MAXREFDES71

Advanced manufacturing, custom production and cost pressures continue to drive factories to higher speed performance and increased flexibility. To meet the requirements of a ‘no compromises’ factory environment, the MAXREFDES71 subsystem reference design provides two high-speed, high-accuracy, 400ksps, 16-bit analog input channels and output channels. All input and output channels support ±10V and ±20mA signals plus 20% margin, providing flexibility for low- and high-speed systems using either voltage or current signals.

The MAXREFDES71 design utilizes four dual fast-settling high-voltage op amps (MAX9633); two 16-bit 500ksps ADCs (MAX11166); two low-noise, fast-settling precision 16-bit DACs (MAX5316); two ultra-high precision 4.096V voltage references (MAX6126); seven high-speed digital isolators (MAX14850); an H-bridge transformer driver for isolated supplies (MAX13256); and regulated +18V, -18V, +5V, and -1.25V power rails (MAX8719, MAX8881). By using high-accuracy and high-speed components, this subsystem performs well in both process control applications, such as sensor inputs, and control applications, such as servo drives, resolvers, and encoders.

The MAXREFDES71 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 MAXREFDES71 reference design.

MAXREFDES71 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 MAXREFDES71 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 MAXREFDES71 uses the MAX13256 (U23) to generate the isolated +23V, -23V, 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.

MAXREFDES71 connects to FMC-compatible field-programmable gate array (FPGA)/microcontroller development boards. MAXREFDES71 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.

Required Equipment:

• Windows PC with two USB ports
• MAXREFDES71 (MAXREFDES71) board
• MAXREFDES71-supported platform (i.e., ZedBoard kit)
• Industrial signal source

Download, read, and carefully follow each step in the appropriate MAXREFDES71 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 MAXREFDES71 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.