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Chapter 3 Theory of Operation
© National Instruments Corporation 3-7 AT-MIO-64F-5 User Manual
Dither Circuitry
When you enable the dither circuitry, you add approximately 0.5 LSB rms of white Gaussian
noise to the signal to be converted by the ADC. This addition is useful for applications involving
averaging to increase the resolution of the AT-MIO-64F-5 to more than 12 bits, as in calibration
or spectral analysis. In such applications, noise modulation is decreased and differential linearity
is improved by the addition of the dither. For high-speed 12-bit applications not involving
averaging or spectral analysis, you may want to disable the dither to reduce noise. Enabling and
disabling of the dither circuitry is accomplished through software (see Chapter 4,
Register Map
and Descriptions ).
When taking DC measurements, such as when calibrating the board, enable dither and average
about 1,000 points to take a single reading. This process removes the effects of 12-bit
quantization and reduces measurement noise, resulting in improved resolution. Dither, or
additive white noise, has the effect of forcing quantization noise to become a zero-mean random
variable rather than a deterministic function of input. For more information on the effects of
dither, see "Dither in Digital Audio" by John Vanderkooy and Stanley P. Lipshitz,
Journal of the
Audio Engineering Society, Vol. 35, No. 12, Dec., 1987.
ADC FIFO Buffer
When an A/D conversion is complete, the ADC circuitry shifts the result into the ADC FIFO
buffer. The FIFO buffer is 16-bits wide and 512-words deep. This FIFO serves as a buffer to
the ADC and is beneficial for two reasons. Any time an A/D conversion is complete, the value is
saved in the FIFO buffer for later reading, and the ADC is free to start a new conversion.
Secondly, the FIFO can collect up to 512 A/D conversion values before any information is lost;
thus software or DMA has extra time (512 times the sample interval) to catch up with the
hardware. If more than 512 values are stored in the FIFO without the FIFO being read from, an
error condition called FIFO overflow occurs and A/D conversion information is lost. When the
ADC FIFO contains a single A/D conversion value or more, it can generate a DMA or interrupt
request to be serviced.
Analog Input Calibration
Measurement reliability is assured through the use of the onboard calibration circuitry of the
AT-MIO-64F-5. This circuitry uses a stable, internal, +5 VDC reference that is measured at the
factory against a higher accuracy reference; then its value is permanently stored in the EEPROM
on the AT-MIO-64F-5. With this stored reference value, the AT-MIO-64F-5 board can be
recalibrated without additional external hardware at any time under any number of different
operating conditions in order to remove errors caused by temperature drift and time. The
AT-MIO-64F-5 is calibrated at the factory in both unipolar and bipolar modes, and these values
are also permanently stored in the EEPROM. Calibration constants can be read from the
EEPROM then written to the calibration DACs that adjust pregain offset, postgain offset, and
gain errors associated with the analog input section. There is an 8-bit pregain offset calibration
DAC, an 8-bit postgain offset calibration DAC, an 8-bit unipolar offset calibration DAC, and an
8-bit gain calibration DAC. Functions are provided with the board to calibrate the analog input
section, access the EEPROM on the board, and write to the calibration DACs. When the
AT-MIO-64F-5 leaves the factory, locations 96 through 127 of the EEPROM are protected and
cannot be modified. Locations 0 through 95 are unprotected and can be used to store alternate
calibration constants for the differing conditions under which the board is used. Refer to
Chapter 6,
Calibration Procedures , for additional calibration information.