Analog Output Circuitry - National Instruments PCI E Series Programmer's Manual

Chapter 2 Theory of Operation

Analog Output Circuitry

The general model for analog output on the PCI E Series boards includes
two channels of double-buffered analog output with programmable
polarity, reference source, and reglitching circuit, as well as a FIFO to
buffer the data. However, not all of the PCI E Series boards contain every
one of these features.
Each analog output channel contains a 12-bit DAC, an amplification stage,
and an onboard voltage reference, except for the PCI-MIO-16XE-10,
PCI-6052E, and PCI-6031E, which have a 16-bit DAC. The output voltage
will be proportional to the voltage reference (Vref) multiplied by the digital
code loaded into the DAC. Note that the output will be set to 0 V on power
up. The polarity, reference source, and reglitching circuit are all configured
in the AO Configuration Register.
The voltage reference source for each DAC is selectable from the onboard
reference or a voltage supplied at the EXTREF pin on the I/O connector,
except for the PCI-MIO-16XE-50, PCI-MIO-16XE-10, PCI-6024E,
PCI-6025E, and PCI-6031E which only supports the onboard reference.
The onboard reference is fixed at +10 V. The external reference can be
either a DC or an AC signal. If you apply an AC reference, the analog
output channel acts as a signal attenuator and the AC signal appears at the
output attenuated by the digital code. For unipolar output the voltage is
simply attenuated. Four quadrant multiplication occurs in bipolar output,
where the signal will not only be attenuated but also inverted for negative
digital codes.
The DAC output can be configured to produce either a unipolar or bipolar
output range, except for the PCI-MIO-16XE-50, which supports only
bipolar output. A unipolar output has an output range of 0 to +Vref
–1 LSB V. A bipolar output has an output voltage range of –Vref to
Vref –1 LSB V). For unipolar output, the data written to the DAC is
interpreted in straight binary format. For bipolar output, the data is
interpreted as two's complement format. One LSB is the voltage increment
corresponding to an LSB change in the digital code word. For a 12-bit
DAC, 1 LSB = (Vref)/4,096 in unipolar mode, and 1 LSB = (Vref)/2,048 in
bipolar mode. For a 16-bit DAC, 1 LSB = (Vref)/65,536 in unipolar mode,
and 1 LSB = (Vref)/32,768 in bipolar mode.
Using the 12-bit DAC and onboard 10 V reference will produce 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
operation. Using 16-bit DAC and onboard 10 V reference will produce an
output range of 0 to 9.9998 V in steps of 153 µV for unipolar output and an
© National Instruments Corporation 2-21 PCI E Series RLPM