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## Re: [Discuss-gnuradio] ADC Specs

**From**: |
Scott A. McDermott |

**Subject**: |
Re: [Discuss-gnuradio] ADC Specs |

**Date**: |
Mon, 15 Dec 2008 17:17:44 -0500 |

**User-agent**: |
Thunderbird 2.0.0.14 (Windows/20080421) |

>* "The full range on the ADCs is 2V peak to peak, and the input is 50 ohms*
>* differential. This is equates to 40mW, or 16dBm."*
>* *
>* Please may someone enlighten me, how the 40mW and 16dBm values were*
>* calculated?*
Hello Aadil ---
The impedance (effective resistance to ground) seen from the input pin
on the USRP coax, is 50 ohms. The calculation you quote assumes a
sinusoidal input at Vpp = 2 Volts peak-to-peak, which means the root
mean square amplitude is Vrms = (Vpp/sqrt(2)) = (2/sqrt(2)) = sqrt(2).
This RMS voltage is going through a 50 ohm resistor, so the power
dissipated is Vrms^2/R = (sqrt(2)^2)/50 = 2/50 = 0.04 Watts, or 40
milliWatts. To convert linear mW to logarithmic (decibel) mW, or dBm,
we calculate 10*log(40) = 16dBm.
That's the short answer, please feel free to ask for more detail.
--Scott McDermott