Digital output added to open-loop Hall effect transducers

10th May 2016
Mick Elliott

Digital output versions of HO and HLSR open-loop Hall effect current transducers with analogue to digital (A/D) conversion performed by an on-board sigma-delta modulator, giving a 1-bit serial bitstream output have been launched by LEM. These new components for nominal current measurements of 10, 32, 50, 80, 100, 120, 150, 200, 250 ARMS in 3 different mechanical designs (PCB and panel mounting) provide up to 12 bit resolution with 20kHz bandwidth.

The single-bit output minimises the connections required, enabling highly compact transducers, and the digital output allows the user to choose the filter used on the bitstream to optimise between resolution and response time, according to the application.

Digital outputs are also intrinsically immune to noise in hostile environments.

For a typical transfer function the average bitstream density is 50% for zero primary current, and 10% or 90% for maximum currents in the negative or positive directions.

In the first samples available in one mode the clock is output from the sensor at 10MHz and both the clock and the data are single-ended signals with CMOS levels. Alternatively the output may be Manchester coded on 2 pins, meeting the RS422 standard. The footprints of these sensors are the same as the analogue HLSR and HO transducer families.

The new transducers can use a supply voltage of 3.3V or 5V and the operating temperature range is from -40°C to +105°C.

Several different filters may be used on a given bitstream. As an example: if a sinc3 filter is used with an over-sampling ratio (OSR) of 128 the effective resolution of a 50 A sensor is 12 bits, and the response time is about 38µs.…./…

Alternatively, a sinc2 filter with an OSR of 16 would give a response time of 4,6µs from the same bitstream, but the resolution would be reduced to 6 bits.Transducers in the HO family have additionally an Over-Current Detect (OCD) feature which measures the current level before the A/D converter. The response time of the OCD is 2us.

Future members of the family that are currently being developed will add 2 extra pins so both the clock and data may be differential signals that meet the RS422 and LVDS standards.

Additionally, the transducer clock may be configured as an input in the range 5 – 12.5 MHz to allow a single clock to be used throughout the system. The single-ended and Manchester modes available today will still be supported.

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