# dBFS to dBm Calculator (with Examples)

This tool converts deciBel Full Scale (dBFS) to deciBel milliwatt (dBm).

Enter:

• The worst spur level is the dBFS value. It is also specified in dB
• Max Peak-to-Peak voltage supported by the analog to digital converter
• Input impedance

The calculator then gives the highest spur level in dBm.

## Background

Spur Free Dynamic Range (SFDR) is defined as the ratio of the root-mean-square (RMS) value of the peak signal amplitude to the RMS value of the amplitude of the peak spurious. When running an Analog-to-digital Converter (ADC) evaluation board, you can observe these experimentally by computing an FFT on the collected time domain samples.

Most Analog-to-digital converter data sheets specify a peak-to-peak voltage. For example in the picture below.

If however, an RMS voltage is specified, then use this calculator to first convert from V RMS to V peak-to-peak.

### What is an RF Spurious Signal or Spur?

A Spur is an unwanted signal artifact or error. In any equipment – measurement, processing, transmission or reception it’s important to minimize spurs. If this is not done it will result in functional or measurement errors.

A spur level that’s the same as an expected signal in a receiver for instance, will result in a high level of interference and communication errors.

In general a design goal for an Analog-to-Digital converter is to maximize the value of dBFS and minimize the spur level in dBm. Ideally the spur level falls below the noise floor of the receiver although this is seldom the case.

## Example Calculation

For an 87 dBFS specification with a 2 Volt peak-to-peak input sinusoid and 50 ohm input impedance, the max spur level works out to -77 dBm.