The MAXREFDES1246 is a 4-channel universal analog input that measures voltage or current signals. Each channel can be configured for voltage input at -10V to 10V or current input at -20mA to 20mA. A 24V power supply can be switched out to power an external 2-wire, 3-wire, or 4-wire sensor.
The MAXREFDES1246 provides a complete system solution for industry voltage and current measurement. The analog front end (AFE) includes Beyond-the-Rails™ analog switches, and operational amplifiers are used to achieve voltage/current input selection and signal conditions. The 24-bit MAX11270 analog-to-digital converter (ADC) is the heart of the system and achieves highly accurate measurement.
The microcontroller communicates with MAX11270 and MAX7317 10-port I/O expander to control the AFE and accept ADC conversion results. The 24V field input power supply is isolated with the AFE and with the microcontroller, which is powered by a USB power supply. This isolated design enhances system robustness in harsh industry applications.
The MAXREFDES1246 also integrates five Beyondthe- Rails SPDT analog switches (MAX14763), a dual Beyond-the-Rails 4:1 analog multiplexer (MAX14778), two ultra-precision, low-noise op amps (MAX44251), 2.75kVRMS data isolators (MAX14930 and MAX14932), a STM32F1 microcontroller, a microprocessor supervisory circuit (MAX6730), a FTDI USB-UART bridge, a high-efficiency iso-buck DC-DC converter (MAX17681), and isolated/regulated +5V and +3.3V power rails (two MAX16910s) and a -5V power rail (MAX1697). The entire system typically operates at less than 500mW and fits into a 5cm x 10.6cm space. While targeted for industrial PLC applications, the MAXREFDES1246 can be used in any application that requires high-accuracy analog-to-digital conversion.
This document describes the hardware and firmware. It provides a detailed system design, especially for the AFE. This reference design has been built and tested, details of which follow later in this document.