Inverter and pH Display
The output of the amplifier stage is 1 volt
per pH unit, but it is inverted, so a pH of 6 produces an 8 volt output, and a pH
of 4 gives 10 volts out. The inverter's sole task is to change polarity of
the amplifier's signal before it is sent to the pH display so that the pH reads
correctly. Note that the non-inverting input, pin 5, is not connected to the
minus supply but to the reference ground. 47k resistors were used to keep
the loading on the amplifier stage light to help limit heating and drift.
Another 10 uF unpolarized capacitor, C400, has been included here to reduce noise.
C401 is a bypass capacitor. It's leads should be short, and it should be kept
close to pins 8 and 4.