Engineer Christoph Redecker has created a custom 1x 100MHz 35V common-mode differential probe for his oscilloscope, to aid with ESC development — building on an earlier design by Bud Bennett.
"In BLDC motor ESC development, it's sometimes necessary to look at differential signals," Redecker explains. "My DS1054Z can do that with two input channels and math, but the results are atrocious. So instead of picking one from a catalogue and ordering it, I adapted Bud [Bennett]'s design and built one."
"The differential signals I'm looking into are a BLDC motor's feedback voltages and its virtual center, divided down to 3V3 (absolute) or 1V2 (differential) for a differential ADC with 1V2 reference. So I asked Bud if he'd help me with adapting his probe to my needs, and bud was very helpful."
The probe is designed to fix an issue with trying to show the difference between two signals — which results in an incredibly noisy display, and ties up two input channels. Using the differential probe, only one input is required for the probe's input — while the represented signal is dramatically improved.
"Supply voltage to motor: 5V, duty cycle about 60%," Redecker writes of a screenshot showing the oscilloscope display while using the probe. "Ch1: 3-state PWM input to motor phase A; Ch2: bemf from phase A, through a 10k/2k2 voltage divider; Ch3: virtual center (all phase feedback signal tied to a common node through 2k2 resistors); Ch4: Ch2-Ch3, using the diff probe."