Chapter 3
Theory of Operation
© National Instruments Corporation
3-11
AT-MIO-16D User Manual
Each analog output channel contains a 12-bit digital-to-analog converter (DAC), output operational
amplifiers (op-amps), reference selection jumpers, and unipolar/bipolar output selection jumpers.
The DAC in each analog output channel generates a current proportional to the input voltage
reference (V
ref
) multiplied by the digital code loaded into the DAC. Each DAC can be loaded with
a 12-bit digital code by writing to registers on the AT-MIO-16D board. The output op-amps
convert the DAC current output to a voltage output provided at the AT-MIO-16D I/O connector
DAC0 OUT and DAC1 OUT pins. The analog output of the DACs is updated to reflect the loaded
12-bit digital code in one of two ways: immediately when the 12-bit code is written to the DACs,
or when an active low pulse occurs on the Am9513A OUT2 pin. The update method used is
selected by the LDAC bit in Command Register 2.
Analog Output Range
The DAC output op-amps can be jumper configured to provide 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.
A bipolar output provides an output voltage range of -V
ref
to +V
ref
-1 LSB V. For unipolar output,
0 V output corresponds to a digital code word of zero. For bipolar output, the form of the digital
code input is jumper selectable. If straight binary form is selected, 0 V output corresponds to a
digital code word of 2,048. If two's complement form is selected, 0 V output corresponds to a
digital code word of zero. 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.
Analog Output Data Coding
The voltage reference source for each DAC is jumper selectable 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 attenuated by the digital code divided by 4,096 for unipolar output.
Bipolar output with an AC reference provides four-quadrant multiplication, which means that the
signal is inverted for digital codes 0 to 2,047 and not inverted for digital codes 2,049 to 4,095. In
straight binary mode, a digital code word of 2,048 attenuates the input signal to 0 V. This
attenuation is equivalent to multiplying the signal by (digital code word - 2,048)/+2,048. In two's
complement mode, a digital code word of zero attenuates the input signal to 0 V.
The internal voltage reference is a buffered version of the 10 V reference supplied by the ADC.
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 V to +9.9951 V in steps of 4.88 mV for
bipolar output.
MIO-16 Digital I/O Circuitry
The MIO-16 circuitry of the AT-MIO-16D provides eight digital I/O lines, while the DIO-24
circuitry provides 24 lines of digital I/O (discussed later in this chapter). The eight lines of digital
I/O from the MIO-16 circuitry are divided into two ports of four lines each and are located at pins
ADIO<3..0> and BDIO<3..0> on the I/O connector. Figure 3-5 shows a block diagram of the
digital I/O circuitry.
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