Product Brochure - Probes and Accessories for Rohde & Schwarz Oscilloscopes (Rohde&Schwarz) - 4

Dynamic range
Selecting the right The dynamic range of a probe is defined as the maximum measurable input voltage. It is specified for DC voltage and often decreases as the frequency increases. In the case probe of differential probes, a distinction is also made between common mode and differential mode dynamic range. The common mode dynamic range determines the valid in- The first step in selecting the right probe is to put voltage range for a single differential input, measured with reference to ground. The differential mode dynamic analyze the measurement task. Is a single-ended range defines the maximum measurable input differential or a differential measurement needed? Which voltage. maximum frequency components need to be transferred? What is the maximum input voltage To accurately measure steep, large-amplitude slopes, a that can occur? sufficiently wide dynamic range must be available at high frequencies. When measuring the residual ripple of DC switching power supplies, very small signals with a large
Differential or single-ended measurement
DC component must also be measured. To make the full Differential probes make measurements possible when A/D converter resolution available, modern probes have neither of the two test points is connected to ground. An the option to feed in a DC offset. example is voltage measurements on components without a connection to ground, which is necessary when charac- In the case of high-voltage probes, operator safety is a key terizing switching power supplies. Differential probes are consideration. High-voltage probes therefore have special also needed for low-noise measurements on differential insulation, protection against accidental contact and other signals. protective mechanisms. These probes are characterized by the maximum voltage to ground and by the measurement Differential probes can in fact also be used for single- category. The measurement category defines the measure- ended measurements. Single-ended probes often offer a ment environments in which the operator is still protected. higher input impedance, a lower input capacitance and the A probe may only be used in the measurement categories advantage of a wider dynamic range. for which it is defined.
Bandwidth and rise time Load on the device under test
Bandwidth is one of the most important parameters when A measurement system must not excessively load the selecting a probe. It defines the cutoff frequency after circuit under test, both to prevent degraded signals and which a signal will be displayed more than 3 dB (approx. to ensure that the functioning of the DUT is not impaired. 30 %) weaker than it actually is. For an accurate signal The key is to use probes with a high input impedance and representation, the cutoff frequency of the measurement a low input capacitance. The resulting input impedance system (oscilloscope and probe) must be greater than the is highly dependent on the frequency and is typically less highest frequency component to be displayed. When mea- than 500 Ω at the probe's cutoff frequency. suring digital signals, the measurement bandwidth should be 3 to 5 times greater than the clock rate (For debugging Passive probes typically have an input impedance of a digital design, a bandwidth that is 3 times greater is suf- 10 MΩ and an input capacitance of > 10 pF. Active probes ficient. For conformance tests on digital interfaces, the typically have an input capacitance of < 1 pF at an input bandwidth must be 5 times greater than the clock rate.). impedance of 1 MΩ and are especially suited for measure- ments on circuits with high-speed signals > 100 MHz. For When measuring fast slopes, such as when characterizing the measurement, it is important to select the right probe switching power supplies, the critical parameter is the rise accessories for contacting with the DUT. Long pins and time of the measurement system (oscilloscope and probe). leads increase the capacitance and inductance, lower the For precise measurements, the rise time of the measure- maximum measurement bandwidth and lead to excessive ment system should be a factor of 3 to 5 times lower than overshoot and ringing artifacts at the pulse slopes. the rise time of the pulse being measured.
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