Question E4A09:
Which of the following is good practice when using an oscilloscope probe?  

Choices:  
A: Minimize the length of the probe's ground connection  
B: Never use a high-impedance probe to measure a low-impedance circuit  
C: Never use a DC-coupled probe to measure an AC circuit  
D: All these choices are correct    

Correct Answer is  A: Minimize the length of the probe's ground connection  

Correct Answer: A — Minimize the length of the probe's ground connection

1) Why A is correct
- The probe’s ground lead has inductance. A long ground lead plus the probe capacitance and circuit impedance can create ringing, overshoot, and inaccurate measurements, especially on fast edges or RF.
- Inductive reactance increases with frequency, so even small inductance becomes significant at HF/VHF/UHF.

Key formulas and rule-of-thumb:
- Ground lead inductance: roughly 5–10 nH per cm.
- Inductive reactance: $$X_L = 2\pi f L$$
- Example: 10 cm ground lead at 10 nH/cm → $L \approx 100\,\text{nH}$
  - At $10\,\text{MHz}$: $X_L \approx 2\pi(10^7)(100\times10^{-9}) \approx 6.3\,\Omega$
  - At $100\,\text{MHz}$: $X_L \approx 63\,\Omega$
- Practice: Use the shortest possible ground (e.g., the probe’s spring ground), or a coax pigtail/ground sleeve, to minimize loop area and inductance.

2) Why the other choices are incorrect
- B: “Never use a high-impedance probe to measure a low-impedance circuit”
  - Incorrect. High-impedance (10×) probes are designed to minimize loading and are commonly used on both high- and low-impedance circuits. A low-impedance source will generally drive a high-impedance probe just fine.
  - Capacitive loading matters more at high frequency: $$X_C=\frac{1}{2\pi f C}$$
    Example: $C \approx 15\,\text{pF}$ at $10\,\text{MHz}$ → $X_C \approx 1.1\,\text{k}\Omega$ (small effect on low-Z nodes).
- C: “Never use a DC-coupled probe to measure an AC circuit”
  - Incorrect. DC coupling works for AC; AC coupling is optional when you want to remove DC offset. AC coupling introduces a high-pass effect that can distort low-frequency content.
  - High-pass cutoff (scope AC coupling): $$f_c=\frac{1}{2\pi R_{\text{in}} C_{\text{coupling}}}$$
  - Therefore “never” is wrong—choose coupling based on whether you need to block DC.
- D: “All these choices are correct”
  - False because only A is correct.

3) Useful rules of thumb
- Keep the probe ground lead as short as possible; prefer a ground spring for HF/RF or fast edges.
- Use a 10× probe to reduce circuit loading; be mindful of probe capacitance at high frequency.
- Choose AC coupling only when you need to remove DC; otherwise use DC coupling to preserve low-frequency content.