MSP430G2755: How to filter noise input to analog input of MCU?

ArTzy

Genius 16680 points

Part Number: MSP430G2755
Other Parts Discussed in Thread: OPA344

Tool/software:

Hi Experts,

We'd like to ask assistance if there is any way to calculate what cut off frequency needed on this circuit:

With scope shot:

Without MCU (red graph) means the RC (R3 & C1) is removed and no MCU connected.

Using OPA344 as a buffer and have a voltage divider with output under 3.3V to also protect the MSP430G2755 from burning.

Do you have anything to suggest?

Regards,
Archie A.

0 Keith Barkley 2 months ago

Guru 41995 points

Whats the maximum BW of the signal?

What kind of signal processing are you going to do? (i.e., do you need a Nyquist filter?)

0 Bruce McKenney 2 months ago

Guru 96345 points

I don't see a red graph (my color vision isn't very good). The blue graph seems to have 1 cycle every 0.2ms, or about 5kHz.

Are you trying to keep this or get rid of it?

0 Keith Barkley 2 months ago in reply to Bruce McKenney

Guru 41995 points

For some reason the red and green graphs are way off scale. All we see are transitions.

0 David Schultz 2 months ago

Guru 23005 points

ArTzy said:
We'd like to ask assistance if there is any way to calculate what cut off frequency needed on this circuit:

If you know how much attenuation you require above the Nyquist frequency, it is trivial.

+1 Luke Ledbetter 2 months ago in reply to David Schultz

Just FYI, Nyquist frequency would be half your sampling rate. So if you're sampling at a 2Mbps select the RC filter to filter at least 1MHz.

I recommend checking out our ADC precision labs videos - below is the one on anti-aliasing filters.

0 Keith Barkley 2 months ago in reply to Luke Ledbetter

Guru 41995 points

Probably 0.9 MHz to be sure - depending on the filter and how much aliasing you can tolerate.

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