Analog Output (AOUT) Modules are essential PLC modules to drive analog actuators. The analog output has the highest cost per channel among all IO modules and possibly the most challenging to protect. There are multiple topologies to implement analog output modules to achieve required performance at a designed for cost. Understanding these topologies and why designer can choose a module is a key factor in selecting the correct parts for your AOUT project.

Like analog input modules, the output modules features large number of performance specifications, which results in diversity in module types. This diversity is not possible to cover by a single architecture.

Figure 2-1 AOUT Module Specifications

As Figure 2-1 shows, the specifications can be clustered into groups covering output signal and interface, accuracy, precision, speed, reliability, and diagnostics. Although the specifications are numerous, few important parameters are determining the designed for architecture to chose. among the most important specs of AOUT module are:

• Output type (voltage or current) and the range (±10V or 4-20mA are common) as well as load range
• Number of channels, and channel isolation
• Conversion time and output noise.

The Figure 3-1 application page on TI.com provides a rich source of information about TI designs for PLC analog output. The page supports the generic structure of the AOUT module as shown in the following figure.

Figure 3-1 Analog Output Module Structure

The core of the AOUT module front-end is a digital-to-analog DAC converter, which drives output buffers either voltage or current. DAC requires a voltage reference, and HART modem is optional for current outputs. Power for AOUT front-end either comes from the field side, or less common from the backplane over isolated stage. This is a functional diagram, the different functions can either be integrated into one device or spread over multiple devices.

This article focuses on the front-end architectures, and what can be the correct architecture to use based on different requirements.

In principle, there are three common architectures for Multi-channel AOUT modules.

1. Fully integrated DAC: this approach offers the smallest area and possibly the highest performance for the highest cost per channel as this approach requires high voltage DAC.
2. Low-voltage DAC plus an output buffer: this approach provides scalability regarding number of channels, and flexibility in choosing the DAC and the buffer features at lower cost than the integrated design.
3. Track and hold multiplexed single channel DAC: this approach offers the lowest cost per channel in the expense of more complex design as channel sequencing needs an MCU. This implementation generally has longer settling time, and higher noise than the other implementations.

Choosing the designed for architecture is a function of the performance, cost, and speed targets

Table 4-1 Multi-Channel AOUT Architectures

	Fully integrated DAC	Voltage DAC+ buffer stage	Multiplexed Track&Hold
Block diagram
Features	High integration, accuracy, smaller area but Higher cost per channel	Scalable in performance and number of channels and low cost per channel but larger area	Lowest cost per channel, but requires MCU and complex design and slower settling
Devices	DAC8775, DAC8140x	DAC8050x, XTR111, XTR305, TLV9302, OPA2990S	DAC80501+TMUX1108/4, DAC8760+TMUX6208

The DAC8755 provides one chip design for 4-channel,16b, bipolar voltage and current with 10us settling time, with highly integrated output front-end including the DAC, the reference, the buffers, as well as the buck-boost stages to implement adaptive power for current output from a wide Vin single supply of 12-65V. Less Than 1-W, Quad-Channel, Analog Output Module With Adaptive Power Management is a reference design showing the powerful features of this device, while Quad-Channel Industrial Voltage and Current Output Driver (EMC/EMI Tested) is going into the details of passing the EMC/EMI tests for the design.

Figure 4-1 shows how this integrated DAC simplifies board design (only one channel is shown), still providing flexibility of software output configuration, and adaptive power management.

Figure 4-1 Schematic of DAC8775 AOUT Circuit

When only voltage output is required, DAC81404 offers wide range of resolution and channel count to choose from. With integrated reference and 12us settling time, DAC81404 is able to cover the highest voltage AOUT demands. The DAC814xx family is capable of high voltage output up to ±20V or unipolar 40V needed for special applications. The 4-channel device DAC81404 provides voltage sense pins which helps maintaining output accuracy when protection devices are placed in series.

This is popular architecture as low voltage DAC are readily available. Even the non-buffered DAC can be used making more parts available to select from. DAC80504 is an excellent DAC for such application. DAC80504 is a family of unipolar 5V buffered voltage output DAC with integrated REF (2ppm/°C), only 5us settling time, programmable output range, and INL=1 LSB in small package. The DAC does not have sense pin, but with output buffers this is not necessary, as the output compensation is done by the buffer feedback loop.

The large family of devices makes sure the designer can find the resolution and the number of channels required. The output buffer options are discussed in detail in a later section.

The DAC plus buffer architecture is a must if higher output current drive is needed, as most integrated DACs have limited current drive. Also if very fast settling is required as a designer can combine fast low voltage DACs and wide bandwidth buffers, but hardly can find any integrated high voltage DAC with faster than 10us settling time.

In this architecture, a single-channel voltage DAC is connected to a de-multiplexer with hold capacitor on each output channel. The MUX switch in addition to the hold capacitor acts as track and hold circuit. If the DAC output is sequentially changed in synchronicity with the multiplexer, separate static or dynamic outputs can be generated from a single channel DAC output.

This is a cost effective implementation, but requires careful design as the settling time of the DAC output, as well as the MUX leakage puts the minimum and maximum limits of the MUX hold time and scanning frequency. These tradeoff are well explained in Multi-Channel Analog Output Module With Multiplexed Single-Channel DAC for PLCs reference design.

Figure 4-2 Block diagram for track and hold multiplexed output architecture

Both low voltage DACs like DAC80501 (16b) ,DAC70501 (14b), DAC60501 (12b) or high voltage DACs like DAC8760 (16b) or DAC7760 (12b) can be used in this architecture. When low voltage DACs is used, the necessary gain is fulfilled by the output buffer. This architecture uses voltage and current buffers like the previous architecture, which are described in a later section.

When only a single-channel output is required, as in the case of channel-to-channel isolated modules, there are three options similar to the multi-channel case, although not exactly, and also different devices of choice.

1. Fully integrated DAC: this approach offers the smallest area and possibly the highest performance for the highest cost per channel as this approach requires high voltage DAC.
2. Low-voltage DAC plus an output buffer: this approach provides scalability and flexibility in choosing the DAC and the buffer features at lower cost than the integrated design.
3. Pulse-width modulation (PWM DAC) plus a buffer: this approach offers the lowest cost per channel but has longer settling time, and higher noise than the other implementations.

Table 5-1 Single-Channel AOUT Architectures

	Fully Integrated DAC	Voltage DAC+ Buffer Stage	PWM DAC + Buffer
Block diagram
Features	High integration, accuracy, smaller area but Higher cost per channel	Lower cost, and can achieve higher drive and faster settling.	Lowest cost per channel, but slower settling and higher noise
Devices	DAC8760, DAC7760, DAC8750	AFE882H1/201, DAC80501, DAC70501, DAC60501	MSPM0L, MSPM0G

As mentioned previously, the fully-integrated DAC provides compact, accurate design with slightly higher cost. Single channel high voltage DACs like DAC8760 is capable of bipolar voltage (+/-10V) and unipolar current output (0-20mA) with TUE=0.1%FSR and DNL=1 LSB. The DAC accepts power supply up to +/-20V. The DAC7760 is the 12-b version for less demanding applications.

The application note, Combined Voltage and Current Output with the DACx760 shows how to use DAC8760 for combined voltage and current output as done in the reference design, Combined Voltage and Current Output Terminal for Analog Outputs (AO) in Industrial Applications. The reference design, Single-Channel Industrial Voltage and Current Output Driver, Isolated, EMC/EMI Tested shows separate V/I outputs instead.

If single-channel current-only output is needed, then DAC8750 (16b) or DAC7750 (12b) is the device of choice.

A single-channel DAC, AFE882H1 family offers feature-rich, integrated reference, and HART modem option both in 14b and 16b versions. The AFE882H1 is especially important for safety-related applications. With TUE=0.08 %FSR this family offers the highest accuracy for such implementation.

Figure 5-1 AFE882H1 Device Block Diagram

The DAC80501 family, including the 12b,14b, and 16b variants of single channel ADC offers high linearity (INL=1LSB), fast settling (5us), and excellent reference drift (2ppm/°C) in a very small package, making DAC80501 an device of choice for such architecture when no safety or HART support is needed.

Pulse-Width Modulation (PWM DAC) is a special architecture used when the stand-alone DAC is to be avoided, and MCU is available in the system. The PWM can be converted to an analog voltage signal using a low-pass filter. Advanced techniques can be used to reach 16b of resolution, and relatively fast output settling time as described in Designing high-performance PWM DACs for field transmitters, and High-Performance 16-bit PWM to 4- to 20-mA DAC for Field Transmitters.

Figure 5-2 2-Path PWM-Based DAC Block Diagram