Precision interpolation circuit for 16-bits with automatic correction

February 15, 2013 // By Julien Happich
The high-resolution signal processor iC-TW8 released by iC-Haus is used to evaluate sine/cosine sensors, minimizing angular errors and jitters with automatic functions. It is also utilized for initial, push-button calibration and to permanently adapt signal path parameters during operation.

The angular position is calculated at a programmable resolution of up to 65,536 increments per input cycle and output as an indexed incremental signal. A 32-bit word including the counted cycles is available through the SPI.

As an application-specific DSP, iC-TW8 has two analog/digital converters that sample simultaneously at a rate of 250 kSPS, fast CORDIC algorithms and special signal filters, and an analog front end with differential PGA inputs which accepts typical magnetic sensor signals from 20 mVpp upwards. Signal frequencies of up to 125 kHz permit high rotary and linear speeds for position measuring devices and are processed at a constant latency period of 24 µs.

No measuring tools are needed to use the 12-bit measurement accuracy provided by the device. The device learns the necessary signal corrections for the offset, amplitude, and phase errors independently, storing these in an external EEPROM. If these start values alone are not sufficient, for example with changes in amplitude or an offset drift across the temperature, permanent automatic corrections can be selected. In addition, for sine signals distorted by harmonics a table with 64 breakpoints allows the angular data to be statically corrected.

The iC-TW8 provides incremental encoder quadrature signals of up to 8 MHz, where the maximum output frequency can be limited to guarantee countability across large edge distances. An analog or digital reference mark sensor, such as an MR, GMR, or Hall sensor, can be connected up for the electronically generated, programmable zero signal.

PWM signals and serial interfaces are provided for controller applications (1-wire interface, and an SPI interface for clock rates of up to 32 MHz). The 32-bit position register supplies the angle data and period count; a second register provides information on the current angular speed.

The chip has two configuration modes. Preset functions and interpolation factors can be retrieved through pins and the device calibrated at the push of a button. No programming is required