At this year’s PCIM show, LEM has announced the completion of its 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.
These new components for nominal current measurements of 16, 32, 40, 50, 80, 100, 120, 150, 200, 250 ARMS in four different mechanical designs (PCB and panel mounting) provide up to 12-bit resolution with 20kHz bandwidth.
They also supply various possible digital outputs. These include, the single-bit output two wires CMOS (with clock in or out modes), RS422 Manchester or LVDS Manchester which minimizes the connections required. Or the four wires mode according to the LVDS or RS 422 (Clock in or out) standards.
The digital output allows the user to choose the filter used on the bitstream to optimize 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 negative or positive directions.
With HLSR-PW models 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 two pins, meeting the RS422 standard. The footprints of these sensors are the same as the analogue HLSR.
HO-NPW, HO-PW and HO-SW models can have the single-ended and Manchester modes as with the HLSR-PW models but they add two 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 five 12.5MHz to allow a single clock to be used throughout the system.
The new transducers can use a supply voltage of 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, for a ‘current control loop’ function: if a sinc3 filter is used with an over-sampling ratio (OSR) of 512 the effective resolution of a 150A sensor is 12-bit, and the bandwidth 5kHz.
Alternatively, for an ‘out of range detection’ function, a sinc2 filter with an OSR of 16 would give a response time of 5.2µs 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.
