The purpose of the DC Amp is to wxpand the AD8307’s 0.7 – 2.5V range to fill the Arduino’s A-D converter acceptance range of 0 – 5.0V.
The potentiometer is used to adjust the nominal 0.7VDC offset so that the opamp output swings down to 0V.
The op-amp shown is a TL974, only because I had one. Any rail-rail amp that can run from 5V will probably be acceptable.
W3JDR Bright Ideas 
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