Checking Reference Startup After Adding a Noise-Reduction Capacitor

A capacitor added to reduce reference noise can also change the time required for the reference to become accurate after power-up. In a duty-cycled instrument, that delay can affect both the first measurement and the energy budget. Review startup accuracy alongside the steady-state noise improvement.

Identify the capacitor’s function

The REF50xx data sheet describes the TRIM/NR connection and notes the startup-time effect of its noise-reduction capacitor. This component has a different role from ordinary supply bypassing or the reference’s output capacitor.

Record the selected reference, capacitor value, tolerance and connection. Follow the part’s documented guidance for the noise-reduction pin and output loading. Do not interchange recommended capacitor locations simply because each appears to connect a reference circuit to ground.

Define when accuracy is required

Specify the allowed reference error at the first retained conversion. Choose a measurement criterion tied to the product requirement, rather than merely waiting until the reference crosses a broad percentage of its nominal value. A voltage that looks settled on a full-scale display can still contain an important precision error.

Include the reference load during startup. An ADC, DAC or buffer can begin drawing current before the reference has reached its final operating state. Record the enable order and any load changes associated with the first conversion.

Measure the complete recovery history

Capture the reference after a long power-off interval and after the shortest permitted off interval. Residual charge can make those cases different. Include the minimum supply ramp and temperature conditions expected in the product, with the intended capacitor fitted.

Compare the first measurements with a later stable value using a method capable of resolving the specified error. Keep probe loading and instrument drift in the uncertainty assessment. A conventional oscilloscope trace can show sequence timing while a more sensitive method evaluates the final precision settling.

Connect delay to energy and firmware

Measure the improvement in steady-state noise and the additional time spent waiting for an accurate reference. Calculate energy over the full wake-measure-sleep cycle. The lowest continuous noise setting is not automatically the best setting for a product that takes brief, infrequent readings.

Document the validated delay, reference-loading state and capacitor option together. Make firmware timing changes traceable to that record. Repeat the check when the capacitor value, reference source or converter sequence changes. This preserves the intended relationship between power control and measurement accuracy and avoids treating the reference’s steady-state specification as a guarantee that its first available output voltage is already ready for precision conversion.