Planning an ADC Anti-Alias Test Around Known Interferers

An unwanted tone can appear inside a sampled measurement band even when its original analog frequency lies outside that band. An anti-alias review should therefore begin with plausible interference sources and the actual sampling architecture. Looking only at the desired signal bandwidth leaves an important path untested.

Map the frequencies of concern

Analog Devices tutorial MT-002 explains sampling, aliasing and the relationship between sampling rate and analog filter requirements. Its discussion provides a basis for deciding which out-of-band frequencies can fold into a useful baseband.

List switching converters, clocks, external transmitters and sensor disturbances that can reach the analog input. Record their frequency ranges and credible amplitudes. Include tolerances and operating modes that move the interferer rather than treating each source as one exact nominal tone.

Create an illustrative alias map

For a simple sampler running at 10 kS/s, a 9 kHz input tone can appear at 1 kHz in the sampled baseband. If the useful signal includes 1 kHz, digital filtering after that sampling event cannot identify the tone merely from its apparent frequency.

Use the converter’s real architecture when building the final map. A device with internal oversampling and decimation has more relevant rates and filter responses than its output word rate alone suggests. Read its analog-input and digital-filter documentation before choosing test frequencies.

Translate the error budget into attenuation

Define the largest allowed unwanted component in the reported measurement. Compare it with the estimated disturbance at the filter input. Use their ratio to determine the required attenuation at each relevant frequency, then include component tolerances and source impedance.

Check passband amplitude, phase and settling requirements at the same time. A filter that strongly rejects interference may still be unsuitable if it distorts the desired signal or takes too long to settle after a channel change. Keep both requirements visible.

Inject and observe deliberately

Apply controlled test tones within the circuit’s allowed input range and observe their sampled aliases. Sweep around the predicted frequencies rather than testing only one nominal point. Confirm that the source and measurement setup do not generate additional harmonics that confuse the result.

Record filter configuration, sampling clocks, input amplitude and observed in-band response. Repeat for product modes that change the sampling rate or interference spectrum. This produces evidence that the analog path suppresses the disturbances the product can encounter, rather than relying on a generic statement that its RC network is an anti-alias filter.