It is seems that amplifier with 1k output impedance are sufficient.
First channel (green) connected to pwm pin 11 via resistor, second channel (yellow) - to pwm pin 3 directly.
PWM frequency - 3906 Hz. ADC speed - 2x100 ksamp/sec.
1k:
10k:
110k:
Now this amplifier used (R1 = 2.7 .. 110k, Q2 - BC547B), but op amp with rail-to-rail output may be better:
