Alameda MAXREFDES24

In PLC and DCS systems, analog output currents and voltages provide critical control and actuation functions. The Alameda (MAXREFDES24#) reference design shown in Figure 1 features four flexible and programmable analog outputs that meet industrial control requirements.

The MAX5134 lies at the heart of the system. This four-channel, 16-bit, high-accuracy digital-to-analog converter (DAC) provides voltage outputs that drive the inputs of four MAX15500 signal conditioners. These signal conditioners produce user-programmable accurate current or voltage outputs. The MAX15500 also provides extensive error reporting. The MAX6126 produces an ultra-high-precision voltage reference for the DAC and the output conditioners. The MAX14850 galvanically isolates data communication from the field side and the system controller.

The Alameda also integrates an isolated, wide DC input range, flyback converter power supply. The peak current-mode flyback controller, MAX17498B, efficiently drives an isolated transformer and generates ±24V and +8V outputs. The MAX1659 low-dropout (LDO) linear regulator then regulates the +8V output to a +5V low-noise output. The entire system requires only a 24V input for power.

The subsystem features all typical bipolar current and voltage output ranges, and appropriate subsets, with less than ±0.1% typical total unadjusted error (TUE). The circuit also provides open-circuit detection, brownout detection, overtemperature protection, short-circuit and overcurrent protection, which are all critical for industrial applications. Flexible power-up options make Alameda an ideal choice for robust industrial control systems.

Features

Competitive Advantages

• Flexibility
• System safety
• Small solution size

Applications

• PLCs
• DCS
• Distributed I/Os
• Industrial control and automation

Required equipment for MAXREFDES24:

• Windows^® PC with two USB ports
• Alameda (MAXREFDES24) board
• Alameda-supported platform (i.e., LX9 development kit or ZedBoard kit)
• One 24V, 150mA minimum DC power supply
• One 750Ω, 0.25W resistor

Equipment used:

• Alameda (MAXREFDES24) board
• FPGA development kit
• One 750Ω, 0.25W resistor load
• Agilent 3458A digital multimeter
• Agilent E3631A DC power supply (any 24V, 150mA minimum DC power supply works)
• National Instruments GPIB card and cable
• Thermonics T-2800 precision temperature forcing system
• Perl script for controlling the FPGA development kit and measurement equipment
• Windows PC

INL, DNL, and TUE are the most important specifications for PLC and other process control systems. The MAX15500 is highly flexible and configurable to meet the needs of various applications. The data was taken at +25°C. In the following pictures, the DNL, INL, and TUE for the first 320 DAC codes are shown as 0 because codes 0 to 320 are in the deadband (0 to 0.02V) of the MAX5134.

Measurements of DNL, INL, and TUE for the -10V to +10V voltage output mode, with 20% overrange are shown in Figure 3, Figure 4, and Figure 5, respectively.

Measurements of DNL, INL, and TUE for the 0 to 10V voltage output mode, with 20% overrange are shown in Figure 6, Figure 7, and Figure 8, respectively.

Measurements of DNL, INL, and TUE for the -20mA to +20mA current output mode, with 20% overrange are shown in Figure 9, Figure 10, and Figure 11, respectively.

Measurements of DNL, INL, and TUE for the 0 to 20mA current output mode, with 20% overrange are shown in Figure 12, Figure 13, and Figure 14, respectively.

ARM is a registered trademark and registered service mark of ARM Limited.

Audio Precision is a registered trademark of Audio Precision, Inc.

Cortex is a registered trademark of ARM Limited.

Eclipse is a trademark of Eclipse Foundation, Inc.

Windows is a registered trademark and registered service mark of Microsoft Corporation.

Xilinx is a registered trademark and registered service mark of Xilinx, Inc.

ZedBoard is a trademark of ZedBoard.org.

Zynq is a registered trademark of Xilinx, Inc.

Alameda connects to Pmod-compatible field-programmable gate array (FPGA)/microcontroller development boards. Alameda requires a 3.3V supply voltage from the Pmod connector and uses the SPI pin assignments as illustrated on the right.

The power requirements are shown in Table 1. Note that the external +24V power supply is required for full system operation. The currently supported platforms and ports are shown in Table 2.

*Tested with ZedBoard Rev C.

The MAX15500 (U1–U4) is a single-channel, low-cost, precision analog current/voltage output conditioner developed to meet the requirements of PLCs and other industrial control and automation applications. The MAX15500 operates from a ±15V to ±32.5V power-supply range.

The MAX15500 can generate both unipolar and bipolar current and voltage outputs. In current mode, the device produces currents of -1.2mA to +24mA or -24mA to +24mA. In voltage mode, the device produces voltages of -0.3V to +6V, -0.6V to +12V, or ±12V. To allow for overrange and underrange capability in unipolar mode, the transfer function of the MAX15500 is offset so that when the voltage at A_IN is 5% of full scale, I_OUT is 0mA and V_OUT is 0V. Once VA_IN attains full scale, V_OUT or I_OUT becomes full scale +5% or +20% (depending on the state of the FS_MODE pin) and I_OUT becomes full scale +5% or +20% (depending on the value of the R_SENSE resistor). Common output modes and respective overrange control jumpers are summarized in Table 3.

The MAX15500 protects against overcurrent and short-circuit conditions when OUT goes to ground or a voltage up to the supply voltage of ±24V. For many systems, however, additional port protection is required. Refer to application note 6008, "Protection: How Much Is Enough for an Analog Output" for more information on protection circuits and components. The device also monitors for overtemperature and supply brownout conditions. The supply brownout threshold is programmable between ±10V and ±24V in 2V increments. The MAX15500 provides extensive error reporting of short-circuit, open-circuit, brownout, and overtemperature conditions through the SPI interface and an additional open-drain interrupt output (ERROR). The MAX15500 also includes an analog 0 to 3V output (MON) to monitor the load condition at OUT.

The MAX5134 (U5) is a quad 16-bit, buffered voltage-output, high-linearity DAC. The device features 4-channel, very low deadband (0.02V max) rail-to-rail outputs. For most applications, no negative biasing power supply is required.

The MAX6126 (U6) drives the analog output conditioners and the DAC's reference inputs with an ultra-high-precision 4.096V voltage reference with 0.02% initial accuracy and a 3ppm/°C maximum temperature coefficient (tempco).

The DAC's outputs directly drive the conditioners' inputs with no external components, making the interface simple.

The MAX17498B (U8) provides an isolated, functional insulation class power solution that accepts single +18V to +32V DC voltage and converts it to ±24V and +8V using an isolation transformer in flyback architecture. Post-regulation is accomplished using the MAX1659 LDO (U9) for the 5V output.

Data isolation between the subsystem and the controller is accomplished using the MAX14850 (U7) digital data isolator. The combined power and data isolation achieved is 600VRMS.

Detailed Description of Firmware for LX9 and ZedBoard Platforms

Table 2 shows the currently supported platforms and ports. Support for additional platforms may be added periodically under Firmware Files in the All Design Files section.

The Alameda firmware released for the LX9 development kit targets a Microblaze™ soft-core microcontroller placed inside a Xilinx^® Spartan-6 FPGA. The Alameda firmware also supports the ZedBoard kit and targets an ARM^® Cortex^®-A9 processor placed inside a Xilinx Zynq system-on-chip (SoC).

The firmware is a working example of how to initiate the system and wait for a user's input. A user can select the output mode and type in the DAC input codes. The simple process flow is shown in Figure 2. The firmware is written in C using the Xilinx software development kit (SDK) tool, which is based on the Eclipse^™ open source standard. Custom Alameda-specific design functions were created utilizing the standard Xilinx XSpi core version 3.03a. The SPI clock frequency is set to 2MHz on the LX9 platform and 3.125MHz on the ZedBoard platform.

The complete source code is provided to speed up customer development. Code documentation can be found with the corresponding firmware platform files.