The Pitfalls of Manual Calibration
All analog circuits exhibit inherent minor variations in their performance from one component to the next. In addition, their characteristics naturally drift slightly over time and more importantly in response to temperature changes. Because of these factors, analog circuits generally require calibration in order to provide accurate performance.
Manual Calibration: Most boards are calibrated manually one time at the factory prior to shipment. A precision voltage source and voltmeter are connected to the board, A/D samples are taken of those voltages, and potentiometers on the board are adjusted until the readings are within some accepted tolerance. This method provides accurate performance when the board is new and when the board is operating at the same temperature at which it was calibrated. But manual calibration introduces many opportunities for errors in the board's performance:
- Input range: Most boards have multiple input ranges (example 0-10V, +/-5V). Each input range will have its own errors. But a manually calibrated board typically can only be calibrated for one input range, which leaves errors in the other ranges.
- Time: As the board ages, it will slowly drift away from its initial calibration, resulting in a gradual decrease in accuracy. This is why, for example, test equipment such as oscilloscopes and multimeters generally require calibration on an annual basis.
- Temperature: The largest source of error is temperature drift. Since the board was calibrated at a single temperature (typically near room temperature of 20C), it may exhibit dramatic errors if it is subjected to temperature swings that are typically experienced by embedded systems.
- Human error: The person performing the calibration procedure may make an incorrect measurement or an error in wiring, resulting in errors in the calibration.
- Cost: Manual calibration also increases the cost of maintenance, since it requires physical access to the board, which may be very difficult once the board is installed inside another piece of equipment.
To resolve these problems, higher-end A/D boards offer automatic calibration, or autocalibration. (See next slide.)