The Tektronix S5 Sampling Head is a low-noise, 1 ns risetime sampling unit with a 1 Mohm, 15 pF input impedance. When used with the included passive probe, the input impedance increases to 10 Mohm, 10 pF while maintaining the 1 ns risetime at the probe tip. A switch on the S5 selects either ac or dc coupling of the input.