Analog circuit design for IoT with consideration of various analog sensor inputs

Various sensors are used in our familiar products. In recent years, sensors have been used for a variety of purposes in the IoT.

It would be easier if the outputs of the sensors were unified, but of course it is not.

A modular type has a digital output, and even an analog type has a different amplitude, differential / single output, different output impedance, and so on.    

Terminal equipment that collects sensor information has an analog input to obtain sensor information. What should I do with the design of this part? Of course, it is easy if the sensor is fixed. You just need to make it according to it.

However, there are terminal devices that can be connected to a wide variety of sensors. What should we do in such cases?

First, consider an analog sensor input with a different amplitude.

Basically, we will adjust the amplifier circuit of the operational amplifier which is the basis of the operational amplifier.

The amplification factor is determined by the ratio of R1 to R2 above, so you can adjust this resistance to accommodate the input of various amplitudes,

It's better to use a digital potentiometer because it's easier to change the resistance in a soft way during mass production.

Analog Devices offers a variety of digital potentiometers, so you can use it as a reference.

Analog Devices' Digital Potentiometers

You can also arrange the resistors and use an analog switch to select the resistor.

However, in any case, it is true that we must consider various things such as selection of resistance value, consideration of dispersion of resistance, consideration of layout, etc. Although it is a simple circuit, there are many things to think about.

Here, I'll tell you a simpler way.

Analog Devices provides a reference circuit called CFTL (Circuits from the Lab).

To briefly describe CFTL, it is a reference circuit set for realizing some function. The circuit is actually provided as an evaluation board as well as a desk top, and the circuit diagram, layout information, and experiment data are also provided. The user can use some or all of the operational circuits as they are.

Circuits form the Lab

In this application, CFTL called CN 0385 can be used. The block diagram is as follows.

Let me briefly introduce the configuration of the signal line.

• ADG5027：8ch input differential multiplexer 
• AD8251：Instrument amplifier with PGA
• AD8475：ADC driver with the function of converting single input to differential output
• AD4003：18 bit, 2.0 MSPS SAR type AD converter
• ADuM141E：Digital isolator to isolate digital signals

In particular, the AD8251 has a PGA (Programmable Gain Amp), so it can handle analog inputs of various amplitudes. Specifically, it can handle the following general input ranges.

With the CN0385, the amplitude problem is solved, and the AD converter and insulation are also possible.

Of course, you can replace it if you don't need such a high-precision A/D converter, or you can skip that part if you don't need insulation.

We believe that the use of a proven CFTL will greatly contribute to shortening the design period

Details of CN0385

Assuming that the amplitude problem has been cleared, there are various other analog sensor outputs. Below are some notes.

It is a sensor with a warning such as the output of a photodiode or "use a shielded cable that is not affected by noise.". In this case, it is essential to use a low-bias amplifier in the first layer.
It is desirable to arrange low-bias operational amplifiers such as ADA 4610 in the first layer.

This is a little troublesome. There is a problem of what to do with the 0 point guarantee circuit. Of course, it is possible to make it only for thermocouples, but I would like to make it for general use if possible. Analog Devices has a dedicated amplifier for thermocouples, you can use AD8494/95/96/97 depending on the type of thermocouple.

The amplitude of the load cell input is very small. As the gain may have to be 100 times or more, the range is not compatible with the CN 0385 introduced above. When the gain is high, the offset is also affected by the gain, so the keyword is "low offset".

I recommend a chopper type amplifier that can cancel the offset, such as ADA 4638.

It is common to input the current by converting it into a voltage with a resistor.

Current source and AC coupling circuit are required.

As described above, the amplifiers that can be used vary depending on the application, so it is difficult to deal with various analog inputs only with a software configuration.

One idea is to use a general-purpose package (such as the SOIC 8 pin) that can be used only by replacing the IC. (For example, a pattern for the SOIC 8 pin is prepared in front of CN0385, and the IC is selected according to the sensor.)

Although it is quite difficult to find a universal product that can connect all analog sensors, I hope that Analog Devices will be able to follow up on most of its CFTL reference circuits and diverse analog product lineup.

Circuits from the Lab reference design

Macnica has applied the Circuits from the Lab reference design to develop its original product, the Analog Sensor Terminal, which can be connected to a variety of analog sensors and can be easily transmitted to a PC or gateway, offering the potential for edge computing with a wealth of CPU power.