DMM-16R-AT User Manual V1.31
www.diamondsystems.com
Page 35
12. AUTOCALIBRATION OPERATION
DMM-16R-AT includes a sophisticated auto calibration circuit that manages the calibration of both the A/D
and the D/A circuitry. Operation is as follows.
12.1 Reference Voltages
The board contains a precision reference voltage chip that is selected for high stability over time and
temperature. The value of the voltage output from this chip is measured at the factory. The board also
contains some precision resistor divider ladders that produce intermediate voltages derived from the original
reference. All these voltages are measured at the factory and their values are stored in an EEPROM on the
board.
12.2 A/D calibration
When the A/D is calibrated, it measures these voltages using an extra input multiplexor reserved for
calibration. The calibration software compares the measurements to the stored values and makes
adjustments to the board to bring the measurements into tolerance (less than 2 LSBs max, in most cases less
than 1 LSB). The adjustments are produced by controlling 4 8-bit DACs that are inserted at various points in
the circuit. The DAC data is then stored in the EEPROM. Each of the valid A/D input ranges has its own set of
calibration values, since each input range has slight offset and gain differences compared to each other
range.
12.3 D/A Ca
l
ibration
When the D/A is calibrated, the board performs a similar operation. The output of DAC 0 is routed through the
calibration multiplexor. The offsets of the other three DACs relative to DAC 0 are measured at the factory and
stored in the EEPROM. During calibration the average offset is added to the measured output of DAC 0, and
this value is used as the comparison value to minimize overall errors. During D/A calibration the board can
automatically determine the jumper configuration of the DACs based on their response to various output
codes.
If the board is configured for programmable output range, the calibration software will first set and calibrate
the output range, then calibrate the D/A.
12.4 Universal Driver Software Support
Calibration is simple when using the Diamond Systems Universal Driver software. Several functions are
provided to manage the entire operation, and a demo program is included. For application developers
targeting an operating system not supported by Universal Driver, the source code is included so you can
incorporate it into your own program.
With the Universal Driver software, you have the option of recalling calibration values each time you change
the input range, or leaving the current ones in place. Leaving the current ones in place will match the
performance of other A/D boards which also use only a single set of calibration values. Recalling the values
specific to the new input range will improve performance by a few LSBs but will result in a time delay since
the data must be recalled from the EEPROM and loaded into the DACs.
A/D and D/A maybe calibrated separately. Calibration takes a few seconds and may be performed as often as
desired, for example at system startup, once a day, etc.
NOTE:
When calibrating the D/A channels, the output voltage of DAC0 will fluctuate between
– full scale and
+ full-scale as part of the procedure. Any circuitry connected to the DAC during this time may be affected and
produce unwanted results.