Figure 7—The first and second stages must use micropower rail-to-rail op-amps (LT1490s are used here). A programming port is located at J3 for downloading and flashing the PIC16C873 processor. The gain resistors in the second stages R12 through R19 and those in the pre-amp stages should be 0.1%-tolerance types to achieve the better than 1% accuracy that’s possible with the PSD.