APPLICATION NOTE 7134

MAX22000 Analog IO Configurations


Abstract:

This application note describes the internal building blocks of the MAX22000 which enable software configurable, universal analog I/O functions. Circuit configurations are shown for common industrial analog I/O modes.


Introduction

The MAX22000 is a software configurable analog input/output (I/O) device targeted to industrial applications such as process control and the I/O modules found in equipment such as programmable logic controllers (PLCs). Traditionally control engineers selected fixed function I/O cards (analog input, analog output, voltage, or current mode) and hardwired their PLC systems according to the sensor or actuator requirements. With the introduction of software configurable devices such as the MAX22000, it is now possible to design I/O modules to allow any common industrial I/O function (analog voltage input, analog current input, analog voltage output, analog current output, and temperature measurement with RTD or thermocouple), which can be software configured in the field. This flexibility translates directly into faster project installation and commissioning with a common module configured using software to adapt to the system I/O requirements.

The acronyms and abbreviations used in this application note are listed in Appendix A.

The MAX22000 Analog I/O Building Blocks

The analog I/O functionality of the MAX22000 is shown in Figure 1a and Figure 1b. Figure 1a features the simplified functional blocks defining the MAX22000, while Figure 1b is the detailed block level detailing internal functions of the MAX22000. As shown in Figure 1b, the key blocks highlighted enable the system designers to implement universal analog I/O modules using a single-chip solution based on the MAX22000. The MAX22000 integrates an 18-bit DAC with fast settling time, a multilayer 24-bit delta-sigma ADC, a precision voltage reference, programmable gain amplifiers (PGAs), and high-voltage analog conditioning functions. The analog output can provide a voltage in the range of ±10V, provide a current in the range of ±20mA, measure a voltage in the range of ±10V, or measure a current in the range of ±20mA. A separate dedicated analog input measures a voltage in the range of ±10V. Another differential analog input has a dedicated PGA, offering ranges of ±12.5V, ±2.5V, ±500mV, ±250mV, or ±125mV. The selected input can be sampled from 1sps to 115.2ksps. The MAX22000 is controlled using a SPI interface for all configuration and operation.

Simplified view of the MAX22000 analog I/OFigure 1a. Simplified view of the MAX22000 analog I/O.

The MAX22000 block diagramFigure 1b. The MAX22000 block diagram.

HVDDO/HVSSO supplies are necessary to power the analog output section. HVDD/HVSS supplies are necessary to power the analog input section of the device. The analog input and output sections each have different supplies as it makes the system design simpler and more easily configurable. For example, the analog inputs reading ±12.5V utilize ±15V at HVDD/HVSS. The analog output section might be required to generate 4mA – 20mA of current for a loop transmitter application with a typical 250Ω load. In this case ±8V might suffice.

The 18-bit DAC and 24-bit ΔΣ ADC is powered by low-voltage DVDD and AVDD. The communication interface with the external controller is powered by DVDD and is typically galvanically isolated from the controller in the application system for safety purposes.

Universal Analog Input/Output Configurations and Features

The common industrial analog I/O configurations and signal levels are shown in Figure 2, and implementations possible with the MAX22000 are listed in Table 1.

The MAX22000 analog input/output configuration and featuresFigure 2. The MAX22000 analog input/output configuration and features.

Figures 3 to 10 show the different configurations for the high-voltage analog I/O ports and how they can be implemented with the MAX22000, corresponding to the eight analog I/O configurations outlined in Table 1.

Configurations Comments
1 1 ACO + 2 AI (DE: V/I) For analog-current output using AI1 and AI2 for current feedback through external low ppm, low temperature coefficient sense resistors.
2 1 ACO + 4 AI (SE: V/I) For analog-current output using AI1 and AI2 for current feedback through external low ppm, low temperature coefficient sense resistors.
3 +24V Field Supply with ACI + 1 AI (DE: V/I) + 1AI (SE: V/I) Field supply with analog-current input (high side) at AI1 and AI2. Use low ppm, temperature coefficient sense resistors.
4 +24V Field Supply with ACI + 3 AI (SE: V/I) For analog-voltage output using AI3 for voltage feedback.
5 1AVO + 1AI (DE: I) + 3AI (SE: V/I) For analog-voltage output using AI3 for voltage feedback.
6 1AVO + 2AI (DE: V/I) + 1AI (SE: V/I) For analog-voltage output using AI3 for voltage feedback. (AI1, AI2) (analog-current input only).
7 1AVO + 5AI (SE: V/I) For analog-voltage output using AI3 for voltage feedback.
8 Universal analog I/O mode Software configurable analog input/output at a terminal block.

Current and Voltage Feedback

The 18-bit DAC in the MAX22000 can be configured as an analog-voltage output (AVO) or analog-current output (ACO). Configurations 1, 2, and 8 from Table 1 show ACO mode, where inputs AI1 and AI2 are used with a 50Ω sense resistor across the internal current-sense amplifier (CSA). This feedback ensures precision tracking of current output throughout different loads over temperature. Configurations 5, 6, 7, and 8 use AVO mode with AI3 used as voltage feedback for precision voltage tracking to maintain set voltage output.

1 ACO + 2 AI (DE: V/I)Figure 3: 1 ACO + 2 AI (DE: V/I).

Figure 3 depicts a single analog-current output at AI2 and two channels of differential voltage or current-input configuration at AI3, AI4 and AI5, AI6. The components highlighted in green feature the voltage sensor connection, and the components highlighted in red feature the current mode. A typical 250Ω resistive load placed across the differential inputs represent a typical current-loop connection.

1 ACO + 4 AI (SE: V/I)Figure 4. 1 ACO + 4 AI (SE: V/I).

Figure 4 depicts a single analog-current output at AI2 terminal with four channels of single-ended voltage or current-input configuration at AI3 to AI6. The components highlighted in green feature the voltage sensor connection, and the components highlighted in red feature the current mode. A typical 250Ω resistive load at inputs AI3 to AI6 in current mode represents a typical receiver-terminated current configuration.

+24V field supply with ACI + 1 AI (DE: V/I) + 1AI (SE: V/I)Figure 5. +24V field supply with ACI + 1 AI (DE: V/I) + 1AI (SE: V/I).

Figure 5 depicts a single analog current input channel with +24V field supply at AI3 terminal, one channel single ended voltage or current input at AI4, and one channel differential input voltage or current configuration at AI5, AI6. The MAX22000 DAC is capable of sourcing 0mA –20mA output current when the AI1, AI2 is configured as an analog input to handle varying loads in the current loop. This mode is generally preferred with the current-loop sensor that is loop powered with external +24V field supply absent.

The components highlighted in green feature the voltage-sensor connection, and the components highlighted in red feature the current mode. A typical 250Ω resistive load across AI5 to AI6 in current mode represents a current input measurement for typical current-loop connection. The typical 250Ω resistive load at AI4 to GND represents current input measurement for typical receiver terminated current configuration.

The voltage feedback signal at AI3 ensures a stable supply is provided by the DAC and the high side measurement of this current from the +24V field supply is measured across AI1 and AI2.

+24V field return with ACI + 3 AI (SE: V/I)Figure 6. +24V field return with ACI + 3 AI (SE: V/I).

Figure 6 depicts a single analog-current-input channel with voltage output configuration at AI3 and three channels of single-ended voltage or current input configuration from AI4 to AI6. The common terminal connected to AI1, AI2, AI3, and DAC is capable of sinking 0mA– 21.5mA input current while the port is configured as an analog output with a set voltage at 0V. Figure 6 with a voltage output is similar to Figure 5, but in this case the voltage output necessitates a stable return voltage to provide a sinking path for the current loop. This current loop is powered by an external +24V field supply. The number of loads in the loop determine the drop across the sensor and terminal at AI3. The DAC output should be set at either 0V or slightly negative in order to compensate for the voltage drop across the 50Ω sense resistor.

The components highlighted in green feature the voltage sensor connection, and the components highlighted in red feature the current mode. A typical 250Ω resistive load to GND at AI4 to AI6 in current mode represents a current-input measurement for typical receiver terminated current configuration.

1AVO + 1AI (DE: I) + 3AI (SE: V/I)Figure 7. 1AVO + 1AI (DE: I) + 3AI (SE: V/I).

Figure 7 depicts a single analog-voltage output channel at AI3, one differential current-input channel at AI1, AI2, and three channels of single-ended voltage or current input configuration at AI4 to AI6. AI1 and AI2 are differential inputs across the 50Ω sense resistor to limit the maximum differential voltage to ±1.25V.

The components highlighted in green feature the voltage sensor connection, and the components highlighted in red feature the current mode. A typical 250Ω resistive load across AI4 to AI6 in current mode represents a current-input measurement for a typical receiver-terminated current configuration.

1AVO + 1AI (DE: I) + 3AI (SE: V/I)Figure 8. 1AVO + 2AI (DE: V/I) + 1AI (SE: V/I).

Figure 8 depicts a single analog-voltage output channel at AI3, two differential current-input channels at AI1, AI2 and AI5, AI6, and one channel single-ended voltage or current input configuration at AI4. AI1 and AI2 are differential inputs across the 50Ω sense resistor to limit the maximum differential voltage to ±1.25V. AI5 and AI6 support differential input measurement of voltage/current input.

The components highlighted in green feature the voltage sensor connection, and the components highlighted in red feature the current mode. A typical 250Ω resistive load across AI5 and AI6 in current mode represents a current-input measurement for a typical current-loop configuration.

1AVO + 5AI (SE: V/I)Figure 9. 1AVO + 5AI (SE: V/I).

Figure 9 depicts a single analog voltage output channel at AI3 and five channels of single ended voltage or current input configuration AI1, AI2, AI4, AI5 and AI6. The components highlighted in green feature the voltage sensor connection, and the components highlighted in red feature the current mode. Typical 250Ω resistive load for single ended modes at AI1 to AI6 to GND in current mode represents a current input measurement for typical receiver terminated current loop configuration.

Universal Analog Input/Output Modes

Figure 10 shows the MAX22000 connected for universal analog I/O modes, covering input or output, voltage or current, and also temperature measured with a RTD or a thermocouple.

1AVO + 5AI (SE: V/I)Figure 10: Universal analog input/output modes.

With the DAC output either set as current or voltage output and AI5, AI6 differential inputs sharing the same way terminal block, a truly universal analog input/output can be achieved at the same terminal through software configuration.

Temperature Measurement Using a RTD or Thermocouple Element:

As shown in Figure 10, 2-wire, 3-wire, and 4-wire RTD measurement is enabled through the universal mode. When using a thermocouple, cold junction compensation is accomplished at low voltage ports AUX1 and AUX2 inputs. The low voltage (±2.5V) ports are not high voltage protected and must not be used with higher voltage field signals. AUX1 and AUX2 can be used separately in case of cold junction compensation or have flexibility to be used for other on-board diagnostic measurements.

For more information on the MAX22000 performance with RTD and thermocouple, refer to Application Note AN7186.

Application Examples for the MAX22000

Application 1: The MAXREFDES200# - Go-IO Factory Automation Card
Go-IO is an industrial internet of things (IoT) reference design designed for rapid prototyping and development of configurable industrial control systems. Go-IO is a combination of boards; the MAXREFDES215# is a carrier board with the I/O connectors, and the MAXREFDES211# is the application processor board. The MAXREFDES200# is an I/O card designed for analog and digital I/Os commonly found in factory automation applications. This I/O card plugs into the carrier card and is controlled by the application processor board.

Four types of MAX22000 analog /I/O are used in the MAXREFDES200#. I/O pin AO1+ is a universal analog I/O (UAIO), meaning it can be configured using software as an input (voltage or current mode) or as an output (voltage or current mode). AO1+ is capable of supporting input and output ranges of ±12.5V or ±25mA depending upon the software configuration—no hardware changes are required. This is accomplished using the MAX22000 pins for the DAC (AOP and AON), ADC inputs (A1 and A2) for the current-sense amplifier (CSA) and ADC input (A3) for the voltage-sense amplifier.

In addition to the UAIO port, other ADC inputs are brought out to the I/O connectors on the MAXREFDES215#. Inputs AI1+ and AI1- connect to ADC inputs A5 and A6 on the MAX22000. The programmable gain amplifier (PGA) within the MAX22000 can be set to different input voltage ranges, covering ±12.5V, ±2.5V, ±500mV, ±250mV, and ±125mV. Input AI2+ connects to ADC input AI4 with a voltage range of ±12.5V. Input AI3+ connects to ADC input AI1.

MAXREFDES200#Figure 11. The MAXREFDES200#.

Application 2: The MAXREFDES177#: IO-Link Universal Analog I/O Reference Design
The MAXREFDES177 aids in quick prototyping and evaluation of the MAX22000 in real-world industrial sensor and actuator applications. Featuring the MAX22515 IO-Link transceiver the design is fully isolated between the analog I/O side and the IO-Link interface side.

The analog port supports:

  • Voltage Output ±10V (±12.5V)
  • Current Output ±20mA (±25mA)
  • Voltage Input ±10V (±12.5V)
  • Current Input ±20mA (±25mA)

The MAXREFDES177# universal analog I/OFigure 12. The MAXREFDES177# universal analog I/O.

Summary

The MAX2200 features and configurability provide a great degree of freedom to design a configurable I/O module. The system with the MAX22000 is no longer constrained by fixed functionality, like input or output, or voltage or current. It is truly a universal analog I/O with complete software configurability. The internal blocks which feature 18-bit DAC and 24-bit ΔΣ ADC blocks enable the user to obtain (less than) < ±0.1%FSR performance for any unknown load over a -40°C to +125°C temperature range.

Appendix A – Acronyms and Abbreviations

ACI – Analog-Current Input
ACO – Analog-Current Output
AI – Analog Input
AVI – Analog-Voltage Input
AVO – Analog-Voltage Output
CSA – Current-Sense Amplifier
DE – Differential
I/O – Input/Output
Op-Amp – Operational Amplifier
PPM – Parts Per Million
Rs – Resistors
RTD – Resistive-Temperature Detector
SE – Single-Ended
V/I – Voltage or Current
VSA – Voltage-Sense Amplifier