Pi and T Attenuator Calculator (Resistive Pad Values)
The resistor values of a matched T or pi attenuator for a given attenuation and impedance, and the power the pad has to dissipate.
Results
The dashed line marks the value you entered.
What this tool does
A resistive pad lowers a signal by a set number of decibels while keeping the same impedance at both ends, so it can be dropped into a 50 Ω or 75 Ω line without causing reflections. With K = 10^(dB ÷ 20), the T pad needs two series resistors Z(K − 1)/(K + 1) and one to ground 2ZK/(K² − 1); the pi pad is the same network drawn the other way round. A 6 dB pad in 50 Ω uses about 16.6 Ω and 66.9 Ω (T), or 150.5 Ω and 37.4 Ω (pi). Real resistors have tolerances, so the match is a little worse than the numbers, and the pad must be rated for the power it absorbs.
Formula
K = 10^(A ÷ 20) · T pad: R₁ = Z (K − 1) ÷ (K + 1), R₂ = 2 Z K ÷ (K² − 1) · pi pad: R₃ = Z (K + 1) ÷ (K − 1), R₄ = Z (K² − 1) ÷ (2K)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
att | Attenuation wanted | dB |
z | Impedance of the source and of the load | Ω |
pin | Power going into the pad | W |
R1 | T pad: each series resistor | Ω |
R2 | T pad: the resistor to ground | Ω |
R3 | Pi pad: each resistor to ground | Ω |
R4 | Pi pad: the series resistor | Ω |
PH | Power the pad turns into heat | W |
Worked example
- Attenuation wanted6 dB
- Impedance of the source and of the load50 Ω
- Power going into the pad1 W
- T pad: each series resistor16.61 Ω
- T pad: the resistor to ground66.93 Ω
- Pi pad: each resistor to ground150.48 Ω
- Pi pad: the series resistor37.35 Ω
- Power the pad turns into heat0.749 W
Limitations
- Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.
- The calculation runs at full precision and only the display is rounded. If you copy an intermediate value and retype it, small differences can appear.