This is a photograph of a 20 Tera Ohm Insulation Tester I made around 20 Years Back. It had even Polarization Index for Transformer Insulation.
The Product above is made with two circuits below.
Insulation Tester or Tera Ohm Meter with Polarization Index
Also using the ICL7650 Chopper Stabilized amplifier must be done as shown in data sheets with grounded guard rings. pA will leak on the PCB itself if layout is not done keeping this in mind.