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Avoiding scope errors (Part 2)

Posted: 04 Oct 2013 ?? ?Print Version ?Bookmark and Share

Keywords:probe? coaxial cable? high Q? capacitance? bandwidth?

Part 1 tackles insufficient bandwidth and/or sample rate.

The three most common probe issues are not calibrating the probe (either for capacitance or for time skew), ringing due to the ground wire, and using 50-Ohm coaxial cable connected to a high-impedance input. The scope probe is represented as a high impedance in parallel with a high Q capacitance. Any series inductance (either connected to the tip or the ground) will result in a high Q tank circuit and ringing. The ringing frequency will generally be lower than the stated bandwidth of the probe.

Figure 1: The probe selection impacts the measurement rise time and the fidelity of the edge. A ring is created due to the probe connection.

An unterminated coaxial cable offers shielding, but it will still resonate with the scope input capacitance and with high Q. This example, using a 36" coaxial cable, can achieve very high Q at high frequencies, which results in ringing that is due to the probe, rather than the measurement.

Figure 2: The forward transfer gain of a 50-Ohm coax cable terminated into 1 MegOhm.

Figure 3 shows the transient response measurement of a point of load (POL) in response to a current pulse. The current pulse is shown in the lower panel, with a step of 100-500 mA and rise and fall times of approximately 8 nS. The upper panel shows the voltage response with the same coax connection AC coupled through a Picotest J2130A bias injector into a 50-Ohm scope input. One trace (M1) shows the responses measured with the coax cable terminated into 50-Ohm. The other voltage trace (M4) shows the same response with the scope input set to an input impedance of 1 MegOhm. Note the severe ringing, which is due only to the unterminated coax cable.

Figure 3: Voltage response of a POL regulator to a step load using a 50-Ohm coax cable connection terminated into 50-Ohm and into 1 MegOhm.

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