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PDFActive Probes
So far, all the probes discussed have been simple passive circuits with no active components such as transistors or integrated circuits. In cases where extremely low capacitance is required for high-frequency measurements, an ACTIVE PROBE may be used. An active probe has a small amplifier built into it that is designed to have very little capacitance on its input. The output of the amplifier is usually matched to drive the 50-ohm input of the oscilloscope. This allows a length of 50-ohm cable to be used between probe and scope without any additional capacitive loading effects.Table 4-4 summarizes the typical specifications of the various types of scope probes that have been discussed. Actual characteristics will vary according to manufacturer and model.
Some scopes have floating or differential inputs that allow both leads of the input to be connected away from ground. In this case, the grounding problem is avoided.
A two-channel scope with the ability to display channel 1-2 (the difference between the two channels) can be used as a one channel floating input scope. The oscilloscope is set up to display 1-2. Channel 1 is connected to the point in the circuit taken to be the more positive voltage. Channel 2 is connected to the other voltage point, and the oscilloscope ground is connected to the circuit ground. Thus, the scope displays the difference between the two voltage points with neither one required to be at ground.
A DIFFERENTIAL PROBE eliminates this problem by providing two scope probe inputs which can be floating relative to the scope's ground (Figure 4-38). The output voltage of the probe is the difference between the voltages on the two input terminals, allowing it to drive the ground-referenced input of an oscilloscope. The differential amplification is not perfect, and the error is specified in terms of COMMON MODE REJECTION RATIO (CMRR). To measure CMRR, both inputs are driven with the same signal. Ideally, the output (which is the difference between the two inputs) is always zero. But in a real probe there is some small output voltage.
Typically, the CMRR of a differential probe is best at low frequencies and degrades at higher frequencies. CMRR is often expressed in dB.
Figure 4-38 A differential probe with a bandwidth of 200 MHz (Photo courtesy of Hewlett-Packard Company.)
High-Voltage Probes
Current Probes
Current probes generally use one of two technologies. The simplest uses the principle of a transformer, with one winding of the transformer being the measured wire. Since transformers work with only AC voltages and currents, current probes of this type do not measure direct current.
Figure 4-39 A typical high-voltage probe. (Photo courtesy of Hewlett-Packard Company.)
The other type of current probe works using the principle of the HALL EFFECT. The Hall effect produces an electric field in response to a current present in an applied magnetic field. This technique requires the use of an external power supply, but does measure both alternating and direct current (AC and DC).
Since current probes measure the current enclosed by their jaws, several techniques can be used that are unique to the current probe. If the sensitivity of the probe and oscilloscope combination is too low for a particular measurement to be made, several turns of the current carrying wire can be inserted into the jaws. The probe will effectively have a larger current to measure (the original current times the number of turns). In a similar manner, the difference between two currents can be measured if the two wires in question are inserted, but with the currents flowing in opposite direction (the sum will be measured if the currents are flowing in the same direction). Of course, the physical size of the wires and the current probe will be a factor in determining how many wires can be inserted. Although the current does not require a direct electrical connection, it still removes energy from the circuit under test. Normally, this small amount of energy loss will not disturb the circuit, but can be a factor in some cases.
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Attenuating Probes
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