A multi-stage passive capture adapter (PCA) circuit is configured to sense and recover digital signals present on a high-speed serial bus for capture and analysis in external test equipment. A first stage of the PCA circuit includes a differentiator that functions as a high impedance probe that contacts the serial bus to capture an original input signal waveform of the high-speed digital signals. The signal waveform is fed to a dual-slope comparator/driver that includes a plurality of high-speed comparators and drivers. The second stage includes a differential receiver/shaper that converts logic levels of differential receiver outputs to input signals that set and reset a signal restorer whose output signals are fed to a driver of a driver/shaper. The output of the driver is then fed to a shaper network configured to substantially match an output signal of driver/shaper to the input signal waveform sensed from the high-speed serial bus.