Auto-Calibration of Advantech's CompactPCI® and DAQ Cards

Since most passive components such as resistors, capacitors and indicators change characteristics over time; calibration is necessary to ensure accuracy. Traditionally this has been accomplished with the help of variable resistors (VR).

To perform a calibration, use a 4.5-digit digital multi-meter and a voltage calibrator, or a stable, noise-free DC voltage source. Regular and accurate calibration procedures ensure maximum accuracy.

A calibration program can be used as guidance through the whole A/D offset and gain adjustment procedure. The basic steps where PCI-1710 (12-bit resolution) is used as example, are described below:
1. Set the analog input channel AI0 as single-ended, bipolar, range ±5 V, and set AI1 as single-ended, unipolar, range 0 to 10 V.
2. Connect a DC voltage source with a value equal to 0.5 LSB (-4.9959 V) to AI0.
3. Adjust VR2 until the output signal from the card’s AI0 flickers between 0 and 1.
4. Connect a DC voltage source with a value of 4094.5 LSB (4.9953 V) to AI0.
5. Adjust VR3 until the output codes from the card’s AI0 flickers between 4094 and 4095.
6. Repeat step 2 to step 5, adjusting VR2 and VR3.
7. Connect a DC voltage source with value equal to 0.5 LSB (1.22 mV) to AI1 (using a precision voltmeter to calibrate the D/A outputs is recommended).
8. Adjust VR1 until the output signal from the card’s AI1 flickers between 0 and 1.

In most cases it is difficult to find a good enough DC voltage source for A/D calibration. This is why Advantech provides a self-adjustable A/D calibration program. The steps of self-calibration are then simplified:
1. Connect DA0_OUT to AI0.
2. Connect AGND to AI1, AI2, AI3, AI4 and AI5.
3. Run the self-calibration program.
4. First calibrate the D/A channel. Adjust VR3 until the DA0_OUT output voltage approaches +10V.
5. Perform A/D calibration. When the DA0_OUT output voltage is +9.995V, adjust VR2 until the output signal from the card’s AI0 is focused on FFE (at least 70%) and adjust VR1 until the output signal from the card’s AI1, AI2, AI3, AI4 and AI5 are focused on 7FF (at least 70%).

To simplify the calibration procedure, Advantech has built-in a microprocessor and a custom circuit on its cards that works together with the calibration utility. All that is required for calibration is to launch the calibration utility, and the procedure is completed. Moreover, all parameters will be stored in the EEPROM, so no more variable resistors are needed. No precise DC voltage source is needed, and it is not necessary to open the chassis. The VR’s characteristics also change over time, so getting rid of this component ensures even more accurate calibration.

The benefits of Advantech’s auto-calibration are primarily time and costs, but it can also avoid common operating mistakes. Furthermore, maintenance efforts can be reduced, and accuracy can be improved. Almost all of Advantech’s PCI-bus plug-in data acquisition cards and CompactPCI® data acquisition modules have built-in this innovative function.