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Texas Instruments Bridging the Divide: A DAC Applications Tutorial (Precision Signal Path) Application notes
ADC161S626,DAC122S085,LM7705,LMP2015,
LMP2016,LMP7702
Bridging the Divide: A DAC Applications Tutorial (Precision Signal Path)
Literature Number: SNAA129
SIGNAL PATH designer
®
Expert tips and techniques for energy-efficient design
No. 121
Bridging the Divide: A DAC Applications Tutorial
— Bill McCulley, Applications Engineer
D
igital-to-Analog Converters (DACs) are used
to transform digital data into an analog signal.
Since the decades following the NyquistShannon sampling theorem, engineers have developed
and used DACs in applications, but it is only in the
past 25 years that monolithic DACs have become
widely available. According to the theorem, any
sampled data can be reconstructed perfectly—provided
it meets bandwidth and Nyquest criteria. So, with
proper design, a DAC can reconstruct sampled data
with precision. The digital data may be generated from a
microprocessor, Application-Specific Integrated Circuit
(ASIC), or Field-Programmable Gate Array (FPGA),
but eventually the data requires conversion to an analog
signal in order to have impact on the real world. The
world of erratic and dynamic analog signals cannot be
handled easily in a pristine 3.3V digital world. In that
regard, the DAC serves as the bridge from digital to
analog domains – and hopefully ends with an accurate
and true representation of the signal.
applications, such as calibration, the use of DACs may
not be immediately apparent.
Calibration
Depending on the application, a number of parameters
such as voltage offset, gain adjustment, or current bias
may require adjustment to ensure consistent results.
The ability to adjust these parameters is important for
applications such as sensors, factory line systems, or
test and measurement equipment. This adjustment or
correction can be done manually by plant engineers as
part of a periodic maintenance inspection. However,
as the industrial world becomes more automated,
the adjustment of those parameters needs to become
dynamic. This drives the need to immediately measure
an error at the output of a system, and then resolve it
by introducing a “correction” at the start of the process
flow. Since that correcting signal is analog in nature, a
DAC is a good fit for calibration in many applications.
A basic application
is a pressure sensor
of a pressure sensor
The system takes a
that may require calibration
system. A graphical depiction
system is shown in Figure 1.
low-level voltage signal from
DAC Applications
DACs are used in a wide spectrum of applications.
While IC manufacturers are integrating more features
into a microprocessor or FPGA every
year, there will always be some type
Pressure Sensor
Excitation
of analog conversion for interfaces.
The DAC therefore will maintain an
Bridge
important role among applications in
Transducer
the electronics industry. While no list
Signal
Serial
Conditioning
Interface
of applications is exhaustive, Table 1
Block
ADC
shows a number of common DAC
Instrumentation
Amplifier
applications, along with a description
of typical functions. In some
applications, the function of a DAC
Figure 1. Pressure Sensing System Diagram
is relatively straightforward. In other
national.com/spdesigner
Microcontroller/
Processor/FPGA
SIGNAL PATH designer
Bridging the Divide: A DAC Applications Tutorial
Application
DAC Function Summary
Audio Amplifier
DACs are used to produce DC voltage gain with microcontroller commands. Often, the DAC
will be incorporated into an entire audio codec which includes signal processing features.
Video Encoder
The video encoder system will process a video signal and send digital signals to a variety of
DACs to produce analog video signals of various formats, along with optimizing of output levels.
As with audio codecs, these ICs may have integrated DACs.
Display Electronics
The graphic controller will typically use a "lookup table" to generate data signals sent to a video
DAC for analog outputs such as Red, Green, Blue (RGB) signals to drive a display.
Data Acquisition
Systems
Data to be measured is digitized by an Analog-to-Digital Converter (ADC) and then sent to a
processor. The data acquisition will also include a process control end, in which the processor
sends feedback data to a DAC for converting to analog signals.
Calibration
The DAC provides dynamic calibration for gain and voltage offset for accuracy in test and
measurement systems.
Motor Control
Many motor controls require voltage control signals, and a DAC is ideal for this application
which may be driven by a processor or controller.
Data Distribution
System
Many industrial and factory lines require multiple programmable voltage sources, and this can
be generated by a bank of DACs that are multiplexed. The use of a DAC allows the dynamic
change of voltages during operation of a system.
Digital
Potentiometer
Almost all digital potentiometers are based on the string DAC architecture. With some reorganization of the resistor/switch array, and the addition of an I2C compatible interface, a fully digital
potentiometer can be implemented.
Software Radio
A DAC is used with a Digital Signal Processor (DSP) to convert a signal into analog for transmission
in the mixer circuit, and then to the radio’s power amplifier and transmitter.
Table 1. Common DAC Applications
the sensor and sends it to a processor for further
action. In more detail, the bridge transducer in
the diagram receives a signal (excitation) from a
pressure sensor and produces an output voltage
based on the pressure level. Altogether, the sensor/
bridge function is considered a pressure bridge
transducer1. Due to the low amplitude of the
transducer’s signal, an instrumentation amplifier
(in amp) is typically used for the signal conditioning
function. This adds gain for small differential signals
with low noise. Depending on the application,
additional amplifiers may be used to filter the
signal for anti-aliasing or buffering the signal to an
ADC for sampling. The ADC then transmits the
data code to a microcontroller or FPGA. Typically,
the interaction between a mixed-signal IC like an
ADC or DAC will be through an I2C compatible,
SPI, or Microwire® serial interface.
2
Applications such as pressure sensing usually
require high precision and accuracy over a range
of conditions such as temperature, parasitic errors
across circuit boards, or lot-to-lot tolerance of passive
components. Over time, the errors introduced in a
system can become large when gain is added with
each measurement. With a DAC, that calibration
can be implemented into the system to dynamically
correct the error as the system operates. A graphical
depiction for calibration of the pressure sensor
system is illustrated in Figure 2 on the following
page. It should be noted that this is not an actual
schematic, but an illustration on how calibration
could be achieved with DACs. While it does not
include aspects such as circuit power supply, passive
components, bypassing, and voltage reference
circuits, this diagram shows how calibration may
be implemented.
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VREF
DAC A
Gain
Adjust
VOUT A
DAC B
Transducer
Offset
VOUT B
DIN
SYNC
CLK
+5 VA
+5VA
Gain Error
Correction
DAC122S085
VOUT A
In Amp
Pressure Sensor
Excitation
+5 VA
+5 VA
VREF
DOUT
-.23 VA
ADC161S626
Bridge
Transducer
CS
MCU
FPGA
SCLK
In Amp
Transducer
Offset
Correction
+5 VA
-.23 VA
+5 VA
VOUT B
In Amp
-.23 VA
LM7705
-.23 VA
Figure 2. Simplified Example of a Calibrated Pressure Sensing Diagram
As shown in Figure 2, a gain-adjust DAC and a
transducer-offset DAC receive data code inputs for
calibration by the microcontroller which monitors
the outputs of the ADC. The microcontroller can be
programmed easily (with either an internal lookup
table or software comparison routines) to send the
appropriate calibration data to the DACs based
on the measured data errors. From the DACs, the
calibration signals are passed through a pair of in
amps to allow pre-scaling and buffering, and then
they are applied as an adjustment to the primary in
amp inputs. The DAC functions in this illustration
are implemented with a 2-channel, 12-bit DAC
(National’s DAC122S085). DAC A sends the
corrective gain signal through the gain pre-scale stage
(in amp) which is then fed to the primary in amp
to the ADC. DAC B sends a corrective offset signal
for transducer DC error through another pre-scale
stage (in amp) and then fed to the primary in amp.
Switches, as graphically depicted, allow the change
between calibration modes. Precision op amps such
as the LMP7702 or LMP2015/LM2016 op amps
can be used to implement the in amp functions or
an integrated device like the LMP8358 amplifier
could be used, depending on the application.
Finally, rail-to-rail output amplifiers—as good as
they perform—do not produce a true ground (0V)
when operating from a single-supply rail. This
can result in error due to the amplifier’s output
saturation voltage being amplified by following
stages. One way to mitigate this is to introduce a
small negative supply voltage that prevents the
3
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Bridging the Divide: A DAC Applications Tutorial
amplifier output from saturating at zero volts,
helping maintain an accurate zero and a full scale
for the 16-bit ADC, ADC161S626. In this circuit,
a small negative voltage of -0.23V has been added
using the LM7705 negative-bias generator.
Motor Control
While a complete review of motor types will not
be covered in this article, it is important to know
some of the most common motors used today.
The primary types include: DC motors (brushed,
brushless), AC motors (synchronous, inductive),
electrostatic motors, and other variants. While
there are several motor applications that require no
closed-loop control, most motor systems do require
some method of control. That control is typically
achieved with a controller, DAC, motor driver, and
a feedback path that contains the data measured by
a sensor.
One of the most popular motors is the DC brushless
motor (BLDC). It has some significant advantages
over the DC brushed motor, including higher
efficiency, less mechanical wear, and lower cost of
service and maintenance. The DC brushless motor
itself has several sub-types, including stepper- and
reluctance-type motors. DC brushless motors have
become very common among consumer products,
industrial and factory systems, robotics, tools, and
other applications. The DC motor is typically used
with a Variable Reluctance Sensor (VRS) or a HallEffect sensor, which is used to measure the position
and speed of the motor. In the newest DC motors
available, the sensor electronics may be integrated
into the entire mechanism of the motor.
DACs, when not integrated into a customized
IC, are often used as a key function for a motor
driver control system. Figure 3 shows a depiction in
which a motor control system could be integrated
with a DAC. It should be noted that this is not an
actual schematic, but a depiction of how a motor
control system could be architected.
Typically, the DAC will be driven by a microcontroller or a specialized controller. The DAC
will receive the input data code and convert the
data code into current outputs to the appropriate
VREF
Current
Outputs
Parallel
Interface
Microcontroller or
Specialized Controller
8 to 12
IOUT
8- to 12-Bit
DAC
IOUT
Velocity and Position (A, B);
Index Pulse Signal (IN)
E
M
Encoder
Motor
Figure 3. Motor Control Example
4
Motor Driver Power Amplifier
Circuit
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motor driver circuitry. The motor driver circuit
can be implemented in several ways. While there
are integrated driver ICs for that task, it can also
be designed with a power operational amplifier. The
architecture of the motor driver circuit will depend
on the requirements of the DC motor—including
the total power, continuous and maximum current,
and voltage range. During operation, the motor with
an encoder sends velocity and position signals to the
microcontroller. Depending on the encoder, these
signals may also include an index pulse signal. The
microcontroller then adjusts the speed and direction
of the motor by changing the data codes sent to
the DAC. The DAC’s important role in “closing
the control-loop” can be seen by the number of
manufacturers integrating DAC functions into
their motor driver ICs, thereby adding value to
their products.
Conclusion
The DAC, just like the ADC and the operational
amplifier, plays a key role in a myriad of applications.
If one considers the ubiquitous op amp as the “glue”
between mixed-signal components, then it can be
concluded that the three central components on a
signal path are the op amp, ADC, and DAC. As
the signal passes from the analog domain to digital
and back again, the DAC could be considered the
dénouement of a circuit. The signal, having done
its work, returns to the analog domain. The DAC
will continue to play a key role in many electronics
applications—including ones that have not yet been
envisioned.
Reference Footnotes:
(1) Pressure Bridge Transducers - http://wiki.xtronics.
com/index.php/Pressure_Transducer_Primer
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