Mic (with 4466) cap coupling and negative voltages

[adamhi]

Hi,

I'm using the Electret Mic with MAX4466 amp http://www.adafruit.com/products/1063 and am planning on running the output thru a switched-capacitor bandpass filter (MAX7490 - datasheet here: http://datasheets.maximintegrated.com/e ... AX7491.pdf) before reading with an Arduino analogIn. I understand that the output from the mic can vary from rail to rail, centered on Vcc/2. So it seems that there should be no hazards in hooking up the mic module directly to the MAX7490 if using 3.3v to the mic, as the MAX7490 can handle from -0.3 v up to Vdd + 0.3 V.

My question is - what happens if i were to couple the output of the Mic module thru a capacitor to the filter's input pin? Would i then risk damaging the MAX7490 because the voltage would drop below -0.3 v because i got rid of the dc offset

Thanks in advance.

--adam, wh6m

[adafruit_support_mike]

You've run into one of the conceptual stumbling blocks of capacitive coupling: you have to provide a DC pathway to the output end of the capacitor. Without some kind of DC path, the node's voltage is undefined, and could theoretically go anywhere.

What you need to do is connect the MAX7490's input pin to the center of a voltage divider, then hook the cap to that. Choose RC values for the coupling cap and voltage divider that are compatible with the filtering you want to do, but for sake of argument you could use a 1k-1k voltage divider and a 10nF cap. That would give you a corner frequency near 32kHz, and would give the MAX7490's input a DC reference point of VCC/2.