Thanks for your reply, Johann,
I want to use PIC16F84 to control the calibration of DA/AD Converter, in another word, I would like to control the value of the feedback variable resistor to change the amplify times via this MCU, and by this way, I can check the output of DAC is exactly equal with the input of ADC.But the problem is, that I can not communicate the output of DAC with the input of ADC through the I/O port of PIC16F84.
If you are familar with this kind of MCU device, hope you can help me to check the circuit which I attached. Thanks.