I am trying to interface an ecg circuit with the Arduino. The circuit produces both positive and negative-going pulses. I need to add a pull-up resistor and capacitor in conjunction with the circuit's 9v power. To get the signal biased within the 0 to 5v range of the Arduino I need to know the resistance across the ADC of the Arduino. Has anyone measured this? I suspect that a bias of 1.5v will work.
Document http://www.atmel.com/dyn/resources/prod_documents/DOC2559.PDF, page 10, says it is 100 Mohms