Isolated ADC or Isolated Amplifier: Which to Use in Your PCB

Low-level analog measurements or low-SNR measurements, especially in high-voltage environments, often benefit from isolation as this can protect component interfaces and ensure user safety. Even when measurements are not being taken in a high-voltage environment, there is still a benefit to enforcing isolation in an analog measurement purely from a noise control perspective.

Unfortunately, most designers will likely split the ground under their ADC when working in this application. This gives some kind of noise isolation (in terms of ground currents), but it does not give any galvanic isolation and it creates the well-known dipole antenna EMC failure mode.

To address this problem, semiconductor manufacturers have developed two types of components: isolated ADCs and isolated amplifiers. Both are highly useful for taking isolated analog measurements, including in situations where high withstand voltage is required. Using these components properly also eliminates the EMC risk that comes with splitting grounds into different nets. I’ll give an overview of these components and how they can be used in a PCB in this article.

Isolated ADCs vs. Isolated Amplifiers

I have covered the usage of isolated ADCs in another article, and I recommend readers refer to that article for information on these components. Rather than repeat those technical points, I would prefer to compare isolated ADCs and isolated amplifiers.

Isolated ADCs and isolated amplifiers are two types of galvanically isolated components which are used for analog measurements and sensing. An isolated ADC gives a direct measurement of a signal on its own, and the output is digital data over a standard interface (usually I2C or SPI). Isolated amplifiers do not give the same kind of direct measurements, instead they have an analog output that must also be sensed by a standard ADC or further manipulated with a standard opamp/comparator circuit.

Isolated amplifier (PN: AMC4311D)

Isolated ADC (PN: AMC0106M05)

Although they are used in different ways, they have some common characteristics:

• Low peak-to-peak input voltage (only a few V)
• High galvanic isolation (up to 5 kV in some components)
• Separate power supplies are required on both sides of the isolation barrier
• Often tested to specific safety and/or EMI standards
• May have single-ended or differential outputs

These characteristics make both components a good choice when sensing an analog signal that is at low level, originates in a high-voltage environment, or both. As an example application, take a look at the datasheet for part number AMC4311D from Texas Instruments. In this case, the component is being used directly for a measurement of a high-voltage bus by dropping the voltage over a resistor divider, and the output is given to an ADC.

Peculiarities of Isolated Amplifiers

Isolated amplifiers are unity gain buffers with high galvanic isolation, typically providing much larger isolation than small transformers. As these are fixed unity gain buffer components, they do not provide the designer with any ability to actually amplify a signal. Instead, this would have to be done with an additional op-amp circuit (possibly with filtration).

The additional amplification and filtering could be done on the input or output side of an isolated amplifier, as shown in the image below.

Which of these options is implemented depends on if the amplifier is being used in a high-voltage or low-voltage environment. In a high-voltage application, it’s best to simply use an isolated amplifier as a buffer as this will protect the other circuitry in the analog interface. However, in a low-voltage environment where there is little risk of fault, some additional components could be placed safely on the input side.

PCB Layout For Isolated Amplifiers and ADCs

Although these components are quite different, isolated ADCs and isolated amplifiers have similar layout requirements, and these are implemented with the primary goal of maintaining galvanic isolation across the internal isolation barrier.

To do this, there are some simple guidelines to follow:

• Clearly define two different ground nets for the input and output sides of the amplifier/ADC
• The split between the two grounds should span through the entire stackup
• Keep input side routing above the input-side ground plane (same for the output side)
• If additional amplifier/filter circuits are needed, consider placing these on the output side in most cases

Note that these components can require supplies on both sides of the component package (see the component schematic diagram above). Unless you have an isolated DC/DC converter with an auxiliary coil, the input and output sides will need physically distinct supplies. These are low-current components, so LDOs will be acceptable supplies. Another option in isolated DC/DC converters with isolated ADCs/amplifiers is to use an auxiliary winding on the transformer; this can step up/down the input into an acceptable range.

Aside from these points, the layout requirements are quite straightforward. We created a dual-ADC module project which includes an isolated ADC; an isolated amplifier can closely follow the same process we show on video. To learn more about how to use isolated ADCs and isolated amplifiers in a PCB layout, watch the video below.

Whether you need to build reliable power electronics or advanced digital systems, use Altium’s complete set of PCB design features and world-class CAD tools. Altium provides the world’s premier electronic product development platform, complete with the industry’s best PCB design tools and cross-disciplinary collaboration features for advanced design teams. Contact an expert at Altium today!