Analysis and comparison: analog and digital isolation technology

Engineers who design industrial circuits use isolation technology to address safety issues, regulatory oversight, and ground plane issues. If you have isolation in your circuit, you can exchange information and power between two points without actual current flow. Isolation has two major benefits. First, it protects people and equipment from potentially dangerous surge currents and voltages. Second, it prevents accidental ground loops from interfering with signals from data links and other interconnects. 
 
Analog input/output, instrumentation, motion control, and other sensor interfaces typically use single-channel isolated signal chains. These isolation channels are used to separate sensor circuits in the harsh environment of the factory floor from signal processing stages in the quiet control room environment.

Depending on the isolation requirements, analog isolation amplifiers, isolated power supplies, digital couplers, or optocouplers can be used. The barrier conduction of these isolation circuits uses capacitive, magnetic, or optical technology.

The single-channel isolated temperature measurement circuit in Figure 1 uses a capacitively coupled analog isolation amplifier. In the circuit, an RTD (resistance temperature detector) converts temperature to a resistance value. A 100-μA current source converts the RTD resistance value to a voltage. The INA114 instrumentation amplifier amplifies the RTD/100-μA voltage and eliminates the RTD wiring resistance RL. The gain of the instrumentation amplifier matches the input voltage range of the isolation amplifier.
 

The precision isolation amplifier uses duty cycle modulation to transmit the output signal of the instrumentation amplifier through a capacitive isolation barrier. The isolation amplifier can achieve a maximum isolation of 1500V. The input signal bandwidth of this isolation device is approximately 50 kHz and the minimum power supply requirement is ±4V. The package uses a 28-pin PDIP or SOIC.

Another design approach is to use an ADC. The ADS1247 ADC has two internal current sources, a PGA (programmable gain amplifier), and a delta-sigma modulator. The current sources convert the RTD resistance to a voltage and cancel the effect of the three wiring resistors. The PGA boosts the RTD input signal, and the converter provides a digital output signal.

The ISO7241 and ISO7221 digital isolators transmit analog-to-digital structures and conversion results across the isolation barrier. The isolator in Figure 2 uses internal capacitive isolation technology to transmit digital signals across the isolator and is available in 8-pin and 16-pin packages.

It is difficult to say which isolation strategy will be right for your application. Each of these analog or digital isolation strategies can be applied to any signal that needs to be isolated in a circuit, which uses a variety of sensors to measure temperature, pressure, current, etc.

Isolation amplifiers may be a suitable approach, as this allows you to stay in the analog domain. However, they have higher power requirements. Digital isolators may also be a good option, as the signal will eventually be converted to the digital domain.