National Instruments AT-MIO-64F-5, User Manual

Page: 74 / 266

Theory of Operation Chapter 3
AT-MIO-64F-5 User Manual 3-14 © National Instruments Corporation
Analog Output Configuration
The DAC output amplifiers can be configured through one of the AT-MIO-64F-5 registers to
generate either a unipolar voltage output or a bipolar voltage output range. A unipolar output has
an output voltage range of 0 to +V
ref
- 1 LSB V and accepts straight binary input values. A
bipolar output has an output voltage range of -V
ref
to +V
ref
-1 LSB V and accepts two's
complement input values. One LSB is the voltage increment corresponding to an LSB change in
the digital code word. For unipolar output, 1 LSB = (V
ref
)/4,096. For bipolar output,
1 LSB = (V
ref
)/2,048.
The voltage reference source for each DAC is selectable through one of the AT-MIO-64F-5
registers and can be supplied either externally at the EXTREF input or internally. The external
reference can be either a DC or an AC signal. If an AC reference is applied, the analog output
channel acts as a signal attenuator, and the AC signal appears at the output multiplied by the
digital code divided by 4,096 for unipolar output or 2,048 for bipolar output. The internal
reference is a 5 V reference multiplied by 2. Using the internal reference supplies an output
voltage range of 0 to 9.9976 V in steps of 2.44 mV for unipolar output and an output voltage
range of -10 to +9.9951 V in steps of 4.88 mV for bipolar output. Gain calibration for the DACs
is intended only for the internal reference; it will only add a variable offset to the external
reference. Offset calibration can be applied to both references.
Analog Output Calibration
Output voltage accuracy 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 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 factory calibrated 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 offset and gain errors associated with each analog output channel.
For each DAC channel, there is an 8-bit offset calibration DAC, and an 8-bit gain calibration
DAC. Functions are provided with the board to calibrate the analog output 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.
DAC Waveform Generation Timing and Circuitry
There are primarily two modes under which the DACs in the analog output section operateÐ
immediate update and posted update. Immediate update mode is self-evident. You write a value
to the DAC and its voltage is
immediately available at the output. In posted update mode, the
voltage is not available at the output until a timer trigger signal initiates an update. This mode
has advantages in waveform generation applications which need precisely timed updates that are
not software-dependent.