Isolation amplifier

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Isolation amplifiers or isolation amplifiers are always used for galvanic isolation when this is not possible or desired using a transformer . In contrast to transformers, isolation amplifiers can also transmit DC voltage signals. The term amplifier does not necessarily mean that you also perform a voltage amplification.

Working principles

Basic circuit diagram of an isolating amplifier

The circuit of an isolation amplifier consists of a transmitting and receiving part. The transmitter works on the measuring potential, while the receiver works on another potential. A floating power supply is required on the transmitter for this. The ground connection of this floating power supply is the reference potential of the transmitter.

The power supply is galvanically isolated via a transformer . Isolated, so-called DC / DC converters are used, which work with a high-frequency oscillator in the range of a few 100  kHz . As a result, very low coupling capacities, often less than 10  pF , are achieved.

The signal voltage must be transmitted to the receiver in an electrically isolated manner. The following types of electrical isolation are used for this purpose:

With inductive or capacitive coupling, the measured voltage must be modulated onto a carrier with a sufficiently high frequency . Frequency modulation or pulse width modulation is mostly used here.

Circuit for the transmission of an analog value using an optocoupler: both optocouplers can also use the same transmitter

Direct voltages can also be transmitted with simple optocouplers. The non-linearity of the optocoupler is canceled by a suitable wiring - by comparison with a reference optocoupler of the same construction. There are also optocouplers especially for this purpose with a transmitting diode and two receiving diodes, which have particularly narrow tolerances with regard to their CTR .

Components

Isolation amplifiers with inductive, capacitive and optoelectronic coupling are commercially available as finished components. Many types have built-in DC voltage converters. External DC voltage converters are therefore usually only used if they are used to operate several isolation amplifiers with a common floating ground.

Important isolation amplifiers
Type Manufacturer Signal transmission Isolated power supply Cutoff frequency Insulation
voltage
Remarks
AD202 Analog Devices inductive internally 0… 2 kHz = 2000 V self-catered entrance
AD210 Analog Devices inductive internally 0… 20 kHz = 2500 V Differential input, supply isolated from I and O
AD215 Analog Devices inductive internally 0… 120 kHz = 1500 V self-catered entrance
ISO100 Texas Instruments , Burr-Brown optically external 0… 5/60 kHz = 750 V low noise, discontinued model
ISO103 Texas Instruments, Burr-Brown capacitive internally 0… 20 kHz = 1500 V self-catered entrance
ISO113 Texas Instruments, Burr-Brown capacitive internally 0… 20 kHz = 1500 V self-supplied output
ISO121 Texas Instruments, Burr-Brown capacitive external 0 ... 60 kHz = 3500 V high insulation strength
ISO122 Texas Instruments, Burr-Brown capacitive external 0 ... 50 kHz = 1500 V
HCPL-7840 Avago Technologies Optocoupler external 0 ... 100 kHz = 2500 V Transient resistance 15 kV · µs −1
HCPL-788J Avago Technologies Optocoupler (digital) external 0 ... 30 kHz = 600 V Transient resistance 25 kV · µs −1 , direct connection to A / D converter, IEC, UL, CSA approval
HCNR200
HCNR201
HP Double
optocoupler
external 0… 1 MHz = 1400 V high linearity

Finished components that require an external power supply are operated with isolated DC / DC converters.

literature

  • Dieter Zastrow: Electronics: text and exercise book for basic circuits in electronics, power electronics, digital technology / digitization with a revision course in electrical engineering . 9th edition. Vieweg + Teubner, 2010, ISBN 978-3-8348-0887-5 , Chapter 9.7: Isolation amplifier.