Overview
A motor drive closes a control loop around a fast current measurement, and the accuracy and the speed of that measurement decide the smoothness, the efficiency and the reliability of the drive. ADI provides the sense amplifier, the converter and the power rails that the loop needs, and BeiLuo supplies them with genuine traceability and FAE support. This page shows how the parts fit together in a motor-control design.
The Current Sense
The first stage senses the phase current across a shunt or a sense element. A low-noise instrumentation amplifier such as the AD8421 recovers a small differential voltage that rides on a large, fast-switching common-mode voltage, so its common-mode rejection and its bandwidth are both important. The high rejection keeps the switching edge from corrupting the measurement, and the wide bandwidth reproduces the fast edge without distortion, which the control loop needs to react in time.
Choosing the Shunt and the Gain
Size the shunt for the current range and the power loss, then set the amplifier gain so the full-scale current fills the converter input. Keep the shunt leads symmetric and the gain resistor low-drift, and sense directly across the element with a Kelvin connection to avoid the trace resistance.
The Converter
The converter digitizes the current signal for the control loop. A fast pipelined part such as the AD9226 delivers a 12-bit result at 65 MSPS, which gives the loop enough resolution and enough speed to close cleanly, and the on-chip reference and sample-and-hold simplify the analog front end. The clock quality matters, because the loop timing follows it, so the clock is kept clean and jitter low.
Loop Timing
The converter latency and the amplifier bandwidth together set the achievable loop bandwidth. Choose both with margin over the switching frequency, and confirm the timing in the control firmware counts the converter latency correctly.
The Power Rails
A motor-control board needs several rails: a logic rail, a gate-drive rail and a quiet analog rail for the sense chain. A multichannel regulator such as the ADP5054 provides them from one input with an interleaved switching that spreads the noise, while a low-noise LDO such as the ADP7118 cleans the analog rail behind the switcher. A single-rail design can use the ADP2370 for efficiency. Keep the switching loop small, place the input and output capacitors close and separate the analog ground from the power ground.
Getting It Right
A motor drive performs well when the sense, the conversion and the rails are designed together. Sense the current with a fast, high-rejection amplifier, digitize it with a converter quick enough for the loop, and power the board from rails that keep the switching noise out of the measurement. BeiLuo supplies the ADI parts from stock, with the documentation to prove their origin and an FAE team that will review the current-sense and power layout with you.