Take a look at mixed-signal systems operating at low noise, especially those used for instrumentation and sensitive measurements. They seem to be able to carefully resolve low-level signals without succumbing to excessive noise. At certain frequencies and in certain voltage ranges, it may be necessary to break some of the well-accepted rules surrounding a grounding in mixed-signal systems in order to acquire accurate representations of a signal.
ADCs are the main components in mixed-signal systems where the digital and analog worlds meet, and best practices for using these components dictate the use of a single ground net to support digital and analog routing anywhere on the PCB. But what happens when we need to have isolation in the system, either for noise control or for user safety?
In this case, we can use an isolated ADC to enforce galvanic isolation. Another option is to use some component that provides coupling between galvanically isolated regions in a system, which then interfaces with a standard ADC. Both options provide an opportunity to control noise and prevent hazardous electric shock. In this design guide, we’ll show how to use these options to provide isolation, starting with isolated ADC component options.
An isolated ADC includes two galvanically isolated regions between the digital I/O section and the input analog interface. These components are built by following a simple concept: in a system that requires galvanic isolation, two sides of a system will be built over separate ground regions. In an isolated ADC, galvanic isolation is enforced between the analog and digital sides of the system. In this way, digital signals will not interfere with the analog signals on the analog side as long as those digital signals are confined to the digital ground plane.
Some situations where these components could be used include:
High voltage systems with a digital section, where some measurement is needed in the system
Specialty instrumentation involving the measurement of very low SNR analog signals
Electromechanical systems or switching systems where fast pulses create power surges that could damage components
Depending on where the signal to be measured is being routed in a PCB, an isolated ADC might make more sense to use than an optocoupler or transformer for isolation.
An example block diagram for an isolated ADC is shown below. In this example, the two GND nets in the component (AGND and DGND) are isolated from each other by some gap that is built into the component. This means that on the PCB, AGND and DGND are connected to physically separate pieces of copper.
When I write “physically separate pieces of copper,” I’m referring to different sections of copper pour that are assigned to different nets. Only the input channels on the analog interface (A_IN_1…A_IN_N) will have their signal levels referenced to the AGND net. The digital I/O block will have some galvanic isolation rating that extends up to some maximum voltage and some maximum frequency.
This separation in the block diagram must be implemented on the PCB. The best way to do this is to place two separate contiguous regions in the PCB to implement galvanic isolation, and only create a bridge between them with the isolated ADC. This will ensure the successful implementation of galvanic isolation but without the issue of creating a multipole antenna by trying to interleave separated ground planes.
An isolated ADC has a few important specifications that must be considered to ensure noise control and safety in mixed-signal systems with galvanic isolation.
Channel count - There will be multiple input analog channels that can be used to sample incoming signals
Isolation voltage - In high-voltage systems, the sampled signal could originate on the analog side, so galvanic isolation will have some high voltage rating reaching kV values
Output bus - This will typically be an SPI bus at low sample rates, or it could be a JESD204 bus at high sample rates (reaching GHz)
Internal vs. external reference and regulation - Some isolated ADCs have internal DC/DC conversion and precision referencing
Dynamic range - This is important when sampling low-level signals, and some low-level signals may require amplification in order to fill the ADC’s dynamic range
Resolution - This has to be weighed against dynamic range and SNR; for low SNR signals, low resolution will provide higher noise immunity
One of the big problems with split-plane systems, such as the PCB that would be used with an isolated ADC, is the fact that one or both grounds are floating. In other words, there can be some difference in the ground potential measured between the two sides of the system. This is a non-trivial problem to eliminate by designing isolated power supplies, and the ground offset could be a function of frequency. The result is that, at certain frequencies, the potential difference between these grounds could oscillate, and this would create radiated EMI.
The simplest solution here is to use a safety capacitor to connect the AGND and DGND ground regions. A safety capacitor (e.g., ceramic or metalized paper) can provide a low-impedance path for currents created by a ground potential offset, thus they will return to the system power supply in a low-inductance loop rather than radiate into free space. The placement shown below is the same placement that would be used alongside a transformer in an isolated power supply.
If a safety capacitor will be used across the ground, then three specifications should be examined:
Moderately high capacitance
Low DC leakage current
High DC/AC voltage limits
The reason for this is to minimize leakage current into a region where a user might interact with a device and possibly receive a slight shock. The capacitance only needs to exceed the parasitic capacitance between the galvanically isolated regions and the capacitance across the isolation gap inside the isolated ADC. Typical safety capacitor values do not exceed ~1 uF with voltage ratings in the 100’s Volts range.
The AMC1333M10 from Texas Instruments provides high isolation with peak voltage reaching up to 8 kV. This component has a built-in clock providing up to 39 kSps sampling (87 dB dynamic range) for a single channel with delta-sigma modulation. The output interface is a simple serial output routed in parallel with a clock output for simplified timing in an MCU. This component is an excellent choice in systems that may be in danger of experiencing large voltage exposure but that do not require a complex digital subsystem to sample and synchronize data.
The MAX14001 and the related MAX14002 from Maxim Integrated employ a 10-bit SAR architecture with a nominal sampling rate of 10 kSps. Configuration, filtered data, and unfiltered data are input/output through an SPI interface. Galvanic isolation in this component is guaranteed up to 3.75 kV RMS voltages, so this component is also suitable for use in a high-voltage environment. Between these two components, only the MAX14001 can limit repetitive inrush pulses to prevent overheating from abnormal input signals that would otherwise trigger a continuous stream of inrush pulses.
Isolated ADCs are very useful components, but they are not the only type of component that can be used to enforce isolation in a mixed-signal system. There are two other components that are traditionally used for isolation: optocouplers and transformers. Both components can be used as part of an isolation strategy, but this can be done without the need for an isolated ADC. Instead, these components are coupling the analog signal between two isolated regions, or they are coupling the digital data out from the ADC.
The table below summarizes when to use different types of coupling mechanisms with analog or digital signals. In short, a transformer should not be used to couple digital data between galvanically isolated regions. The reason for this is that the transformer converts the digital signals to pulses because the transformer only couples electromagnetic energy when the input signal is switching. Therefore, if you must use a coupling mechanism with digital data rather than an isolated ADC, an optocoupler would be preferable.
An optocoupler IC is often used in isolated power supplies to route a feedback line from the output side to the input side while ensuring galvanic isolation between both sides of the system. An optocoupler can be used with digital or analog signals, but arguably the best use case is to couple analog signals across a gap into a non-isolated ADC.
An optocoupler could be used to couple the digital bus output across the galvanically isolated regions rather than the analog signals. However, this may not be the best option for coupling signals because it could alter the edge rate and timing, so there is a slight risk of violating setup and hold times on the receiving component. Therefore, the best usage may be to use an optocoupler only with the analog signals to be sampled.
A transformer is suitable for bridging the analog and digital sides of a galvanically isolated mixed-signal system as long as safety capacitor guidelines are followed. This would be used when an analog signal needs to be sampled in, for example, a high-voltage environment. Another option is to step up the voltage of a low-level signal that is then fed into a non-isolated ADC.
The reason the transformer is only used with an analog signal is that it only couples a signal between the galvanically isolated sides when a signal is switching. If the transformer were being used on the digital side to couple the digital output across an isolation gap, the transformer will lose the data because it converts the digital edge rates into pulses. Therefore, it can only be used with analog signals.
Mixed-signal systems that implement isolation and must provide accurate low-level signal measurements require many other components beyond isolated ADCs. These components range across processors, amplifiers, filters, and much more. DACs are also useful in these systems, although isolated DACs are less common than isolated ADCs. Some of the other components designers might need to include:
Designers that want to find unique components like isolated ADCs for mixed-signal systems can implement a just-in-case supply chain strategy with Octopart. Only Octopart provides advanced search and filtration features to help buyers find components and up-to-date distributor pricing data, parts inventory, and parts specifications. Take a look at our integrated circuits page to find the components you need.
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