Cover story
be readjusted between 10pF and 25pF with a supplied screwdriver. Higher impedance measurements
are no longer used for higher bandwidth applications. The aim here is to achieve an adjustment to the circuit (usually 50Ω) in order to avoid unwanted reflections. If a probe is required for HF applications, active probes can also be used instead of passive ones. Decisive for the right choice is the intention to allow the amplification factor over a certain bandwidth and at the same time not to influence the circuit too much.
Amplifiers and current clamps Active probes have an integrated linear broadband amplifier that needs to be powered. In addition to the bandwidth, the active part achieves a very high signal accuracy with a low input capacitance. The disadvantage is the limited input voltage, (e.g., the maximum that can be measured with PVA8000: is 30Vpeak
).
Current clamps offer a connection option to be able to measure currents. Because an oscilloscope is a voltage meter, these currents must be translated into a voltage value. Since the translation voltage/current is known from the data sheet in the current clamps, the current can be displayed and measured in the oscilloscope using the amplification and the unit [Ampere]. The principle of the current clamps is based on a transformer, with the measuring line representing the primary winding and the current clamp representing the secondary winding. This is how alternating currents can be measured. Direct currents are recorded using a Hall probe.
Differential probes Differential measurements are required when measuring between two potential voltage values. Two variants are available for this. On the one hand a differential HF probe is required for fast differential data signals such as Ethernet or LVDS. On the other hand, there is a need for differential probes in the field of high-voltage measurement, in case you have to measure voltage differences that are not related to ground. For lower
Figure 1: Simplified circuit of passive probe on DUT and oscilloscope
Figure 2: Input impedance of the measurement system
Figure 3: Compensation on a passive probe (example PVP2350)
frequencies, such measurements can also be displayed with two passive probes (channel 1: potential 1, channel 2: potential 2) and via a math function (channel 2 – channel 1). However, these measurements are noisy, runtime errors between the probes can occur and the maximum input voltage of the passive probes must be observed. A key consideration in differential
probes is the common mode rejection ratio (CMRR). Common mode changes are unwanted simultaneous changes in potentials 1 and 2 with respect to ground
that are visible at the output since only the change between potentials 1 and 2 is measured. This is about the amplification factor of the differential mode compared to the common mode, which should be much higher. This value is bandwidth dependent. The PHA2150 high-voltage differential probe has a bandwidth of up to 200MHz and the maximum differential voltage that can be measured is 1500V. CMRRDC is > 80dB and CMRR1MHz is > 50dB.
www.rigol.eu www.electronicsworld.co.uk Septembert 2022 07
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