20. ANALOG CIRCUIT CALIBRATION

The Helios data acquisition circuit contains an advanced autocalibration circuit that can maintain the accuracy of both A/D and D/A circuits to within the specified tolerances regardless of time and temperature. Autocalibration is supported in the Diamond Systems Universal Driver software included with the board.

The autocalibration circuit uses an ultra-stable +5V reference voltage IC as the source for its calibration. Both A/D and D/A circuits are calibrated in the analog domain by using a series of 8-bit “TrimDACs” to adjust the offset and gain settings of the circuits. The data values driving the DACs are stored in an EEPROM and are loaded automatically each time the board powers up.

During the autocalibration process, the board will measure the on-board reference and calibrate the A/D circuit by adjusting the TrimDACs to achieve the best accuracy. Once the A/D circuit is calibrated, the D/A circuit is calibrated by routing the D/A outputs into the A/D converter and adjusting them as well. The new calibration values for the TrimDACs are stored back into the EEPROM so they can be automatically recalled thereafter.

A unique feature of Diamond’s autocalibration process is that each analog input range is individually calibrated for optimum performance. Analog amplifier circuits with 16-bit accuracy exhibit gain and offset errors that vary depending on the gain setting. The settings that work best for one range may not be sufficient to calibrate another. If a circuit is calibrated for maximum accuracy in a particular input range, such as +/-5V, changing the input range to +/-10V or 0-2.5V may introduce errors that exceed the resolution of a 16-bit measurement and will require calibration again.

To counteract this phenomenon, Diamond’s autocalibration circuit provides for a separate complete set of calibration settings for each analog input range. During the autocalibration process, each range is calibrated one at a time, and its set of calibration settings is stored in a separate area of the EEPROM’s memory. One of these ranges is identified as the “boot range”, and this range’s calibration values are the ones that are automatically recalled during power-up. You have the option of specifying the boot range, which should be chosen as the range most commonly used in your application. When you change the input range, you have the option of loading the calibration values for the new input range to maintain optimum accuracy of your measurements.

The autocalibration process is triggered with a single function call in the Diamond Universal Driver software. The process takes about 10-20 seconds to calibrate the complete set of analog input ranges and about the same time for the D/A circuit. Autocalibration can easily be incorporated into your application program, so that you can calibrate the data acquisition circuit as often as necessary while your system is running.