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Techcurious , 07-18-2025, 09:11 PM

Hi,

In the mixed signal hardware design course, there is no clear justification provided for using the 150pF cap between the ADC differential lines. See the attached figure, is it needed for copying the ADC_IN+ signal to ADC_IN- signal line? but 150pF is not short at audio frequencies. Can someone please clarify?

Techcurious , 07-24-2025, 09:22 PM

@Phil Can you pleaes provide response to this question, it is from your mixed signal hardware course

Phil , 08-11-2025, 08:43 AM

This forms a 'balanced' low-pass filter. Although we're only feeding in a single-ended signal (due to our single-ended signal source), the ADC has a differential input. The ADC with this circuit sees approximately the same impedance at the +/- inputs which helps with noise rejection. The 150p cap is not for audio frequencies, but to ensure HF/RF frequencies are attenuated right before we feed into the ADC pins (in the layout, the cap is placed right at the ADC input). It is very likely in practice that it is not needed, but I like putting in placeholder parts, that I can adjust when I get the 'real' boards. Better to have the placeholders than have to re-spin a new revision.