I have what I believe to be a fairly simple schematic, as shown below, to take either a PWM signal from an AVR board to play a song/alarm, or radio signal.
I'm connecting both of these with a summing circuit OpAmp. Only one of these signals will be on at a time. The rails on the OpAmp are +5V and ground, so a voltage divider was used to feed in a 2.5 volt signal to the non-inverting output. The output I want to get from the OpAmp to the Audio Amplifier is about 0.15 volts, so I selected the appropriate resistors to achieve that voltage level. Rf= 10kOhm, R1 = 330kOhm.
It was recommended to put a 0.1 uF in series with R_2 to block any DC signal on the AVR line. When I start probing the signal after the capacitor the signal appears to just disappear. I've tried AC and DC coupling with 50 Ohm termination and haven't been able to see any noticeable signal. I have tried the set-up without the CAP and it will play the song as expected.
Am I using the incorrect voltage gain equation? Are my resistor values way off? Is the capacitor recommendation incorrect?
Any suggesetions are greatly appreciated and let me know if I need to provide any additional information.