FAQ: The MAX22190 Octal Industrial Digital Input Device with Diagnostics


The MAX22190 is an octal industrial digital input device featuring high integration, reduced power and heat dissipation, and fault-tolerant features with built-in diagnostics. This series of frequently asked questions (FAQs) answers the most common questions about MAX22190 operation for industrial applications.

What are the key benefits of the MAX22190?

The MAX22190 offers the following key benefits for the industrial design engineer:

  • High levels of integration, reducing BOM count and board space
  • Reduced power and heat dissipation, making it ideal for fanless operation
  • Fault tolerance with built-in diagnostics, enhancing system reliability and increasing uptime
  • Configurability to support multiple sensor types with a single SKU

What are the target applications for the MAX22190?

MAX22190 is often used in digital input modules found in programmable logic controllers (PLCs).

Is MAX22190 a replacement for previous Analog Devices digital input devices such as MAX31910–MAX31915 products?

The MAX22190 is a functional replacement/upgrade, but it is not a 100% pin-compatible replacement. The MAX22190, however, provides many additional features compared with the older generation of MAX3191x products.

What are the main differences between MAX3191x and MAX22190 products?

MAX3191x products offer only a subset of the features found in MAX22190. Table 1 shows a top-level comparison of the products and highlights the benefits of choosing the MAX22190.

Table 1. MAX3191x vs MAX22190 Comparison
Feature MAX3191x MAX22190 MAX22190 Benefit
Sensor Support Type 1, 3 Type 1, 3, and 2 Reduces SKUs
Wire-Break Detection No Yes, including adjustable wire-break threshold at input channels and open/short detection at REFDI and REFWB pins Increased system integrity and reliability
Fault Detection Field supply and temperature monitors Field supply and temperature monitors, CRC and SPI Frame errors, wire-break detection, Open/Short detection Increased uptime due to fault tolerance and built-in diagnostics
LATCH signal to synchronize all inputs prior to serialization No Yes Simultaneously samples all input signals across multiple ICs
Package TSSOP-28 TQFN-32 Smaller footprint
Integrated Input Protection No Yes Robust, up to ±40V Field Input, Surge Protection

Can I interface to IEC 61131-2 Type 1, 2, and 3 sensors?

The MAX22190 is the first device to support accurate current limiting for Type 1, 3, or 2 sensors. The MAX22190 can be configured as an octal device for Type 1 and Type 3 sensors or a quad device for Type 2 sensors.

How do I select the current limit?

An external resistor connected to the REFDI pin sets the input current limit. An external resistor connected to the REFWB pin sets the current limit for the wire-break alarm threshold.

Can MAX22190 field inputs support both current-sinking and current-sourcing sensors?

No. MAX22190 inputs only support current-sourcing sensors.

How do I electrically isolate the MAX22190?

The easiest way to electrically isolate the MAX22190 is to isolate the SPI port and the control signals (FAULT, READY, and LATCH). Analog Devices has a companion product—the MAX14483 six-channel digital isolator that provides a single-chip solution with 3.75kVRMS isolation. The MAX14483 has the additional benefit of providing voltage-level translation between the MAX22190 and a lower voltage MCU or FPGA.

Can I power the MAX22190 from the low (5V) supply if the higher (24V) field supply is not available?

Yes. MAX22190 can be powered using a 3.0V–5.5V supply connected to the VDD pin, with the VDD24 pin left not connected (floating). This configuration has lower heat dissipation.

What does the Wire-Break Alarms feature do?

This feature is intended to verify the integrity of field wiring between the sensor element and the MAX22190. A comparator with programmable threshold senses the presence of the small input current produced by a two-wire proximity sensor in its open state or of the current from an open switch with a diagnostic resistor placed across it. The wire-break current threshold is set by placing a resistor between the REFWB pin and GND.

What is the Wire-Break Filter Delay?

Electrical characteristic tWBD is the Wire-Break Filter Delay, which is 20ms (typ), or the typical delay from a wiring fault occurring and the relevant bit in the WB register being set.

What does the PCB Fault Alarms feature do?

The MAX22190 monitors the REFWB and REFDI pins to determine if they are open-circuit or short-circuit. In normal operating conditions, these pins are neither open nor short, so no faults are detected. If at least one of the current-limit setting resistors for either of these pins is not installed correctly, however, then the FAULT2 register generates a fault. If the current sensed at one of these pins is less than 6.6µA, for example, an open-circuit fault for that pin is detected; if the current sensed at one of these pins is more than 550µA, a short-circuit fault for that pin is detected. Four bits in the FAULT2 register—two for each pin—can be set to indicate open-circuit or short-circuit faults in the pins.

Can I daisy chain multiple MAX22190s?

MAX22190 has four operating modes, selected by pins M1 and M0. The daisy-chain operation is supported, provided mode 2 or mode 3 is selected. Daisy-chain is NOT supported for mode 0 or mode 1.

Can I mix-and-match MAX22190 with digital output devices such as MAX14912?

Yes, but if the devices are daisy-chained, ensure the operating modes selected for each device are compatible.

How do I write microcontroller code to use the CRC feature in the SPI interface?

In SPI mode 0 and mode 2, a five-bit CRC can be used to confirm data integrity during transfer between the MAX22190 and an external device such as a MCU. This requires the use of 24-bit frames—for SPI write cycles, the external MCU is required to calculate the CRC and clock it into the MAX22190 on the SDI line. The MAX22190 sets an error flag if the received data has a CRC error. For SPI read cycles, the MAX22190 generates the CRC and clocks it out of the MAX22190 on the SDO line. The MCU receiving this data is required to check if there is a CRC error and flag accordingly.

How do I meet EMC Standard Compliance and guarantee performance to IEC 61000-4 specifications?

The MAX22190 EV Kit schematic provides recommended external components to meet IEC 61000-4-2, /-4, /-5 standards for ESD, EFT, and Surge.

What does the term "Energyless LED" mean?

If the sensor is on, the voltage seen at the MAX22190 INx pin is high (typically 6V low-to-high threshold). The LED connected to the corresponding LEDx pin draws current from this field input voltage, as opposed to the MAX22190's field supply. Therefore, the term "Energyless LED" is used because the LED operation does not increase the field supply loading.

How do you recommend I layout my PCB for MAX22190?

The MAX22190 must have a solid ground plane underneath the entire exposed pad (EP) area with multiple thermal vias for best thermal performance.

What does the input filter do?

The filter is intended to debounce and remove glitches from slowly transitioning input signals to prevent false triggering. The programmable filter on each input can be programmed as bypass or with a specified delay. For example, the user selects a delay value of 3.2ms. The total delay time between the input switching and output switching, however, can be >3.2ms if there are glitches during this delay period, because the internal filter counter reverses counting direction during any glitches.

Bypass mode has no "extra" filter delay, and the typical input-output delay is 2µs.

What is the minimum detectable input signal and what is the minimum delay to read the data?

There are three electrical specifications in the data sheet:

fIN = field input sampling rate (1000kHz)

tPW = minimum detectable field input pulse width (3µs)

LATCH delay = delay until IN1-8 input data is frozen (50ns)

Any signal less than 3µs is detected and passes through the input filter. It takes a maximum 50ns for the device to freeze all eight inputs from when a LATCH signal falling edge is detected, prior to input filter and serialization. Assuming a 10MHz SCLK and 16-bit SPI frame, the minimum time to extract data is 1.6µs.

If fIN = 1/tPW why isn't tPW = 1µs?

The sampling rate is 1000kHz (1µs) and there is a minimum delay of 2µs from the filtering circuit (in Bypass mode). Therefore, the minimum detectable pulse width is 3µs.

I like the features of MAX22190, but I need a parallel, rather than SPI interface?

MAX22195 offers similar functionality to the MAX22190 but has eight outputs capable of driving 3.0V–5.5V CMOS loads, providing a simpler, and higher speed interface than SPI.

I only need one or two individually isolated DI channels?

MAX14914 is a configurable digital input/output (DIO) device that also supports Type 1, 2, or 3 inputs.

MAX22191 is a single-channel digital input device that can also support Type 1, 2, 3 sensors. It can be "parasitically" powered and works with both current sinking and current sourcing sensor types.