Sensor ADC Acquisition: Source Impedance, Aliasing, ENOB, and Resolution

Code Range and Engineering Resolution

Nominal bit depth describes available codes, not how many the sensor uses. A 0.5–4.5 V sensor on a 0–5 V ADC occupies about 80% of the input range. Preserve unrounded values internally and apply calibration before formatting engineering units.

Use the Sensor ADC Resolution Calculator for usable codes, ideal LSB, ENOB references, and engineering resolution.

Acquisition Time and Source Impedance

Many SAR and MCU ADC inputs connect a sampling capacitor through a switch. During acquisition, the source must settle from the previous channel voltage to the required error. Datasheet maximum source resistance assumes specific acquisition time, capacitance, and resolution.

A buffer can lower drive impedance, but it must remain stable with the ADC input network and settle after charge kickback. An RC network can supply transient charge and filter noise; verify both op-amp stability and inter-channel settling.

Reference, Ratiometric Measurement, and Range

• The reference sets ADC scale, so its initial accuracy, noise, temperature drift, load regulation, and decoupling enter the measurement.
• Ratiometric excitation can cancel first-order supply variation when the sensor output and ADC reference share the same excitation ratio.
• This cancellation does not remove reference noise, bridge self-heating, amplifier error, ADC nonlinearity, or timing mismatch.
• Allow headroom for sensor tolerance, diagnostic overrange, amplifier output swing, reference tolerance, and protection clamp behavior.

Sampling and Aliasing

Sampling above twice the desired signal frequency is necessary but not sufficient. Sensor noise, PWM edges, mains interference, clocks, and amplifier noise above Nyquist can alias into the measurement band. The analog anti-alias filter must provide enough attenuation at the actual sample rate and interference spectrum.

ENOB, Noise, and Accuracy

ENOB is a dynamic test result, commonly derived from SINAD for a sine-wave test. It is not automatically the number of noise-free DC bits. For slow sensors, code spread, peak-to-peak noise, drift, offset, and calibrated residuals may be more useful. State bandwidth and test conditions whenever quoting resolution.

Calibration and Verification

1. Record raw codes at grounded, reference, and traceable sensor-equivalent inputs.
2. Sweep the full range to measure offset, gain, INL, missing codes, and residuals.
3. Repeat with worst-case source impedance, channel order, and acquisition time.
4. Measure code distribution and spectrum at the actual sample/filter configuration.
5. Inject out-of-band interference to verify anti-alias attenuation.
6. Repeat over temperature, supply, reference load, and sensor warm-up.
7. Verify fault inputs and recovery without hiding raw diagnostic codes.

Use the Sensor Calibration Calculator for slope, offset, residual, and inverse-fit checks; use the Analog Sensor Linear Scaling Calculator for signed endpoint conversion.

Common Mistakes

• Calling nominal bit depth measurement accuracy.
• Assuming the sensor uses every ADC code.
• Ignoring sampling-capacitor settling and source impedance.
• Adding a buffer without checking capacitive-load stability.
• Treating sample rate alone as an anti-alias filter.
• Using ENOB without its bandwidth and input conditions.
• Expecting averaging to remove drift or INL.
• Ignoring reference noise and temperature drift.
• Reusing rounded display values in calibration math.
• Clamping fault codes before diagnostics.

Scope Boundary

This guide provides system-level acquisition design principles, not a universal ADC driver equation. Switched-capacitor behavior, differential inputs, reference drive, isolation, sigma-delta settling, and digital filters are device-specific. Verify critical designs with the converter datasheet, driver models, simulation, code-density or spectral tests, and calibrated hardware.