Probe Types II: Active Probes

Figure 10

Figure 10 shows the diagram of the active probe. The active probe comes in two varieties: the FET probe and the non-FET or bipolar input probe. Both probes have a very high input resistance. This probe differs from the passive probes in that the probe tip contains an active amplifier in addition to the R-C network. This active amplifier drives a 50-ohm cable which is then connected to the 50-ohm input of the oscilloscope. The oscilloscope that is being used with this probe must have a 50-ohm input. The major feature of this probe is its overall combination of resistive and capacitive loading, in addition to a very high bandwidth. These benefits make this the least intrusive of all the probes that have been discussed. With this probe you can look at many different kinds of circuits, including ECL, CMOS, and GaAs. You can also look at typical analog circuits, transmission lines, and basically any circuit that has a source resistance between 0 and about 10-kohm. This probe has the highest cost of all the probe limited dynamic range. This is usually plus or minus 40-Volts dc plus peak ac.

Figure 11

Figure 11 shows a diagram of the differential probe. The significant difference here is that the probe tip has two inputs, a positive (or noninverting) and a negative (or inverting) input. These two inputs feed a differential amplifier, which in turn drives a 50-ohm cable that is connected to the 50-ohm input of the oscilloscope. One benefit of the differential probe, as compared to subtracting two channels with passive probes, is the electrical path for the two signals has been matched as closely as possible. The primary features of this probe are controls for the dc offset, dc reject, and coupling. It also has fairly high common mode rejection (such as 3000:1 at 1 MHz). Note that the higher the common mode rejection ratio (CMRR), the smaller the signals that are possible to view. The benefits are the ease of viewing small signals in the presence of large dc offsets or other common mode signals, and the accuracy that can be achieved in looking at these signals. The applications for this probe are measuring signals that are not referenced to ground, looking at differential amplifiers, and troubleshooting power supplies. The tradeoffs are that the probe is more bulky, is more expensive than two passive probes, less dynamic range and requires the use of an external power and control module.

Figure 12

Practically speaking, CMRR is the ratio of a common mode signal divided by the rejected common mode signal. Figure 12 shows an application for the differential probe in which a small signal is being measured in the presence of a large common mode signal. It shows that a very large 60-Hz line voltage signal is overwhelming the signal of interest. Looking at either one of the signal leads separately, a small signal would be seen riding on top of the common mode 60-Hz signal. The differential probe eliminates this common mode signal resulting in a small signal, a very nice square wave at the output. It would be very difficult to make this measurement without the differential probe.

Figure 13

Figure 13 shows how to measure the CMRR of the differential probe. The first step is to connect both inputs of the differential probe to a signal generator with a 1-V peak-to-peak, 1-MHz sinewave and measure the probe's output amplitude on the oscilloscope. This measurement is V3 and the probe output is 0.33 mV peak-to-peak. The second step is to take the signal generator and feed it differentially into the plus and minus probe inputs. The output, V3, should be approximately equal to 1 V peak-to-peak. To obtain the CMRR, divide V1 by V2. This results in the CMRR for this probe at 1-MHz. For this example, that calculates out to be approximately 3000 to 1.

Figure 14

Figure 14 shows the change in the CMRR as a function of frequency for the HP 1141A differential probe. The dips occur in the upper two curves due to the dual signal path designed into the probe. There is a low-frequency, and a high-frequency signal path. The low-frequency path CMRR is degrading while the high-frequency path CMRR is improving at their crossover, resulting in the dip. Note that the low CMRR caused by the ac coupler only occurs with its use. The dc reject would normally be used unless the dc voltage is too high.

Figure 15

Figure 15 illustrates how to measure signals that are not referenced to ground with the differential probe. Shown is a diagram of a linear power supply in which the differential probe can be used to measure the current flowing through a sense resistor. It is also being used to measure the voltage across a series regulator. The differential probe provides a reliable method of measuring how close the series regulator is coming to its drop-out-voltage specification (the minimum voltage across the regulator required for proper operation).