|
The following article entitled "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
Example PCB Fabrication from Mer-Mar Electronics
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
Mer-Mar Electronics is a California based company, providing complete solution for
contract manufacturing services from PCB Fabrication, PCB Assembly, Cable harnessing
all the way to Box Build, we provide both Consignment (labor only) and Turn-key
services for quick-turn and standard delivery schedules. We also provide custom
system enclosures to sustain harsh environments making them a powerful and reliable
tool to reduce and eliminate costly downtime. As a service provider, we will only
manufacture to highest quality standards and all products must pass our stringent
Quality inspections.
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
|
