R_in / C_in — High-Z Probe Characterization

Works with any oscilloscope or measurement device — no assumed scope values. R_scope is measured directly (step 1A); C_scope cancels by differential sweep (step 2).

1 Measure R_in — Resistive divider

Two measurements, same R_serie and V_gen — only the load changes:
• 1A: load = R_scope only → measures R_scope directly
• 1B: load = R_probe ‖ R_scope → measures R_in_tot
R_probe = 1 / (1/R_in_tot − 1/R_scope)
No nominal scope values assumed. V_out_B < V_out_A because the probe adds parallel load.
Choose R_serie ≈ R_scope (e.g. 1 MΩ) for ~50 % attenuation and best sensitivity.

R_serie — series resistor, same for both measurements

MΩ

V_gen — before R_serie (DC / Vpk / Vrms)

1A No probe — measures R_scope

Connect R_serie → scope input directly. Do not touch probe tip to anything.
Measure V_out_A at the scope input node.

V_out_A — no probe (same unit as V_gen, e.g. if V_gen=1 → type 0.500)

1B With probe — measures R_in_tot

Connect probe tip to signal source. Same R_s, same V_gen.
V_out_B < V_out_A — probe in parallel reduces load impedance.

V_out_B — probe connected (same unit as V_gen, must be < V_out_A)

2 Measure C_in — Differential sweep

Two sweeps, same R_serie. C_scope + stray cancel in the subtraction.
Each R_par uses the actual load for that sweep:

2A C_A = 12π · f_A · R_{par_A} R_{par_A} = R_serie ‖ R_scope

2B C_B = 12π · f_B · R_{par_B} R_{par_B} = R_serie ‖ R_{in_tot}

C_probe = C_B − C_A

Target f_−3dB: 10 kHz – 300 kHz. For 1–2 pF probe: R_serie = 1 MΩ → f_−3dB ≈ 10–20 kHz.

R_serie — series resistor, same for both sweeps

kΩ

2A Sweep A — No probe (baseline)

Connect R_serie → scope input. No probe tip contact.
Sine sweep → find f_A where amplitude = 70.7 % of DC value (−3 dB).
⚠ Run step 1A first

f_A — measured −3 dB frequency (no probe)

kHz

2B Sweep B — With probe

Connect probe tip to source. Same R_s.
Sine sweep → find f_B (−3 dB). f_B < f_A — probe adds capacitance.

f_B — measured −3 dB (with probe)

kHz

R_in_tot from step 1B (auto-filled)

MΩ

3 Summary & |Z_in(f)| frequency response

Parameter	Value	Formula / source
Complete steps 1 and 2 to see the summary.

Complete steps 1 and 2 to draw the chart.

Rin / Cin Measurement — High-Z Probe

1 Measure Rin — Resistive divider

2 Measure Cin — Differential sweep

3 Summary & |Z_in(f)| frequency response

R_in / C_in Measurement — High-Z Probe

1 Measure R_in — Resistive divider

2 Measure C_in — Differential sweep