How to Reduce Leakage Current Using Electrometers, Ammeters, and Picoammeters

Leakage current is generated by parasitic resistance paths between the measurement circuit and a nearby voltage source. This current can significantly reduce the accuracy of low current measurements. To reduce leakage current, use high-quality insulation materials, reduce humidity in the test environment, and employ guarding techniques. Guarding techniques can also reduce the effects of shunt capacitance in the measurement circuit.

When building the test circuit, one way to reduce leakage current is to use high-quality insulating materials. Teflon, polyethylene, and sapphire are examples of high-quality insulating materials, but avoid materials such as Bakelite and nylon.

Humidity can also affect low current measurements. Different types of insulation absorb different amounts of moisture from the air, so it is best to choose insulation that water vapor does not easily form a continuous film on. Sometimes, because the material being measured tends to absorb moisture, it is best to perform the measurement in an environmentally controlled room if this cannot be avoided. In some cases, there may be ionic contaminants on the insulation and can produce spurious currents, especially in high humidity environments.

Guarding is a very effective way to reduce leakage current. Guarding is a technique that forces a low-resistance node in a circuit to be approximately equal in potential to the high-resistance input node. Guarding techniques for electrometers, ammeters, and picoammeters can be best illustrated by the following example.

The guard terminal of the electrometer ammeter and picoammeter is the LO input terminal. The guard can be used to isolate the high impedance input leads of the ammeter from the leakage current caused by the voltage source. Figure 1 shows an example of this kind of guard.

The guarding technique is applied to measure the ion current of the ionization chamber. The unguarded ionization chamber and its corresponding equivalent circuit are shown in Figure 1. The equivalent circuit shows that the entire bias voltage appears across the insulation resistance (RL). Therefore, the leakage current (IL) will be added to the measured ion current (IM = IC + IL). The leakage resistance is caused by the insulation of the ionization chamber and the coaxial cable.

In Figure 2-11b, a metal guard ring is added to the ionization chamber. This guard circuit divides the leakage resistance into two parts. The voltage on RL1 is the voltage drop at the input of the picoammeter, which is usually less than 1mV, so the current generated is quite small. The entire bias voltage appears on RL2. The leakage current will flow in this loop, but it will not affect the measurement.

Protection is also necessary to avoid leakage currents generated by the test fixture.