Introduction

A precision signal chain is a system, and its accuracy depends on how the stages fit together rather than on any single part. The sensor, the amplifier, the reference, the converter and the supply each contribute a term to the error budget, and the layout decides whether the promised performance is reached. This application note explains the practical rules for designing an ADI analog front end.

Budgeting the Noise

Start from the sensor and work forward. Refer every stage's noise back to the input and add the terms to get the total input-referred noise, which sets the smallest signal the chain can resolve. Keep the first stage quiet, because its noise is amplified by the following gain, and set the gain so that later stages contribute little. A high-resolution converter behind a noisy front end gives no more usable resolution than a low-resolution part, so the front end and the converter are chosen together.

Offset and Drift

The offset and the offset drift set the DC accuracy, and a low-frequency measurement is limited by the drift. A zero-drift amplifier such as the ADA4522-2 removes the slow error, and a low-drift reference protects the absolute accuracy over temperature.

The Reference and the Drive

The reference sets the absolute scale of the conversion, so choose a low-drift, low-noise reference or use an on-chip reference where the accuracy allows, and decouple it close to the pin. Drive the converter from a low-distortion amplifier through a small network, keep the source impedance balanced and place the network close to the converter pins, because the dynamic performance depends on both the drive and the clock quality.

The Clock and the Timing

A fast converter's dynamic performance depends on the clock, so keep the aperture jitter low and the clock clean. Confirm the timing margin between the converter, the host and any digital filter, and count the converter latency correctly in the control loop.

Grounding and Layout

Return the analog and the digital currents separately to a single point, keep the digital switching away from the analog inputs and avoid running the switching return through the sensitive area. Place the anti-alias filter and the decoupling close to the converter, guard the high-impedance nodes and keep the reference trace short and quiet. The layout decides the final result as much as the parts, so it deserves attention from the first draft.

Getting It Right

A signal chain meets its accuracy when the noise, the offset, the reference and the layout are managed together: keep the first stage quiet, budget the noise and the drift stage by stage, use a clean reference and drive and return the analog and the digital currents separately. Confirm the result with a measurement rather than an estimate, because the noise and the grounding behavior appear only in the real board. BeiLuo supplies the ADI signal-chain parts from stock and will review the error budget and the layout with you.