Power Electronics

Revving Up Data Rates for More Accurate Motor Control

Three data converters developed by Analog Devices for motor control applications enable more precise positioning of motor shafts because of their fast sampling capabilities. Intended for current sensing on motor inputs, the AD7400 and AD7401 are isolated sigma-delta ADCs that provide 12-bit linearity at up to 20 Msamples/s. Two other devices provide different options for servo feedback, while also providing some improvement in system accuracy or integration of features. The AD7266 is a simultaneous sampling dual-channel, 12-bit SAR ADC that boasts 2-Msample/s performance. The AD2S1200 is a complete monolithic resolver-to-digital converter that offers speeds up to 1000 rps and accuracy up to ±11 arc minutes (see the figure).

The AD7400/7401 are 2-chip devices that provides high-speed A/D conversion and isolation in a single package — a 16-pin SOIC. The ability of the AD7401 to offer 20 Msample/s conversions across an isolation barrier is said to exceed that which is commonly achieved through the use of external optocouplers. Although there are 2-die solutions combining optocouplers and data converters in the same package, those introduced so far offer lower data rates.

In the AD7400, the company applies its iCoupler isolation technology, which is based on planar magnetics rather than optocouplers. This technology provides 3.75 kV of isolation, while limiting current consumption for these CMOS devices to a low 25 mA max. (15 mA typ.) at 5 V.

As with other sigma-delta modulators, the converter's linearity decreases with sample rate. The AD7400/7401 provide 16 bits of linearity at slower speeds. Two other specs indicate the ADC's high-level of dc accuracy: Integral linearity is just 1 LSB at 16 bits, while input offset drift is just 30-ppm/°C. The AD7400, which features an internal clock, is limited to data rates of 10Msamples/s. The faster AD7401 uses an external clock to synchronize multiple converters. Both parts are available in 16-pin SOPs and are priced at $4 in 1000-piece quantities.

With the AD7266, the SAR ADC's 2-Msample/s speed offers an advantage over comparable dual-channel simultaneous sampling SARs, and the higher speed translates to more precise location of shaft position in the application. The company quotes 1 LSB INL at 12 bits for this converter. In addition, the part consumes a maximum of 20 mW, said to be less than half of the next-fastest competitor. The AD7266 and the AD7265 (a 1-Msample/s version) are offered in 32-lead LFCSP or TQFP packages.

The AD2S1200 resolver-to-digital converter is distinguished by its integration of many features otherwise not found in a monolithic device. It contains a 12-bit resolution-tracking resolver, a programmable sinusoidal oscillator, a fault-detection circuit that detects any loss of resolver signals, out-of-range input signals, input signal mismatch or loss of position tracking. The chip communicates via a 12-bit parallel port or 3-wire serial interface.

In addition, the resolver-to-digital converter offers an incremental encoder emulation output with 1024 pulses per revolution with additional directional pulse, eliminating the need for external A and B directional decode logic. Available in a 44-lead LQFP, the AD2S1200 is priced starting at $12 in 1000 piece quantities.

All of these data converters are now sampling. The ADS1200 currently is in full production, and the company expects to be in full production with the other devices late this year.

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