Keysight Technologies N7020A, User Manual

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Using the Power Rail Probe
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N7020A and N7024A Power Rail Probes User’s Guide 41
Requirements for Optimum Probe Performance
Low Source Impedance Required
The N7020A and N7024A are designed explicitly for measuring DC power
supplies. In order to maintain a flat frequency response across the probe's
bandwidth, the DUT must have extremely low source impedance, like that of a
power rail. The probes have a non-flat input impedance that transitions from
50 k Ω at DC to 50 Ω at 1 MHz and above. Therefore, if the DUT impedance is not
low, it will divide with the probe's impedance unequally across frequency, causing
a non-flat measurement. The recommended DUT impedance should be much less
than 1 ohm. At 1 ohm, the DC-to-AC non-flatness is about 2%. Figure 7 shows the
non-flatness for DUT impedances of 0 Ω , 1 Ω , 25 Ω , and 50 Ω .
Figure 7 Frequency Response (N7020A Probe with N7022A Main Cable) Versus DUT Im-
pedance