s. Using INA333 in the path would degrade the signal isn't it? We should have amp having greater BW than the sampling freq isn't it?

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  • \$\begingroup\$ Are you aware of the purpose of these ADC channels in this application? I'm 99% sure they're for health status and engineering / diagnostic purposes, and they're not used in any control loops - they're not part of the power supply regulation / compensation. \$\endgroup\$ Commented Sep 4, 2019 at 7:45

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In general the analog signal chain (combination of passive filter networks and/or active amplifiers) feeding an ADC should have a bandwidth less (preferably significantly less) than half the sampling frequency, to avoid aliasing.

Furthermore, in this type of power rail monitoring application you usually want to reject the power supply switching frequency, so it makes sense to have a cutoff frequency significantly below that.

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  • \$\begingroup\$ So, by what ratio the BW shall be set to reject the power supply switching freq ? Is it 1/10 of Cross-over frequency? \$\endgroup\$
    – seeker
    Commented Sep 4, 2019 at 9:06

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