Probe Loading

Figure 1

In an ideal world, an oscilloscope probe would simply be a nonintrusive (infinite resistive, noncapacitive and noninductive) wire attached to the circuit of interest, and it would provide an exact replica of the signal being probed. But in the real world, the probe becomes part of the circuit and it introduces resistive, capacitive and inductive loading to the circuit. This in turn causes the scope to provide a different measurement with the deviations, depending on how much the probe loads the circuit. Figure 1 shows a simplified diagram of the sources of loading introduced by the probe. Because it is impossible to totally eliminate loading, the goal is to choose a probe that will minimize the loading effects and that will provide the most accurate replication of the signal.

Figure 2

Resistive loading will be significant if the input resistance of the probe is the same magnitude as that of the signal being probed. When a probe is introduced into a circuit, some of the current that was flowing in the circuit will now flow into the probe, reducing the voltage at the point being probed. Basically, resistive loading affects voltage. Figure 2 shows the effects of resistive loading. The effects of resistive loading are attenuation of amplitude, dc offset shift, and circuit bias change. It may also cause a malfunctioning circuit to work, or more likely cause a functioning circuit to become nonfunctional. For these reasons, we recommend that the probe resistance should be greater than 10 times the resistance of the circuit under test, so that the amplitude measurement errors will be less than 10%.

Figure 3

Probe tips also have some capacitance that affects timing measurements. This capacitance introduces measurement errors that are frequency dependent. Figure 3 shows the results of capacitive loading. Delay and rise time measurements are more adversely affected by capacitive loading than by resistive loading. This loading is caused by the tendency of the probe's capacitance to act as a low-pass filter at high frequencies. It shunts the high-frequency information to ground and significantly reduces the probe's input impedance at high frequencies. Bandwidth is also affected. When the risetime is slowed, there is a corresponding decrease in bandwidth. This decrease comes from the equation: bandwidth = 0.35/risetime.

Figure 4

Figure 4 shows the calculations used to derive the 10% to 90% rise time of a single pole R-C circuit. As the calculations show, the rise time of the capacitive loading is equal to 2.2RC. The adverse results of capacitive loading are slowed rise time, reduced bandwidth, and an increased propagation delay. We recommend that the capacitance of the probe tip is minimized in order to reduce its adverse effects on rise time measurements.

Figure 5

Basically, inductive loading distorts the signal being measured. Figure 5 illustrates the adverse effects of inductive loading on the circuit under test. The loading comes from the inductance of the probe ground lead. The inductance of a typical ground path is one nanohenry per millimeter. Thus, a typical probe ground lead of 3 inches has an inductance of 75 nH. The ringing that is introduced due to the loading has a frequency, f, equal to the following: f = 1/(2 Pi SQRT (LC)), where C is the sum of the circuit capacitance and the probe capacitance and L is the ground lead inductance. We recommend that the ground lead used is as short as possible in order to minimize the ringing on top of the measured waveform.