MAXREFDES32

The MAXREFDES32# subsystem is optimized for applications that need both analog inputs and analog outputs with isolated power and data. Figure 1 shows the block diagram of the MAXREFDES32# reference design.

MAXREFDES32# uses an external MAX6126 (U12 and U22) voltage reference for both the ADC and the DAC to provide the highest possible accuracy. The MAX6126 has an initial accuracy of 0.02% and a 3ppm/ºC maximum temperature coefficient (tempco).

The MAX9633 (U1) is a dual high-voltage, low-noise op amp. The op amps attenuate and buffer the input signals to match the input range of the ADC (MAX11166). When measuring a voltage signal, connect the positive terminal and negative terminal of the signal source to the Vx+/Ix+ and Vx-/Ix- connectors, respectively. When measuring a current signal, connect the positive terminal of the signal source to the Vx+/Ix+ connector, and connect the negative terminal to both Vx-/Ix- and Ix- connectors. The input buffer is configured in a difference amplifier configuration. The equation to convert the ADC code to input voltage is:

V = 3.6621 × 10^-4 × (CODE_ADC - 32768)

When measuring the current signal, if the Ix- terminal is connected to the ground of the current loop, the equation to convert the ADC code to current is:

I = 7.4466 × 10^-7 × (CODE_ADC - 32768)

The MAX5316 (U17 and U18) is a low-noise, fast-settling, 16-bit DAC. The range of the DAC outputs is 0V to 4.096V. The dual op amp, MAX9633 (U19), buffers the DAC outputs and rescales the output range to -10V to +10V (±2V) to meet the industrial standard. A second dual op amp, MAX9633 (U20), buffers the outputs of the first MAX9633 (U19). A third dual op amp, MAX9633 (U21), uses the Howland current source configuration to convert the voltage output of the second MAX9633 (U20) into -20mA to +20mA (±4mA) current output. Therefore, the MAXREFDES32# can produce two channels of both voltage and current outputs simultaneously. The equations to convert the DAC code to output voltage and current are:

V = 3.6758 × 10^-4 × CODE_DAC - 12.0055

I = 7.3517 × 10^-7 × CODE_DAC - 2.4011 × 10-2

In standard configuration, the MAXREFDES32# uses the MAX17498B (U23) to generate the isolated +22V, -22V, +7V, and -7V rails from a 24V supply. The MAX8719 (U4), MAX8881 (U5, U6), and LM337 (U7, U8) provide post-regulated +18V, +5V, -18V, and -1.25V rails. Data isolation is accomplished using the MAX14850 (U9–U11, U13–U16) digital data isolators. The combined power and data isolation achieved is 600VRMS. To use external power supplies, disconnect the 24V supply and move the shunts on all jumpers to the 2-3 position. Connect the ground terminal of the external power supplies to the GND connector. Connect the +18V, +5V, -1.25V, and -18V supplies to the corresponding connectors on the board.

MAXREFDES32# connects to FMC-compatible field-programmable gate array (FPGA)/microcontroller development boards. MAXREFDES32# requires a 24V supply and also the 3.3V supply from the FMC connector. The FMC pin assignments are illustrated in Figure 2.

Table 1 shows the power requirements. Table 2 shows currently supported platforms and ports.

Detailed Description of ZedBoard Firmware

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 MAXREFDES32# firmware released for the ZedBoard kit targets an ARM^® Cortex^®-A9 processor placed inside a Xilinx^® Zynq system-on-chip (SoC).

The firmware is a working example of how to interface to the hardware, collect samples, save them to memory, and replicate the input signals on the outputs. Figure 3 shows the process flow. The firmware is written in C using the Xilinx SDK tool, which is based on the Eclipse open source standard. Custom MAXREFDES32# design functions were created utilizing the AXI MAXREFDES32 custom IP core. The firmware supports the maximum ADC sampling rate at 400ksps. The maximum signal replication rate is 60ksps because the settling time for the voltage output signals is 17µs. Because the settling time for current output signals is 77µs, a replication rate of 10ksps or less is recommended.

The firmware accepts commands, configures the ADC and DAC, replicates the inputs on outputs, and is capable of downloading blocks of sampled data to a standard terminal program through a virtual COM port. The complete source code is provided to speed up customer development. The corresponding firmware platform files contain code documentation.

Required Equipment:

• Windows^® PC with two USB ports
• MAXREFDES32# (MAXREFDES32#) board
• MAXREFDES32#-supported platform (i.e., ZedBoard kit)
• Industrial signal source

Download, read, and carefully follow each step in the appropriate MAXREFDES32# Quick Start Guide: MAXREFDES32# ZedBoard Quick Start Guide

Equipment used for analog input tests:

• Audio Precision^® SYS-2722 signal source or equivalent
• Voltage calibrator DVC-8500
• Windows PC with two USB ports
• MAXREFDES32# board
ZedBoard kit

Equipment used for analog output tests:

• Maxim custom FPGA test board
• One 499Ω, 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
• Perl script for controlling the FPGA development kit and measurement equipment
• Windows PC

Take special care and use proper equipment when testing the MAXREFDES32# design. Duplication of the presented test data requires a signal source with higher accuracy than the design being tested. A low distortion signal source is absolutely required to duplicate the presented results. The input signal was generated using the Audio Precision SYS-2722. The FFTs were created using the FFT control in SignalLab from Mitov Software.

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.