Sunday, August 30, 2020

USB Isolation For Instrumentation Applications

 

USB in instrumentation applications is a 21st Century substitute for a 20th Century RS232 serial link, but with faster speeds. It still requires a conductive and grounded connection that can be several feet in length between the measuring device and the PC controller however and that is where the fun starts.

Trouble is, these communication links connect your carefully crafted measuring instrument to a PC or Laptop somewhere that may or may not have its own power supply, and then suddenly you have a massive ground loop through the AC power distribution system and all the other test instruments that may be in use.

Additionally now, nearly every test instrument includes a switching power supply that also produces ground currents due to the common-mode currents generated across their internal transformers. While these common mode currents can be carefully controlled and minimized by adding electrostatic shielding inside the transformer's windings, profit pressures have made such shielding very unlikely now.

This problem is created by common-mode currents generated by switching power supplies running down all the cables and completing the loop through the AC Mains connection. This loop is always many feet in diameter. Remember that a loop many feet in diameter has quite a large inductance and voltage across an inductor is proportional to the current rate of change,

           Vinductor = L * dI/dT

Even 60 Hz leakage current has a rate of change and will react with an AC Mains loop to produce unwanted 60 Hz voltages on your test bench. While this is not a concern and seldom noticed when probing around an embedded processor circuit. Having several switching power supplies in test instruments and the controller PC, etc. produce real havoc with low-level measurements.

Many times you can see the effects of this AC Mains ground loop by moving your hand around and touching various pieces of test equipment, the large capacitance of your body will change the path of the ground loop and you can observe the measurements changing. This is most readily apparent in real-time when looking at Oscilloscope displays when moving your hand about but it can be seen with sensitive DMM's also in that the DC values appear to change as you move your hand around.


What to do?

While running a noise-sensitive process or measurement sequence, the test instruments like a DMM, Oscilloscope, or Logic Analyzer can be disconnected from the measuring circuit, the controller PC remains, and that ground connection from the PC and it's switching power supply.

With a Laptop, the charger can sometimes be disconnected to cut that ground loop there, but this will only work for testing a few hours at best until the Laptop battery runs dead. This is not a solution for any tests, such as encountered when averaging very low noise measurements that can take 24 hours or more to run.

How about isolating the USB using one of those cheap USB isolators? There are many little isolators available for purchase online that do indeed cut the galvanic loop in the USB cable, but these common isolators also introduce noise of their own. This is because they add isolation in the USB digital side and any downstream device needs at least some trickle power to properly function. To get this trickle power these devices use – yes, you guessed it a very low cost and oftentimes low-quality DC/DC converter. These converters lack the necessary shielding on their isolation transformers and hence inject yet another source of noise inline, this time on the USB cable where you want it least.

These noise currents have to go somewhere and they will usually end up flowing back to each other through the AC Mains. That is again a huge ground loop.

 

Figure 2 – A generic USB Isolator based on the very popular Analog Devices ADuM3160 USB Bus Isolator chip [1]. This isolation works on the USB side and still contains another noise generator, namely in a very low-cost DC/DC converter (Black Block at Top of PCB) that is generating its own common-mode noise between the isolated system grounds. The Capacitance between the isolated grounds of this unit was measured at 44 pF.


Measuring the common mode current of the cheap Isolator from Figure 2 reveals that the DC/DC converter produces 18mV p-p across 100 ohms (180 uA p-p) as can be seen in Figure 3.
 

 

Figure 3A – The little DC/DC converter that is typical of these designs is not shielded and produces a lot of common-mode current. Using the test circuit shown in Figure 3B, it is measured as 18 mV p-p across 100 Ohms or 180 uA p-p. As can be seen the noise is impulse like and thus produces harmonics will into the VHF region where any wire length will act as an antenna launching the noise all over your lab.

 

Figure 3B – Typical test circuit for measuring the common mode noise of a DC/DC converter. Running from a battery source and a restive load not grounded to anything else and placed on a metal ground plane bonded to all the test instruments improves the measurement of the true common-mode noise. The voltage measured on the scope is converted to current by the formula: Icm = Vcm/100.

The other worry of these low-cost isolators is what isolation do they really have? Are they even Hipot tested? My suggestion is that if you want to use one of these that you use one that uses the full Analog Devices chipset like the Adafruit 2107 Isolator [2] because it uses a quality DC/DC converter that is Hipot tested by Analog Devices. Yes, it costs 5x more than the cheap designs, but at least you know the quality of the components.


Another Way

There is an alternative way to isolate the USB and that is to do the isolation on the Microprocessor side. This can be built into a system from the start. Since many projects use FTDI like USB to UART bridges [3] to translate the USB to a UART compatible serial signal, an optically coupled Isolator can be used between the FTDI chip UART output and the Microprocessor UART input. This has the advantage of not needing yet another DC/DC converter and that extra noise that that will produce.

A suitable Isolator is the OnSemi FOD8012A [4] as shown in figure 4. This part is rated for 15 Mbit/Sec operation which is well matched to the speed of the FTDI232RL Bridge IC. Normally I run my designs at 115,200 Baud for applications that require only low speeds to 921,600 Baud for systems that need more speed and native Windows Baud Rate compatibility to upwards of 3 M Baud for systems that can use a nonstandard Windows rate but need maximum throughput. The FOD8012A is well suited for these tasks.

 


Figure 4 – The FOD8012A is a simple to use 8 pin device. No additional DC/DC converter is required for operation since the USB side power can be supplied by the existing USB connection and the Host side is supplied by the existing Microprocessor power. Diagram provided by the courtesy of OnSemi.

 
Figure 5 – A complete circuit diagram of the "other, quieter" way to get USB isolation for a measurement product. The FOD8021A is simply added between the FTDI FT232RL USB Bridge chip and the Microprocessor Serial port lines. The FOD8021A can even do voltage translation from a 5 Volt to 3.3 Volt system.

 
Figure 6 – The actual implementation tucked into the corner of a PCB. The White FOD8012 can be seen as straddling the two ground regions. The measurement between the USB and measuring circuit ground shows 10 pF of capacitance between them which could be decreased by further by increased separation of the isolated and instrument ground regions on the PCB.

One thing to remember is that the outer metal shell of the USB connector cannot be connected to chassis ground or this will defeat the isolation. In the design idea presented here I use a 3D printed plastic bushing that I put in a larger hole than normal through the instrument's chassis that still is a tight fit around the USB connector shell. Doing this adds the mechanical support to the chassis necessary to keep the USB connector from being ripped off the PCB, while still maintaining the integrity of the ground isolation by keeping separation of the chassis ground and the USB connector shell.


Conclusion:

By isolating the USB connection after the USB/UART bridge IC another source of noise is eliminated and the overall system noise problems will be simplified. The bonus? Only one low-cost IC is required.
 

Bonus:

More information on how Common Mode Currents are generated in power supplies is available in this article,

http://analoghome.blogspot.com/2020/09/how-common-mode-currents-are-generated.html


References:
[1] Analog Devices ADuM3160 USB bus side isolator.
https://www.analog.com/en/products/adum3160.html

[2] Adafruit USB Isolator Product 2107
https://www.adafruit.com/product/2107

[3] FTDI Chip, USB to USART bridge chips.
https://www.ftdichip.com/Products/ICs/FT232R.htm

[4] OnSemi FOD8012AHigh Speed Digital Opto-Isolator
https://www.onsemi.com/products/optoelectronics/high-performance-optocouplers/low-voltage-high-performance-optocouplers/fod8012a


Article By: Steve Hageman / www.AnalogHome.com

We design custom: Analog, RF and Embedded systems for a wide variety of industrial and commercial clients. Please feel free to contact us if we can help on your next project.

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