A delta-sigma ADC can output samples fast enough for a signal and still attenuate that signal inside its digital filter. Output data rate is not a statement of flat passband. Sinc filters commonly used in precision converters have a gradual response near DC and notches related to their decimation settings.
Use the selected filter response
Analog Devices compares selectable sinc filters in AN-0979. Filter order changes attenuation, noise, and settling together. Obtain the response for the actual data rate, clock, filter option, and any postfilter. A generic sinc curve may be incomplete when the converter combines several stages or uses a special fast-settling mode.
For an idealized high-decimation sinc-cubed response, the amplitude factor near baseband is approximately [sin(pi f/fout)/(pi f/fout)] cubed. At one-tenth of the output rate this is about 0.952, or roughly minus 0.43 dB. The calculation is an example of potentially meaningful passband droop, not a specification for every converter. Compare with the exact transfer function before correcting real data.
Measure the complete amplitude chain
Apply a stable sine wave and sweep frequency while maintaining amplitude at the ADC input. Normalize against a low-frequency point after the system has settled. Include the analog RC filter and driver in the model; an observed roll-off may contain both analog and digital contributions. Verify that neither input amplitude nor common-mode range changes across the sweep.
| Choice | Tradeoff to examine |
|---|---|
| Higher output rate | Less in-band droop but different noise and data volume |
| Lower filter order | Different rejection and settling behavior |
| Digital amplitude correction | Noise amplification and model dependence |
| Narrower measurement band | Reduced droop with reduced application bandwidth |
Keep calibration tied to configuration
A DC gain calibration does not remove frequency-dependent droop. If a correction table is used, version it with the converter settings and include its uncertainty. Check phase response separately when combining channels or calculating real power. A magnitude-only correction is not a universal timing correction.
Combine the passband check with conversion latency scheduling and clock-dependent line-frequency rejection. State the frequency band over which the corrected or uncorrected system meets its amplitude requirement, rather than quoting only sample rate and nominal bit depth.
Move from reading to a build-ready project
Use the fabrication and assembly checklists to prepare drawings, production files and test requirements.
Open the DFM checklist