Ask Michael: Let Your Customer (NOT Your PLC) Choose How to Use Its Channels

Ask Michael: Let Your Customer (NOT Your PLC) Choose How to Use Its Channels

Hi, Michael:

I am trying to reduce the number of different ICs required in our latest model of PLC. We have always used multiple converter ICs for the different input signal types (4-20mA, voltage, RTD, and thermocouple). This approach has constrained the number of channels we can allocate to each, which frustrates some customers. Do you guys have a part that can accept different types of input signals on a single channel that would allow us more flexibility in our design?

Patrick, New York

Hi, Patrick:

The permutations and combinations of numbers and types of industrial sensors that customers want to keep track of can be mind-boggling–no two users ever seem to use a piece of equipment in the same way. Some want lots of channels for a single sensor type while others need a liberal mix of them all. Then, of course, there is the conundrum of how many single-ended versus differential channels to provide…

Thankfully we have a part that I think will make things a lot simpler for you–the MAX11410 multichannel 24-bit ADC, a low-power, high-precision device for accurate sensing. This part is great because it has the flexibility to allow any type of sensor to be connected to any of its input channels. Even better, all of the channels can provide current to external sensors that need it, e.g. an RTD. Figure 1 shows the IC working with a 4-wire RTD (but it also works for 2- and 3-wire types).

Figure 1. MAX11410 multichannel 24-bit ADC delivers accurate sensing.

Its 10-channel input multiplexer has the added advantage that any of the inputs can be used either in single-ended mode or in conjunction with any other channel in differential mode. This means it can accommodate up to 10 single-ended input channels or 5 differential inputs or combinations of single-ended or differential channels. As a result, your customer gets to decide how to use the channels you provide–not the PLC!

Apart from the degree of flexibility it offers in the choice of sensor inputs, this ADC has some other features which you will be interested in. The current sourced from the input channels ranges from 10µA to 1.6mA in 16 programmable steps. It has a programmable gain amplifier (PGA) with gain settings between 1x and 128x, which ensures that even sensors with very low output levels can be amplified to fall within the dynamic range of the ADC. It also provides for a choice of three reference inputs.

Finally, as you'll only be too well aware of, Patrick, an extremely important requirement for equipment operating in noisy industrial environments is the ability to reject mains frequency (50Hz/60Hz) interference. The MAX11410 is well equipped to deal with this–offering at least 90dB of multi-harmonic rejection. Oh, I should also mention that it has a broken-wire/burnout detect functionality that helps find sensors that need to be replaced–a constant headache that will help equipment technicians to reduce process downtime. It provides all of the above in a tiny TQFN package with a small footprint (4mm × 4mm), which consumes only 400µA of current in typical use.

There is an evaluation kit available that will allow you to assess the MAX11410. MAX11410EVKIT is USB-powered and features:

  • Selectable on-board voltage reference (2.5V, 1.25V)
  • Isolated power and digital communication
  • Various sample rates and sample sizes
  • Time domain, frequency domain, and histogram plotting
  • Savable plots and register configurations
  • Optional external clock (2.4576MHz)

Hopefully now you'll only need one type of ADC for the input channels on your new PLC design, Patrick.