Circuit Design: Analog Isolation Techniques for Your Next Circuit Layout

The following article titled "Circuit Design: Analog Isolation Techniques for Your Next Circuit Layout" was prepared for and submitted to RF Cafe by its author, Suresh Patel, of MerMar Electronics. He wrote asking whether he could provide an article on an aspect of PCB design relevant to RF Cafe visitors, so I suggested addressing the issue of placing digital and analog / RF signals on a common substrate. Below is his submission which briefly discusses that as well as power supply isolation.

At MerMar Electronics, we have a dedicated team of employees for manufacturing aerospace printed circuit boards in California, USA. With 40 years of experience and an advancement in circuit board technology, our team provides excellent DFM assistant, expert review of printed circuit board design. We have an in-house engineering team & make sure the final deliverables of aerospace PCB assembly are robust, secure and reliable! MerMar Electronics has an established company for high quality PCB for military for years to come. MerMar -- Leading the Way for over 40 Years!

Mer-Mar Electronics: Analog Isolation Techniques for Your Next Circuit Layout

Essentially there are two types of power supplies that can be designed into a circuit board - isolated and non-isolated. When we speak of isolation, we typically refer to isolation between the inputs and outputs of the power supply. A non-isolated power supply on the other hand relies on integrated circuits, which does not offer the same level of protection.

With an isolated power supply, safety to the user is a given. Especially in medical devices where there is high voltage involved, you certainly want to make sure that the high voltage does not shoot into the user. One needs to take care of this aspect most when PCB Box Build Assembly is done, as the manufacturer takes care of everything in it.

What then are some of the techniques for analog signals to be isolated and importantly what are their relative merits and demerits? It will be worth taking a deep dive into the matter.

Isolation Transformer

An oft-used method is the isolation transformer. It uses coils that can transfer energy through a magnetic field. The two coils are electrically isolated from each other. The issues associated with this isolation methodology, however, are as under:

• There are chances of surge energy being transmitted across the transformer. The large inductance of the transformer means that it can resist sudden energy spikes. However, mains surges will be hard to resist.
•  The other issue is that while transformers operate in the AC realm, DC analog voltages cannot be isolated.
•  Also, since transformers are designed to work at specific frequencies, they are not suitable for measurement of signals with an unknown frequency.

Switching to Digital Signal

Unlike analog signals, digital signals are typically easy to isolate. It is therefore prudent to convert an analog signal into a digital pulse width modulation signal. The PWM signal is then isolated using an optoisolator. Subsequently, the output of the optoisolator is re-converted into an analog signal. The big advantages of this method include the fact that it offers protection from electrical surges and ESDs. However, it needs to be seen that if the PWM is generated using counters, the analog signal is quantized. Simply put, it means that the isolated analog signal will not be true to the original signal. Also, since the circuitry that is used to convert the analog signal isn't isolated, there could be damage from ESD and other sources.

Linear Optical Isolation

This technique involves the use of an optoisolator in its linear range. The disadvantages associated with this method include:

• Optoisolators have a narrow range of linearity.
• Analog voltages have to be converted into a current.
• Optoisolators have different characteristics and could behave differently. To solve this issue, a special range of optoisolators are used which includes two matched IR LEDs that can be used together in an op-amp circuit so that the non-linearity is fed back into the amplifier.

Isolation Amplifier

These are integrated circuits that use any of the above methods for analog isolation. Some isolation amplifiers use an internal transformer to ensure analog isolation. This involves using a voltage-to-frequency convertor. This converts an incoming analog voltage to a carrier wave. Subsequently the carrier wave is passed through an internal transformer and then the carrier wave is reconverted into analog voltage through a frequency-to-voltage converter.

Mechanical Methods for Analog Isolation

Mechanical methods are known to provide a high degree of insulation. However, they are also known to be slow as also not too much suited for high frequency signals. The other disadvantage of mechanical methods is that they are dependent on mechanical parts which are known to undergo wear and tear. One example of such kind of isolation is the use of potentiometer.

To Sum Up

As the above options suggest, analog isolation is a complicated process. With many applications all you may need is protection and not isolation. Hence the need for isolation should be carefully assessed before undertaking it. Where the need for isolation is established, typically a ready to use solution offers ease of implementation although it can come at a high price point. The other option is to go with a custom solution, which can, not only be complex but also very time consuming.

About Mer-Mar Electronics

Additional services we offer include the following, but not limited to:

• SMT & Thru-hole 01005 & 0201
• Component Placement
• Quick-Turn Assemblies (Same Day & Next Day)
• Electro Mechanical Assembly
• Complete Box Builds / System Builds / System Integration
• Press Fit Connectors
• Testing
• AOI / X-Ray Check
• Prototype & Production Quantity

Contact Info

Suresh Patel

Mer-Mar Electronics

7042 Santa Fe Avenue East

Hesperia, CA 92345

Tel : 760-244-6149

Email: sales@mermarinc.com

Website: mermarinc.com

Posted August 30, 2021