Why do I need to use a unity gain amplifier to measure DAC output voltage of a PIC16F1xxx?

The DAC has a source impedance of about 300 kΩ. Load resistance may be low enough that it could draw a large current that would eventually decrease the output voltage. Adding an opamp, in unity gain amplifier configuration, copies the input voltage to its output terminal while drawing lower DAC output current. This is because the input impedance of an opamp is very high and buffers the load from the DAC output. The output of the opamp can then supply the required amount of current based on the opamp output current specifications.

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