Custom Leads

A project log for A 10X 100MHz Differential Probe

A DIY oscilloscope probe to dig small differential signals embedded in high common mode voltages.

Bud BennettBud Bennett 02/12/2020 at 16:540 Comments

The inductance of a 3 inch long 26AWG wire is 1.23µH. It plays havoc with the diff probe if not properly accounted for. I ginned up a simple circuit to help me visualize how to dampen the ring associated with the lead:

A small value of series resistance in the lead will remove most of the ringing caused by the lead inductance. The largest value of series resistance that can be inserted into a 3 inch long lead is 620Ω. Any more than that will reduce the bandwidth of the differential probe. There is also a small, 0.006%, reduction in gain.

With 620Ω, the response to a 1V input step with a 3.5ns risetime is:

There is a 10% overshoot in the probe's response to this fast step. Clearly, if you need fidelity on signals with very fast rising edges the lead length must be as small as possible. But if the rise time is reduced to 10ns the overshoot decreases to only 3.5%:

I'll be making couple of sets of custom leads, with unique values of series resistance. There is no need to add any resistance to the PCB. The lead set to use will depend upon the characteristics of the signal to measure.