Fully-isolated, high-accuracy, Σ-Δ modulator boosts motor-drive efficiency

May 23, 2014 // By Graham Prophet
AD7403 is an isolated sigma-delta modulator that achieves the most-accurate-available current and voltage measurements; you can use it instead of Hall Effect sensors in higher DC-bus voltage power conversion and motor drive applications.

The AD7403 isolated sigma-delta modulator achieves an 81-dB signal-to-noise and distortion ratio (SINAD) (min) @ 78 ksamples/sec over a -40C° to +125C° temperature range, which is claimed to be 11 dB higher than competing devices. The higher SINAD enables more accurate current and voltage measurement which improves the performance of motor drives by reducing torque ripple on the motor shaft. The higher SINAD also increases the power transfer efficiency in DC/AC power inverter applications, such as motor drives, solar and wind inverters.

According to an ADI spokesman, the part moves achieved accuracy into the 15-bit region, with corresponding benefits in noise and dynamic range performance – all with on-board, 1250V peak magnetically-coupled isolation.

The AD7403 isolated sigma-delta modulator integrates ADI’s iCoupler digital isolation technology, creating a single-chip solution for measuring the voltage across a shunt resistor by converting an analogue signal into a 1-bit data stream that can be clocked at up to 20 MHz. The increased isolation working voltage (1250 V peak) allows designers to replace less accurate, more costly Hall Effect sensors in higher DC-bus voltage power conversion and motor drive applications. The higher performance of the isolated sigma-delta modulator enables the use of a smaller shunt resistor, which reduces thermal loss across the shunt while retaining measurement accuracy.

Features include;

5-MHz to 20-MHz external clock input rate

16 bits, no missing codes

Typical offset drift: 1.5 μV/°C

On-board digital isolator

On-board reference

True bipolar analogue input range: ±320 mV

High common-mode transient immunity: >25 kV/μsec

In a 16-lead, wide body SOIC package for 8.3mm creepage and clearance distances, the device costs $2.95 (10,000).

Analog Devices; www.analog.com/AD7403