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Maxim > Design Support > Technical Documents > Sub-System Boards > APP 5562
Keywords: Campbell, MAXREFDES4, subsystem reference design, analog front end, AFE, industrial sensors,
isolated power and data, industrial automation, 4-20mA, current loop, PLC, medical
SUB-SYSTEM BOARD 5562
Campbell (MAXREFDES4#): 16-Bit High-Accuracy 4-
20mA Input Isolated Analog Front End (AFE)
Jan 16, 2013
Abstract: This document explains how the Campbell (MAXREFDES4#) subsystem reference design meets the
higher resolution and isolation needs of industrial control and industrial automation applications. Hardware and
firmware design files as well as FFTs and histograms from lab measurements are provided.
Introduction
In industrial control and industrial
automation applications, high-resolution
data converters are often required.
Although today's field programmable gate
arrays (FPGAs) and microcontrollers may
integrate analog-to-digital converters
(ADCs), in many cases, the resolution is
not high enough and isolation is lacking.
The Campbell (MAXREFDES4#) subsystem reference design is a 16-bit high-accuracy industrial analog front
end (AFE) that accepts a 4–20mA current loop or a 0.2V to 4.096V voltage input signal, and features isolated
power and data—all integrated into a small form factor. The Campbell design integrates a precision low-noise
buffer (MAX44250), a high-accuracy ADC (MAX11100), an ultra-high-precision 4.096V voltage reference
(MAX6126), 600VRMS data isolation (MAX14850), and isolated/regulated 5V power rails (MAX256/MAX1659).
This AFE solution can be used in any application that needs high-accuracy analog-to-digital conversion, but it is
mainly targeted for industrial sensors, industrial automation, process control, programmable logic controllers
(PLCs), and medical applications.
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Figure 1. The Campbell subsystem design block diagram.
Features Applications
High accuracy
4–20mA current loop input
0.2V to 4.096V input range
Isolated power and data
Small printed circuit board (PCB) area
Device drivers
Example C source code
Pmod™-compatible form factor
Industrial sensors
Process control
Industrial automation
PLC
Medical
Detailed Description of Hardware
The Pmod specification allows for both 3.3V and 5V modules as well as various pin
assignments. This module is designed only for a supply voltage of 3.3V and uses
the SPI pin assignments as illustrated on the right.
The power requirements are shown in Table 1. The currently supported platforms
and ports are shown in Table 2.
Table 1. Power Options for the Campbell Subsystem Reference Design
Power Type Jumper
Shunt Input
Voltage (V) Input Current
(mA, typ)
On-board isolated
power JU3: 1–2
JU4: 2–3 3.3 72.5
External power JU3: 2–3
JU4: 1–2 12 10.7
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Table 2. Supported Platforms and Ports
Supported Platforms Port
Nexys™ 3 Platform (Spartan®-6) JB1
ZedBoard™ platform (Zynq®-7020) JA1
The Campbell subsystem is best suited for a high-accuracy 4–20mA current loop or a 0.2V to 4.096V input
voltage analog-to-digital data acquisition system. The hardware design provides both isolated power (MAX256)
and isolated data (MAX14850).
The MAX44250 op amp (U1) input circuit buffers a 4–20mA current-loop sense voltage on a 200Ω load resistor
(with JU2 closed) or a 0.2V to 4.096V (with JU2 open) voltage signal.
The MAX11100 (U2) is a 16-bit, successive-approximation register (SAR) ADC with AutoShutdown™ and fast
1.1µs wake-up features. The ADC's reference input is driven by the MAX6126 ultra-high-precision 4.096V
voltage reference (U3) with 0.02% initial accuracy and a 3ppm/°C maximum temperature coefficient (tempco).
The MAX256 (U4) provides an isolated, functional insulation class power solution that accepts 3.3V and
converts it to 12V using an off-the-shelf TGM-H281NF Halo® transformer with a 1:2.6 primary to secondary
turns ratio plus an external on-board voltage-doubler circuit. Post-regulation is accomplished using the
MAX1659 low dropout (LDO) regulators (U5 for a 12V output, U6 for an analog 5V output, and U8 for a digital
5V output). Data isolation is accomplished using the MAX14850 (U5) digital data isolator. The combined power
and data isolation achieved is 600VRMS.
To use the on-board isolated power supplies, move the shunt on jumper JU3 to the 1–2 position and move the
shunt on jumper JU4 to the 2–3 position. If power isolation is not required, an external 12V DC power supply
can be used. Move the shunt on jumper JU3 to the 2–3 position and move the shunt on jumper JU4 to the 1–2
position. Connect the ground terminal of the external power supply to the GND2 connector. Connect the 12V
DC power supply to the EXT_V connector. See Table 1 for the jumper settings and the input current
requirements.
Detailed Description of Firmware for Nexys 3 Platform
The Campbell firmware design was initially released for the Nexys 3 development kit and targeted a
MicroBlaze™ soft core microcontroller placed inside a Xilinx® Spartan-6 FPGA. Support for additional platforms
may be added periodically under Firmware Files in the All Design Files section. The currently supported
platforms and ports are shown in Table 2.
The firmware is a working example of how to interface to the hardware, collect samples, and save them to
memory. The simple process flow is shown in Figure 2a. The firmware is written in C using the Xilinx SDK tool,
which is based on the Eclipse™ open source standard. Custom Campbell-specific design functions were
created utilizing the standard Xilinx XSpi core version 3.03a. The SPI clock frequency is set to 3.125MHz.
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Figure 2a. The Campbell firmware flowchart for Nexys 3 Platform.
The firmware accepts commands, writes status, and is capable of downloading blocks of sampled data to a
standard terminal program via a virtual COM port. The complete source code is provided to speed up customer
development. Code documentation can be found in the corresponding firmware platform files.
Detailed Description of Firmware for ZedBoard Platform
The Campbell firmware design is also developed and tested for the ZedBoard kit and targets an ARM® Cortex®
-A9 processor placed inside a Xilinx Zynq system-on-chip (SoC).
An AXI MAX11100 custom IP core is created for this reference design to optimize the sampling rate and SPI
timing stability.
The firmware is a working example of how to interface to the hardware, collect samples, and save them to
memory. The simple process flow is shown in Figure 2b. The firmware is written in C using the Xilinx SDK tool,
which is based on the Eclipse open source standard. Custom Campbell-specific design functions were created
utilizing the AXI MAX11100 custom IP core. The SPI clock frequency is set to 4.54MHz when a 189.4ksps
sampling rate is selected. The SPI clock frequency is set to 2.5MHz for all other sampling rate.
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DOWNLOAD DATA TO F07 ommuous SAMPLING BLOCK SAMPLING SAMPLING 5le n - essu 1 - 131072 2 - 524288 a - 1049575 + SAWLINGRAYE o-1wsps I-lksps 2-lDlsvs 3-2015» «4st 5430.415“
Figure 2b. The Campbell firmware flowchart for ZedBoard platform.
The firmware accepts commands, writes statuses, and is capable of downloading blocks of sampled data to a
standard terminal program via a virtual COM port. The complete source code is provided to speed up customer
development. Code documentation can be found in the corresponding firmware platform files.
Quick Start
Required equipment:
Windows® PC with two USB ports
Campbell (MAXREFDES4#) board
Campbell-supported platform (i.e., Nexys 3 development kit or ZedBoard kit)
4–20mA current loop sensor or other signal source
Download, read, and carefully follow each step in the appropriate Campbell Quick Start Guide:
Campbell (MAXREFDES4#) Nexys 3 Quick Start Guide.
Campbell (MAXREFDES4#) ZedBoard Quick Start Guide.
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4D 00 -EU 00 44000 .‘5000 June!) I| oraale FFT 613 Channels SNR 9445‘:st SNH 9275d3c _ Wm“ SINAD ea 32¢an SINAD 82 ezaac THD 97 SQGEE SFDFK 33 ZZdB: News: “DO! 7123 SSdEFS l—l—l—l—l—\ 4 can 5 one = ‘ Frequency [kHl)
Lab Measurements
Equipment used:
Audio Precision® SYS-2722 signal source or equivalent
Voltage calibrator DVC-8500
Windows PC with two USB ports
Campbell (MAXREFDES4#) board
Nexys 3 development kit
12V DC power supply (for external power testing only)
Special care must be taken and the proper equipment must be used when testing the Campbell design. The
key to testing any high-accuracy design is to use sources and measurement equipment that are of higher
accuracy than the design under test. A low distortion signal source is absolutely required in order to duplicate
the results presented below. The input signal was generated using the Audio Precision SYS-2722. The FFTs
were created using the FFT control in SignalLab from Mitov Software.
AC and DC performance for on-board isolated power is shown in Figure 3 and Figure 4. AC and DC
performance for external power is shown in Figure 5 and Figure 6.
Figure 3. AC FFT using on-board isolated power, a 0.2V to 4.08V 1kHz sine wave input signal, high-impedance
input, a 20ksps sample rate, and a Blackman-Harris window.
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Histogram 3 8 I Occurrence i 8 I 15000— 12500 — 10000 — 7500— 5000 — 2500 — 0— I I I I I I I I I I I 32003 00 32003 50 32009 00 32009 50 320m 00 320I050 3201100 320I150 32012 00 32012 50 32013 00 Bins (1 LSBIBin)
Figure 4. DC histogram using on-board isolated power; a 2.0V input signal; high-impedance input; a 20ksps
sample rate; 65,536 samples; a code spread of 6 LSBs with 96.57% of the codes falling within the three center
LSBs; and a standard deviation of 0.702.
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OIEJE‘. FFT gm Channels SINAD 841SdBFS SINAD 82 53dBc THD 88 E4dEc SFDH QB EBdBc Nome F‘DDI A129 SSdEFS 75000 40000 42000 44000 45000 46000 ‘00 Frequency (kHz)
Figure 5. AC FFT using external power, a 0.2V to 4.08V 1kHz sine wave input signal, high-impedance input, a
20ksps sample rate, and a Blackman-Harris window.
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Histogram 400007 37500- 350007 32500— 30000— 275007 250007 225007 20000— Occurrence I I I I 32003 00 32009 00 32010 00 3201100 Bins (1 LSB/Bin) I 3201200 I 32015 00 I 32014 00
Figure 6. DC histogram using external power; a 2.0V input signal; high-impedance input; a 20ksps sample rate;
65,536 samples; a code spread of 7 LSBs with 96.94% of the codes falling within the three center LSBs; and a
standard deviation of 0.721.
All Design Files
Download all design files.
Hardware Files
Schematic
Bill of materials (BOM)
PCB layout
PCB Gerber
PCB CAD (PADS 9.0)
Firmware Files
Nexys 3 platform (Spartan-6)
ZedBoard platform (Zynq-7020)
Buy Reference Design
Campbell (MAXREFDES4#)
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ARM is a registered trademark and registered service mark of ARM Limited.
Audio Precision is a registered trademark of Audio Precision, Inc.
AutoShutdown is a trademark of Maxim Integrated Products, Inc.
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.
Nexys is a trademark of Digilent 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.
Related Parts
MAX11100 16-Bit, +5V, 200ksps ADC with 10µA Shutdown Free Samples
MAX14850 Six-Channel Digital Isolator
MAX1659 350mA, 16.5V Input, Low-Dropout Linear Regulators Free Samples
MAX256 3W Primary-Side Transformer H-Bridge Driver for Isolated
Supplies Free Samples
MAX44250 20V, Ultra-Precision, Low-Noise Op Amps Free Samples
MAX6126 Ultra-High-Precision, Ultra-Low-Noise, Series Voltage
Reference Free Samples
MAXREFDES4 Campbell (MAXREFDES4#): 16-Bit High-Accuracy 4-20mA
Input Isolated Analog Front End (AFE)
More Information
For Technical Support: http://www.maximintegrated.com/support
For Samples: http://www.maximintegrated.com/samples
Other Questions and Comments: http://www.maximintegrated.com/contact
Application Note 5562: http://www.maximintegrated.com/an5562
SUB-SYSTEM BOARD 5562, AN5562, AN 5562, APP5562, Appnote5562, Appnote 5562
© 2013 Maxim Integrated Products, Inc.
Additional Legal Notices: http://www.maximintegrated.com/legal
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