Huge drop in voltage when connecting output to pin of microcontroller?
Are you absolutely sure the pin/port is configured as an analog input, and not a digital output that's been set low?

An accidental digital low on the port could be what's loading the peak detector output. 

I'm not an expert on your particular architecture, but in general, micro I/O pins are multiplexed - you need to explicitly set registers to configure them as inputs, outputs or ADC channels. Setting up an internal ADC still requires you to manually configure its input pin appropriately.