One important performance parameter of the capacitor is the insulation performance. The insulation resistance of a capacitor can be basically modeled as a resistance between the two terminals of the capacitor. The resistance of the integration capacitor needs to be determined to calibrate the measurement. High insulation is one of the first requirements for ultra-low bias current measurements. Therefore, selecting high-insulation material is of primary importance.

Every material has the volume resistivity. Generally, a resistivity is measured one minute after the voltage is applied across the specimen at room temperature. However, due to internal charges with large parasitic resistance, more than one minute is needed to be an equilibrium condition for some materials. The specimen has both surface resistivity and internal resistivity. Some material are more sensitive to surface condition such as humidity and cleanliness.

Polypropylene capacitors have high resistivity and satisfactory high resistance for ultra-low bias current measurements. To understand the parasitic resistance, or Insulation resistance, vendors use a parameter represented by an Ohms-Farad product Ω·F as a unit. For example, a minimum spec of 10,000Ω·F (at 20°C 100Vdc) is extracted as

10,000Ω·F / 1µF = 10 x 10⁹Ω

If we can extrapolate the curve for smaller capacitance, such as 100pF, we expect

10,000Ω·F / 100pF = 0.1 x 10¹⁵Ω

The number indicates that the leakage current is 10aA or less in the condition of the voltage across the capacitor is 1mV. The next section can give us data on smaller capacitance and voltage, which is not described in the data sheet of the capacitor.

Figure 2-8 Capacitor has Leakage Across the Terminals