Planning a DAC Reconstruction Filter From Its Nearest Image

A DAC reconstruction filter must preserve the wanted output while attenuating replicas created by sampling. Designing it only around the sample clock frequency can miss the nearest unwanted image. Begin with the maximum desired output frequency and the actual DAC update rate, including any interpolation mode.

Draw the output frequency map

The Analog Devices discussion of mixed signal communications converters places DAC images around multiples of the sampling frequency. For a baseband output extending to a maximum frequency, the nearest image begins at the sampling frequency minus that maximum.

Keep clock feedthrough, harmonics, and sampling images as separate entries. Their frequencies and amplitudes need not follow the same mechanism, even when a spectrum contains several nearby peaks.

Calculate the available transition

Consider a hypothetical DAC updating at 100 MSPS with wanted content through 20 MHz. The nearest first image begins at 80 MHz, leaving a 60 MHz interval between the wanted band edge and that image edge. If the wanted band extends to 45 MHz, the corresponding interval narrows to 10 MHz.

This calculation establishes frequency space, not the required filter order. Specify allowed passband loss and required image attenuation before choosing the response. Include the DAC’s output mode and spectral envelope in the complete amplitude budget.

Review interpolation and tuning modes

If digital interpolation changes the effective update rate, use the rate and image suppression applicable to that mode. Confirm which spectral components the digital filter suppresses and which remain for the analog network.

Evaluate the entire tuning range. A filter that works well for a low frequency demonstration tone may not provide the required rejection when the output approaches the upper end of its allowed range.

Measure the installed response

Check passband amplitude and image levels with the actual source and load impedances. Extend measurement beyond the first image where the receiver remains sensitive. Real inductors, capacitors, and layout paths can limit high frequency rejection compared with an ideal filter calculation.

Save sample rate, interpolation configuration, wanted band, image map, and measured attenuation. Identify the highest output frequency tested and the measurement dynamic range. A valid reconstruction filter result demonstrates that wanted content and unwanted replicas satisfy separate limits across the operating modes. Merely reducing a peak at the clock frequency does not establish that the nearest image has been removed or that the reconstructed waveform meets its spectral requirement. Include each permitted sample clock setting in the acceptance frequency map.