ToolsElectronics, cables & data
Matched attenuator pads (T, Pi, H, O)
Resistor values for impedance-matched T, Pi, H and O attenuators.
Ω
50 Ω or 75 Ω for RF and video, 600 Ω for legacy audio lines.
dB
- T-pad: each series resistor
- 311.696Ω
- T-pad: shunt resistor
- 421.637Ω
- Pi-pad: each shunt resistor
- 1154.97Ω
- Pi-pad: series resistor
- 853.815Ω
- H-pad (balanced T): each of four series resistors
- 155.848Ω
- H-pad: shunt resistor
- 421.637Ω
- O-pad (balanced Pi): each of two series resistors
- 426.907Ω
- O-pad: each shunt resistor
- 1154.97Ω
These pads keep the impedance matched on both sides, as needed in RF, video and 600 Ω systems. Modern audio uses voltage bridging instead; for a simple line or microphone pad, a voltage divider is usually enough. Use the H or O form for balanced lines.
How it works
A matched pad reduces the level while presenting the system impedance to both the source and the load. This matters where lines are terminated: RF, video and legacy 600 Ω audio.
Formulas
- K = 10^(A / 20)
- T: R_s = Z · (K − 1)/(K + 1), R_p = 2ZK / (K² − 1)
- Pi: R_p = Z · (K + 1)/(K − 1), R_s = Z · (K² − 1)/(2K)
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