Isolation amplifier
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
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:
- inductive coupling
- capacitive coupling
- optoelectronic coupling ( optocoupler or fiber optic cable )
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.
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.
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.