ADC Isolation Research

Abstract

There is a need to isolate external ADCs.

Electrical Isolation

Electrical isolation is a method of physically and electrically separating two distinct parts of a circuit; meaning that current does not flow between the two parts of the system that are being isolated. In regards to measurement accuracy, the greatest advantage of electrical isolation is that isolation breaks ground loops.

It is important to remember that isolation uses physical and electrical barriers to provide safety benefits by keeping high voltages or high transient voltages away from important circuit components (such as the microcontroller).

Brief Overview of Ground Loops

Ground loops are the most common source of noise, and occur when two connected terminals in a circuit are at different ground potentials. This causes current to flow between the two terminals and this potential difference is a source of error in the measured voltage.

Analog Versus Digital Isolation

Analog input or output channels can be isolated using two different methods regardless of the instrument isolation topology. The difference between the two methods lies in the location of the isolation circuitry in the instrument. Analog isolation is where the isolation circuitry is in the path prior to the analog-to-digital convertor (ADC) and it acts on the analog signal. Digital isolation is where the isolation circuitry is after the ADC, because it acts on the newly digitized data. 

Analog Isolation

One large benefit of analog isolation is that it protects the ADC. Because the isolation is provided before the ADC, the ADC is less likely to be damaged by transient or high voltages. Analog isolation does, however, have disadvantages. First, because analog isolation is not perfect and it lies before the ADC, it can add gain, nonlinear, or offset error to the analog signal before it reaches the ADC. This is not ideal and can decrease the accuracy of the measurement. In addition, analog isolation components can introduce longer settling times and are often more expensive than their digital isolation counterparts. 

Digital Isolation

Digital isolation can lead to better performance and accuracy, in comparison to analog isolation circuitry, because the measured signal is less altered before it is digitized by the ADC. Digital isolation circuitry also has advantages over analog isolation circuitry because it is typically lower in overall cost and it performs at higher data transfer speeds. However, because digital isolation circuitry is after the ADC, the ADC is more susceptible to the damage a voltage spike can cause. 

Types of Isolation

Physical isolation is the most basic form of isolation, meaning that there is a physical barrier between two electrical systems. This can be in the form of insulation, an air gap, or any nonconductive path between two electrical systems. With pure physical isolation, you can imply that no signal transfer exists between electrical systems. When dealing with isolated measurement systems, the signal of interest needs to cross the isolation barrier with the bene ts of removing ground loops. This can be done using Capacitive Isolation, Inductive Isolation, as well as Optical Isolation.

Capacitive Isolation

Capacitive isolation uses an electrical field as the form of energy to transfer the signal across the isolation barrier. The electric field changes the level of charge on the capacitor. This charge is detected across the isolation barrier and the charge detected is proportional to the level of the measured signal.

Inductive Isolation

Inductive isolation uses a transformer to transfer a signal across an isolation barrier. The transformer generates an electromagnetic field, proportional to the measured signal, as the form of energy to cross the isolation barrier.

Similar to capacitive coupling, inductive isolation can provide relatively high-speed data transmission rates. In addition to high-speed transmission, inductive coupling uses low power for the data transmission. However, inductive coupling is susceptible to interference from surrounding magnetic fields because it uses electromagnetic fields as the method to cross the isolation barrier. If external magnetic fields do interfere with the electromagnetic field produced by the transformer, this could affect the accuracy of the measurement.

Optical Isolation

Optical isolation uses an LED and a photodetector to transmit the signal information across the isolation barrier. The isolation barrier in optical isolation is typically an air gap and the signal is transmitted using light. The light intensity produced by the LED is proportional to the measured signal. 

Because optical isolation uses light as the energy to transfer the measured signal across the isolation barrier, it gains the advantage of immunity from electrical- and magnetic-field interference. This can make optical isolation an effective technique in industrial areas where strong electric or magnetic fields could be present. The advantages gained by using light are balanced by some disadvantages. Optical isolation typically has slower data transfer rates, which are limited to the LED switching speed. It also has relatively high power dissipation when compared to capacitive and inductive isolation. 

Isolation Types Summary

Source: http://download.ni.com/evaluation/pxi/Isolation_Types.pdf

Optocouplers 

Optocouplers use the concept of optical isolation to provide complete electrical isolation between input and output circuits. Generally the useful purpose of isolation is to provide protection from high voltage, surge voltage and low level noises that could potentially be sources of error.

Example: OPI1268S

• Part: http://www.digikey.com/product-detail/en/tt-electronics-optek-technology/OPI1268S/365-1562-ND/2744562
• Brief product brochure: http://media.digikey.com/pdf/Data%20Sheets/Optek%20PDFs/OPI1268S_Brief.pdf