MAXREFDES18

In PLC and DCS systems, analog output currents and voltages provide critical control and actuation functions. The Carmel (MAXREFDES18) reference design shown in Figure 1 provides a flexible and programmable analog output that meets industrial control requirements.

The buffered voltage output from the MAX5316 16-bit, high-accuracy digital-to-analog converter (DAC) drives the input of the MAX15500, a programmable analog output conditioner with extensive error reporting. The MAX6126 ultra-high-precision voltage reference provides references for the DAC and the output conditioner. The MAX14850 galvanically isolates data communication between the subsystem and the system controller. Optionally, the subsystem also integrates an isolated and regulated power supply by using the MAX13253 transformer driver and the MAX1659 and MAX1735 low-dropout (LDO) linear regulators.

The subsystem features all typical bipolar current and voltage output ranges, and appropriate subsets, with less than 0.105% total unadjusted error (TUE). The circuit also provides short-circuit and overcurrent protection, open circuit detection, brownout detection, overtemperature protection, all of which are critical for industrial applications. Flexible power-up options make Carmel an ideal choice for robust industrial control systems.

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

The power requirements are shown in Table 1. Note that the external AVDD and AVSS power rails are required for full system operation. The currently supported platforms and ports are shown in Table 2.

The MAX15500 (U1) 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 AIN is 5% of full scale, IOUT is 0mA and VOUT is 0V. Once VAIN attains full scale, VOUT or IOUT becomes full scale +5% or +20% depending on the state of FSMODE.

The MAX15500 protects against overcurrent and short-circuit conditions when OUT goes to ground or a voltage up to ±32.5V. 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 MAX5316 (U2) is a high-accuracy, 16-bit, buffered voltage-output DAC. The device features ±1 LSB integral nonlinearity (INL) (max) accuracy and a ±1 LSB differential nonlinearity (DNL) (max) accuracy over the -40°C to +105°C. A separate -1.25V AVSS supply allows the output amplifier to go to 0V (GND) while maintaining full linearity performance. For lower deadband requirements, the feature-reduced MAX5216 DAC can be used instead.

The MAX6126 (U3) drives the analog output conditioner and the DAC's reference input 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 output directly drives the conditioner’s input with no external components, making the interface simple.

The MAX13253 (U4) provides an isolated, functional insulation class power solution that accepts 3.3V and converts it to ±6V using an isolation transformer. Post-regulation is accomplished using the MAX1659 LDO (U5) for the 5V output, and the MAX1735 (U6) for the -1.25V 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.

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 Carmel firmware released for the LX9 development kit targets a Microblaze^™ soft-core microcontroller placed inside a Xilinx^® Spartan^®-6 FPGA. The Carmel 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 code. 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 Carmel-specific design functions were created utilizing the standard Xilinx XSpi core version 3.03a. The SPI clock frequency is set to 3.125MHz.

Required equipment:

• Windows^® PC with two USB ports
• Carmel (MAXREFDES18#) board
• Carmel-supported platform (i.e., LX9 development kit or ZedBoard kit)
• One ±24V, 25mA minimum DC power supply
• One 750Ω, 0.25W resistor
• Download, read, and carefully follow each step in the appropriate Carmel Quick Start Guide.

Equipment:

• Carmel (MAXREFDES18) board
• FPGA development kit
• One 750Ω, 0.25W resistor load
• Agilent 3458A digital multimeter
• Agilent E3631A DC power supply (any ±24V, 25mA 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 total unadjusted error (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. Measurements of DNL, INL, and output error for the reference design are shown in Figure 3, Figure 4, and Figure 5, respectively. The data was taken at +25°C in the -10V to +10V voltage output mode, with 5% overrange.

In the case of current output, DNL, INL, and output error (without calibration) for the reference design are shown inFigure 6, Figure 7, and Figure 8, respectively. The data was taken at +25°C in the 0mA to 20mA current output mode, with 5% overrange.

Cortex is a registered trademark of ARM Limited.

Eclipse is a trademark of Eclipse Foundation, Inc.

Halo is a registered trademark of Halo Electronics, Inc.

MicroBlaze is a trademark of Xilinx, Inc.

Pmod is a trademark of Digilent Inc.

Spartan is a registered trademark of Xilinx, 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.