Obviously, the large value of leakage current would obscure the actual diode reverse current (ID), which is typically much lower than 15nA in modern diodes. One way to solve the problem is to increase the leakage resistance (RL) to 1T, reducing IL to 15pA, which still may be large compared to ID. Another way to reduce the effects of leakage currents is to guard the connection between the diode and the picoammeter so that it is completely surrounded by a conductor connected to the same potential (+15V in this example). The resulting configuration is shown in Figure 10c. Since a typical feedback picoammeter has a maximum voltage burden of only 200V, the voltage across RL is reduced to that value, and the leakage current now becomes: Thus, IL is reduced by four decades and is likely to be insignificant compared to ID.

The current flowing through RG is still 15nA, but it is now supplied by the low impedance +15V source, and it does not present a problem since it is not measured by the ammeter. Another situation where guarding is beneficial is shown in Figure 11a. In this case, the leakage resistance of a coaxial cable whose shield is grounded will reduce the output voltage of a high resistance source, as shown in Figures 11a and 11b. Note that voltage actually measured by the high impedance voltmeter is attenuated by the voltage divider formed by the source resistance (RS) and the leakage resistance (RL).

Although the main advantage of guarding is in the reduction of leakage resistance effects, another is the reduction of effective input capacitance. Measurement rise time depends both on the equivalent source resistance and the effective meter input capacitance, which depends both on the electrometer input capacitance and the connecting cable capacitance. Thus, for high resistance measurements, even a small amount of cable capacitance can result in very long rise times. For example, an input capacitance of 1,000pF (including the input cable) and a resistance of 200G results in a time constant (t = RC) of 200 seconds, and 1,000 seconds (more than 16 minutes) must be allowed for the measurement to settle to within 1% of final value. Modern electrometers, such as the Models 6514, 6517B, and 6430, have built-in provisions to take full advantage of guarding. In the guarded mode, the effective cable capacitance can be reduced to about 20pF, speeding up measurements by a factor of 50.

If instead, the shield is connected to a low impedance source of the same potential as the high resistance source, such as the unity-gain or preamp output of an electrometer voltmeter (Figure 11c), leakage from the center conductor to the shield will vanish, since there is essentially no potential across the insulator (RL). Leakage current (IG) from the outer guard shield to ground may be considerable, but it is of no consequence since the current is supplied by the low impedance preamp output rather than by the high impedance source.