Mapping Audio ADC Input Voltage to Digital Full Scale

An audio ADC’s digital full scale is a numerical reference, while its analog input range is a voltage under specified supply and bias conditions. Connecting the two correctly prevents an input pad or preamplifier from wasting dynamic range or clipping unexpectedly. Write the conversion before selecting gain.

Read the analog range definition

Check whether the data sheet specifies peak, peak to peak, or RMS voltage, and whether it describes one input or a differential pair. Include the required common mode voltage and supply condition.

For example, the TI PCM1808 data sheet lists a nominal 3 V peak to peak full scale input at a 5 V analog supply. That is a device specific reference, not a general definition of line level or the maximum voltage every audio ADC accepts.

Convert the waveform consistently

For an undistorted sine wave, RMS voltage is peak to peak voltage divided by twice the square root of two. Thus 3 V peak to peak is approximately 1.061 V RMS. Other waveform shapes need their own relationship between peaks and RMS value.

If the measurement convention defines that full scale sine as zero dBFS, a sine at half its amplitude is about minus 6.02 dBFS. Verify the instrument’s convention because some digital RMS displays use a different reference from a peak referenced meter.

Build the complete gain map

Start at the external connector and include attenuation, analog gain, ADC gain settings, and any digital scaling before the displayed meter. Record each stage’s maximum signal and bias separately.

For a source with occasional peaks above its nominal level, reserve analog headroom before the converter. Reducing digital gain after conversion changes the displayed value but does not repair samples clipped at the analog input or converter.

Check with a known sine

Apply a low distortion sine comfortably below full scale and measure the actual voltage at the ADC input. Compare the captured sample amplitude with the expected mapping. Increase in controlled steps while monitoring harmonic growth and peak codes, staying within the device’s permitted input conditions.

Repeat for every supported input gain mode. If the voltage and code relationship disagree, inspect gain configuration, channel routing, data alignment, and meter reference before changing the analog circuit.

Save connector voltage, ADC pin voltage, supply, gain state, waveform type, and digital amplitude definition. Include the intended overload margin in the released configuration. A clear conversion table allows hardware and software measurements to describe the same signal; a number labeled dBFS alone cannot identify how much analog headroom remains at the board connector.